×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Three-Lifting Surface
3

Three-Lifting Surface

Three-Lifting Surface

(OP)
A lot of discussion ensues when somebody brings up the topic of "canards", which invariably involves lousy stall characteristics, Mr. Rutan's designs, and landing speeds about 20 knots below Vne.
The solution seems to exist in one very unique Italian aircraft called the Piaggio Avanti which has 3-count-em-three lifting surfaces.  The main wing has flaps.  The forward wing has flaps, too.  The tail wing has some kind of elevator-stabilator mechanism that I don't quite understand that controls pitch with and without flaps.
As far as I can tell, this is the only 3LS aircraft certified by the FAA.
Are there any others?  Whenever I search the internet, all I get are wannabe designers' pipe dreams and scaled-down knock-off's.
Apart from its aerodynamic complexity, what hidden drawbacks might be in the design?

RE: Three-Lifting Surface

Hi.
My interpretation of the Piaggio design is that it is a clever way of achieving a cabin area free from the spar carry-thru, without resorting to putting the same on the outside of the fuselage cross section. As the spar is located behind the cabin, and the nose of the aircraft is long and the tail is short. All cargo is located in front of the wing. To offset this the engines are put on "backwards". To avoid large trim drag an low authority when flaps are extended from the short-coupled tail the front lifting surface is added. The drawback could be questionable post stall behaviour. No doubt this can be solved as the design is certified.

RE: Three-Lifting Surface

(OP)
Cabin space is an issue I didn't think of before.  The Israel Aircraft Westwind has its mid-wing spar carry-through defining its aft pressure bulkhead, too, but this consumes about 1/3 of the potentially available fuselage area.  The ability to move the wing back by many feet vastly expands the usable space.  BTW, there is a small baggage compartment behind the wing, too - definitely not accessible when the engines are running!

STF

RE: Three-Lifting Surface

   Eagle-X, orginally designed in Australia and now owned by the Malaysians is also a three surface configuration. The Eagle-X is certified to FAR-23 standards.

   I will say no more.

   Timbo

RE: Three-Lifting Surface

(OP)
bfuchs,

Don't confuse simple kit canard aircraft and with the more elaborate 3 lifting surface design.

The Velocity is heavier than the Cessna 172, and stalls at 65 knots.  That's more than the LANDING speed of the 172.  Canard aircraft are not as safe to operate as conventional aircraft.  Are any canard aircraft certified?  I don't think so; not many at least.

The three lifting surface design keeps the landing speeds down to levels safe enough for certification of the Piaggio and the Eagle-X.

macoovancancy,

Do you happen to know offhand if the Eagle-X has flaps on its main and forward wings?

STF

RE: Three-Lifting Surface

“…Canard aircraft are not as safe to operate as conventional aircraft…..”
Burt Rutan originally started designing canards in the experimental (homebuilt) industry for exactly the opposite reason. The leading cause of light plane fatal accidents is stall-spin during the landing pattern. A canard is supposed to be stall-proof since the canard surface stalls at lower angle of attack than the wing’s stall angle of attack. The canard then no longer has enough control authority to keep the nose up so the aircraft never reaches the wing’s critical angel of attack. Do I have that right?

Another certified canard was the Beech Starship, certified in 1988 it went into production in 1992, and out of production in 1994 after only 53 were built. It had great performance but cost $5 million, as much as some jets.

RE: Three-Lifting Surface

Canard: Boeing's unfortunately abandoned Sonic Cruiser would most likely have been a canard.  The problem was not controllability: it was keeping the canard from hitting the jetway.  Further, the instability of canards can easily be overcome with modern digital control systems.

3-surface: Check to see if the Piaggio P-180 Avanti is in Jan Roskam's most recent book Airplane War Stories.  It's on the cover.

RE: Three-Lifting Surface

Offhand, the Eagle-X has flaps on both the fore plane and the aft plane. The pilot has only one flap control. Also the aft plane has a wing fence and vortex generators immediately ahead of the ailerons. The wing section outboard of the wing fence has a drooped nose. The FAR certification requirement requires aileron control down to , and into, the stall of the aircraft. The aft plane (the larger one) has the aileron section in the UPWASH of the fore plane. Offhand.

Timbo

RE: Three-Lifting Surface

(OP)
I recall seeing the same drooped leading edge halfway outboard the main wing on the Piaggio, but no fences.
I also noticed that the avionics technicians having difficulty getting around the front wing to work on the electronics bay in the nose.  In compensation, it makes a handy work table (perhaps not such a good thing).

Thanks for all the ideas.  So far the score is:

Certified 3 lifting-surface aircraft=2 (Piaggio & Eagle)
Certified canard aircraft=1 (Starship, but we won't forget the Wright Flyer)
Certified conventional aft-tail aircraft=everything else.

STF

RE: Three-Lifting Surface

SparWeb
Going back to your original question, one drawback of the 3-surface design on aft-engined configurations is that it can make it difficult to access the nose of the airplane, where avionics and other systems service and maintenance are done.  Also makes it hard to put a convenient baggage compartment there.  The Eagle-X, which has its engine in the nose, has a swing-away engine mount to provide access to the engine and firewall.

Another possible drawback is that the forward wing may block a large portion of the pilot's forward vision, which can make navigation difficult and affect safety in the airport pattern.  The Avanti 3rd surface is too small to be much of an issue here, but the Eagle's forward wing is quite large.

In a way, the Eagle-X almost seems to be more of a biplane with large negative stagger than a 3-surface airplane like the Avanti.  The Eagle-X seems to be nicely detailed and Dave Higdon's flight test of it is very positive.  http://www.avweb.com/news/newacft/182746-1.html

Is the company still alive?  It's website isn't.
http://www.eagleair.com.au/

Was the Eagle certified to FAR PART 23 or to JAR-VLA?

RE: Three-Lifting Surface

The fences and drooped leading edges on the main wing of an airplane with a forward wing are usually there to counteract the tip vortex of the front wing. The vortex can significantly increase the angle of attack of the outboard main wing.  Drooping the outboard leading edge is like twisting it nose down.  

The fences help to keep the vortex disturbance and the wing profile discontinuity from affecting the inboard wing too much.

For additional comments on canard and 3-surface configurations, see FAQ2-760.

RE: Three-Lifting Surface

As I see it, at lower speeds, i.e. landing, takeoff and low speed manouvers, with lift shared between the foreplane and the mainplane, and the stabilator providing vertical control, the aircraft will remain stable in the vertical plane, as long as the foreplane stalls before the main plane.  

Unlike a pure cannard, a three surface setup does not rely on a stalled surface for control once the foreplane stalls.  The control is provided by the stabilator, which is not stalled, since to provide meaningful control it must remain in a near neutral lift position.  This removes the main danger of the cannard configuration.  As I said before, trying to maintain control with a stalled airfoil isn't something I really want to experience.

Since both the fore and aft planes on the Avanti have flaps, it follows that during landing and takeoff, the lift from  both is enhanced and this mutual enhancement should be such that the centre of lift remains constant.  However, a thought occured which I will try to check out:  With the flaps on the main wing lowered, the turning moment of the flaps on the aircraft will be resisted by the foreplane.  If the foreplane flaps deploy further, pro rata, than the mainplane flaps, this will increase the lift of the foreplane and counteract the turning moment.  This will also ensure that the foreplane will stall before the mainplane, which is a pre-requisite for stall stability.

In the cruise, it should be possible to trim the aircraft such that the stabilators also contribute to lift.  Whether this will increase of lower drag is a moot point. (Don't point that moot at me!) It could be achieved by increasing the angle of attack of the foreplane slightly and compensating for the nose up moment this would cause by placing a nose down trim on the stabilator.  It would increase lift at the expense of a, probably, slight increase in drag and may explane the Avanti's high cruise altitude ability.

For general information, here some performance figures for the Avanti.

PERFORMANCE
Maximum Cruise Speed
395 KTAS/ 732 km/hr at 28,000 ft/8538 m
Ceiling
41,000 ft/12,500 m
Maximum Range
1,750 nm/3,241 km (6 PAX, VFR Reserve)
Engines
Pratt & Whitney, Canada PT6A-66

EXTERNAL DIMENSIONS
Wing Span
46.03 ft/14.03 m
Length
47.28 ft/14.41 m
Height
13.06 ft/3.98 m

CABIN DIMENSIONS
Height
5.74 ft/1.75 m
Width
6.07 ft/1.85 m
Length
14.93 ft/4.55 m

Weights
Maximum Take-Off Weight
11,550 lbs/5,239 Kg
Standard Empty Weight
7,500 lbs/3,402 Kg

Take-off run - 2,850ft
Climb - 2,950ft./min
Climb - 1,106ft./nm

Useful Load (Incl. Crew)
4,100 lbs/1,860 Kg
Maximum Payload
2,000 lbs/907 Kg

Seating Capacity
Maximum Seating Capacity
9 plus 2 crew
Typical Executive Payload
6 plus 1 crew

I find the climb rate of 1000 fpm on one engine particularly impressive.  Whether the three lifting surface design contributes to this I don't know, but I would suspect that it does.  Take off run of just over a half mile is also quite useful for an aircraft with this performance.

I think there may be two reasons why this design meets with resistance:

1 - It gets confused with pure cannards such as the velocity,

2 - It's different.  don't forget that aviation is a one hundred year old industry and in any industry of such an age there tends to develop a desire to stick with the tried and tested - and familiar.  Sometimes this is a good thing; no-ones come up with a real replacement for the C-130, for example.  sometimes it holds the industry back and prevents people from judging products on their merits, such as performance and comfort.

Actually, the more you look at the three surface design, the better it looks.  I wouldn't be surprised to see an airliner designed on the same principles before long, just don't expect it from Boing or Airbus!

John

RE: Three-Lifting Surface

Miper:

The FAA Type Certificate lists the following as the cert basis (TCDS #A00005LA):

14 CFR part 21, § 21.17(b) using Joint Aviation Requirements - Very Light
Aeroplanes (JAR-VLA) at Amendment 0 dated 26 April 1990, through Amendment
VLA/92/1; § 21.29; and 14 CFR part 36 through amendment 36-21 effective
December 28, 1995.
Noise Control Act of 1972.
Eligible for day-VFR operations and normal category maneuvers only.

I assume that other authorities that have type approved the Eagle probably accepted JAR-VLA as the cert basis as well.  Just guessing.

And on a completely different note, I was quite enjoying the technical discussion in this thread.  Not that it matters.

Regards

RE: Three-Lifting Surface

Rumours of a stretched and/or jet version of the P-180:  anybody heard anything conclusive?

John

RE: Three-Lifting Surface

i278,
Thank you for the information about the Eagle.  Day VFR only would probably make the airplane a hard sell in this country, especially at $120,000 or so for a 2-place.  


-----

John,
I have liked the Avanti from the first time I saw a rendering of it.

There is no doubt some truth in your comment that the Avanti meets with resistance because it is different.  I have heard a few people mention that it's fuselage has an unattractive, almost mouse-like look because there's no "notch" at the windshield.  

Some little-understood fallout from the Starship is that, because it failed to live up to its high technology hype, new technologies lost their credibility among many pilots, customers, and business airplane builders for about 15 years.  After Raytheon had spent over $1Billion on the Starship (about 5 times the cost of developing an equivalent, garden variety jet) and then failed to find a market, no one seemed to care that Piaggio succeeded in its implementation of new technology, because the Avanti didn't sell well either.

And there are other facets to Piaggio's sales problems as well:

It takes a lot of time and money to establish a toehold in the business aircraft market and Piaggio has not yet invested enough there to fully establish its credibility to potential customers.  In particular, a turbine airplane customer who might fly anywhere in the world wants assurances that he can get overnight turnaround on minor maintenance and servicing almost anywhere in the world. Piaggio can't do that yet.

Quite literally, turbine airplanes are sold for luxury and status to the richest .00003% (roughly 1 in 3 million!) of the world's population. With turboprop airplane prices generally equivalent to a same-size business jet, the turbine market usually prefers the quieter cabin and higher speed of a jet.  (It may seem counter-intuitive, but it actually costs a bit more to design and produce a turboprop than a jet, because engine integration and control for the turboprop is more difficult.)  The jet is not as efficient, but it is usually quieter (inside), faster, and more prestigious; and those who can afford them are not much concerned about fuel costs.  In general, the fraction of sales going to turboprops has slipped over the past 20 years compared to jets.

I also wonder how quiet the Avanti cabin is. The props operate in the wing wake and must, therefore, generate considerable external and internal sound signatures.  These can be controlled, but are they? and at what weight penalty for the soundproofing?

Finally, the airplane is so optimized for low drag that it appears to be difficult to maintain.  This may not be the case, but access to systems appears to be more constrained than on competing airplanes.  One example is the heated leading edge of the wing.  Other manufacturers make this a removable assembly because it often requires maintenance or parts replacement (failed temperature sensors, bird strike, hangar rash).  On the Avanti, there is no access provided.  Also, access to avionics in the nose is more difficult because of the 3rd surface.

Unfortunately, while the major companies tend to appear hidebound by their entirely natural tendency to keep doing what has worked before, the new companies with new ideas often find those ideas much more expensive to implement; and then they don't have the capital to establish themselves in the marketplace.  Avanti is slowly overcoming this

---------

Have not heard anything about a possible jet version of the Avanti, but it seems like a natural thing to do, except for one thing.  Swapping out the props for jets might make a worse airplane - not because there would be anything wrong with it, but the turboprop is such a magnificently  optimized design.  It would take a very careful look to be sure that a jet has anything to offer.  

I know this seems to conflict with my comment above about a jet being preferable, but when you have a baseline for the market to compare, you have to show a real difference.  You might remember that when Mooney put a Porsche engine in its airframe, the new airplane cost more, had lower payload, and was slower.  Of course, it didn’t sell.  They could have succeeded if they’d done a new airplane for the engine.  A new design would have allowed them to hide the negative aspects of the engine and fully exploit the positives.  Piaggio may have the same kind of problem with simply putting a jet into the Avanti.

About 50% more fuel volume would be needed in a jet version of the Avanti, and the airplane's gross weight would increase;  so it would probably need a new or modified wing, associated changes in the other two lifting surfaces, and maybe beefed up gear and fuselage components.  That could be a $60 Million program, depending on the scope of the changes.

RE: Three-Lifting Surface

Miper,

Good thoughts on the "jetifying" of the Avanti. One of the things which attracted me to the aircraft in the first place was the fact that it was a turbo-prop.  A turbo-prop with jet performance but prop economics, which is a hard combination to beat.

Apart from the sheer looks of the thing, one of the Avanti's biggest selling points, to me, is the combination of economy and performance.  I'm not among those who will ever become careless about economy, unlike, as you said, many corporate jet owners.  I have investors and it's not, in reality, my money to waste.

Intersting point about noise.  I remember, back when I was young and remarkably stupid, stepping out of the door of a perfectly serviceable Hercules at 2000 ft and, before my 'chute damn near castrated me, being relieved to get out of the noise in the aircraft.  It's said that the Hercules is so quiet on the outside because it concentrates all the noise inside.  

Now I don't have proof of this, but I suspect that the Herc' fuselage is resonant at or near the frequency generated as the compression wave from each prop blade strikes the fuselage.  I know that the noise actually varied as the RPM varied, with quiet and loud points occuring, which would tend to confirm this assumption.  The flat side of the fuselage would simply act as a sounding board for the compression waves and the lack of any real damping material inside (apart from the bodies of myself and the other would be escapees)just made the situation worse.  This is probably close to a worst case as far as turbo-props are concerned.

The Avanti props, on the other hand, will produce compression waves which strike the fuselage at the narrowest point as far as elevation area is concerned, that is, just before the swept back vertical stabilizer.  This won't eliminate resonance effects, but the rounded fuselage and smaller area of compression wave impact will certainly reduce it.  

Another factor is the fact that the Avaint is pressurised to sea level up to 24000 feet.  (Which seems a bit excessive, but I'm not going to complain).  This means that the fuselage, apart from its intrinsic stiffness, will increase in stiffness as the aircraft climbs.  This will reduce fuselage resonance and therefore noise.  Which reminds me - I must check to see if the bagage compartment is pressurised.

One other point:  Piaggio themselves don't claim that the stabilator ever provides lift, so my thoughts about trimming for three airfoil lift in the cruise were wrong.  The tailplane is there to ensure that the aircraft gets the best of both worlds, lack of trim drag and continuance of control if the foreplane stalls.  

There have been some remarks about the possibility of the foreplane wash impinging on the mainplane.  Actually, looking at the front elevation of the aircraft it can be seen that the foreplane is actually much lower than the mainplane.  The foreplane actually has anhedral which, if I remember correctly, will reduce tip vortices.  The downwash from the foreplane will pass safely under the mainplane even in landing configuration, unless the aircraft is in a ludicrous attitude.  The stabilator is mounted at the top of the vertical stabilizer, which means that it is well out of the way of either fore or mainplane turbulance, so generally, there is a complete lack of interference between horizontal surfaces.

I had at first thought that the swept back vertical stabilizer was a bit of Italian design!  About as pointless as the equivalent on the Cessna.  However, if the stabilators are mounted at the top of the vertical stabilizer the sweep back increases the distance between the stabilators and the mainplane, increasing the moment of rotation on the aircraft when the stabilator is operated.  So - cool and practical - very Italian!  

Your points about maintenance are well taken.  It may be that Piaggio have gone too far with the smooth skin effect.  It needs looking into.  However, I would assume (and like all assumptions this may very well be wrong) that Piaggio have got the maintenance details right as well.  For example, were I to design a heated leading edge which was sealed, I wouldn't place any fail prone elements, such as temperature sensors, actually in the leading edge itself.  the skin of the fuselage may not, due to compression effects, be at the exact same temperature as the wing leading edge, but a sensor placed on the fuselage skin and connected to the heater control via a very simple processor will compensate for temperature differences and still turn the heating elements on when required.  The heating elements themselves would be very robust and, obviously, powered via circuit breakers.  As far as the foreplane interfering with avionics maintenance, I'm pretty sure that this is something the techs will get used to.  As long as they don't dent the foreplane with their heads too frequently!  

The more I investigate this aircraft, the better it seems.  If you throw out your preconceptions about what an aircraft "ought" to look like, its a very clever, advanced and practical design.  It's gotten to the point where I'm actually looking for faults - for example, that landing gear is going to take up fuselage space, or, two thousand miles isn't THAT far!  

Regards

John

RE: Three-Lifting Surface

(OP)
A few answers that I can provide:

The Avanti is pressurized to a maximum of 9 psi differential, which is 3 psi greater than the typical turboprop.  The consequence is that the fuselage skins are thicker than usual, too.

There are many things that change when a leading edge is drooped, any one of which could have been the reason for Piaggio doing so on the main wing of the Avanti.

The baggage compartment is behind the aft pressure bulkhead, therefore, not pressurized.  It's not terribly big, either.  The pilot I talked to would have preferred to have an auxiliary fuel tank there.

The foreplane is definitely in the way of avionics work, but in exchange, you get a handy work table at waist height(the anhedral isn't so steep that your screwdriver won't roll off).

STF

RE: Three-Lifting Surface

John,

The economy of  turboprops appeals to many buyers who share your view.  Perhaps the most notable was Sam Walton, who was famous among jet sales teams for refusing to have a jet because he considered them to be wasteful and projected the wrong image to his investors, employees, and customers.

The science of interior noise propagation and suppression is still incomplete, but your observations are correct.  Turbopropeller noise is mostly concentrated in the plane of the prop, so it is important to keep the prop as far as possible from the fuselage.   Piaggio did a better job of this than Rutan on the Starship configuration he sold to Beech. The props on the Starship are only about a foot away from the fuselage, which is a very stiff carbon-epoxy composite sandwich that has scant natural damping.  The sound energy levels in the cabin without noise treatment are said to be painful.

Raytheon ended up with 4 separate soundproofing treatments on the Starship.  1. Small mechanical absorbers that were mounted on the fuselage wall between the windows at ear height.  They vibrated out of phase with the blade passage frequency at cruise.   2. Two layers of heavy, leaded vinyl on the aft pressure bulkhead, on the floor, and on the entire fuselage shell from the aft bulkhead to the cockpit.  3. A fiberglass layer to absorb higher frequency noise.  4. Unusually heavy interior trim panels that may have included more lead for dampening low frequency noise.

Detailed analyses of canard configurations by Ilan Kroo and others show that, from a minimum interference drag standpoint, the best vertical location for a canard is well above the main wing.  This is similar to the result from biplane theory, that shows positive stagger to be better than negative stagger.  Canard test results confirm this conclusion, even if it isn’t intuitive.  Like you, I had thought a low canard was better before I saw these results.  In any case, few configuration lend themselves to a high canard, and the proposed Boeing Sonic Cruiser is one of the few I’ve seen.

The front view of the Avanti shows that the canard tips are approximately in line with the inboard edges of the nacelles. This arrangement certainly minimizes the effect of the canard tip vortex on the wing.  

I have learned not to be too sure of where the canard and wing wakes will go.  We rely on water tunnel and wind tunnel tests for that, and even then we get surprised.  The aft fuselage strakes on the Avanti are probably not part of the original design.  They are, no doubt, a late fix added in flight test to pass some extreme attitude certification point.  Was the problem due to impingement of one wake on another surface?  Probably yes, but only the people involved know for sure.

Again, I consider the Avanti configuration to be one of the most elegant and sophisticated aerodynamic designs ever done.  Please understand that I only pointed out some possible drawbacks because the original question in this thread asked for them.  Whether these things are real or important drawbacks is another thing entirely.

RE: Three-Lifting Surface

SparWeb,

My comments about outboard leading edge droop were made with the Eagle-X in mind.  Does the Avanti have droop?  I can’t see any in the photos I looked at on the web.  The usual use of drooped leading edges on the outboard wing is for improved spin entry / recovery on light planes .  Jim Patton and Paul Stough had excellent success with this solution in a famous NASA G.A. spin program (late 1970s – early 1980s).  The Cirrus airplanes and several others use the outboard drooped leading edge.  Anyway, as you say, many things change with droop, so there can be several reasons for doing it.

Fuselage tanks are sometimes necessary, but designers and operators don’t like them much because they add significant complexity to an otherwise simple fuel system.  Also, in-flight fuel and CG management is often required, which pilots complain about.  FAA & JAR rules require that the tank not be placed in the rotor burst plane of any turbine engine wheels.  That’s taken as a (plus & minus)5 degree disk about the engine centerline at each wheel.  This limits possible locations for a tank.  Right now, the P-180's aft baggage is in line with the turbine wheels, so Piaggio would probably have to move almost everything in the tailcone to accommodate a fuselage tank.

RE: Three-Lifting Surface

Miper,

First, thanks forthe extra information, especially regarding the possibility of a fuel tank in the aft baggage compartment.  I guess I'll just have to get familiar with Keflavik, not that there's much there to get familiar with.

"Detailed analyses of canard configurations by Ilan Kroo and others show that, from a minimum interference drag standpoint, the best vertical location for a canard is well above the main wing.  This is similar to the result from biplane theory, that shows positive stagger to be better than negative stagger.  Canard test results confirm this conclusion, even if it isn’t intuitive.  Like you, I had thought a low canard was better before I saw these results.  In any case, few configuration lend themselves to a high canard, and the proposed Boeing Sonic Cruiser is one of the few I’ve seen."

As I recall from my reading, the reason for positive stagger in biplanes was to preventthe high pressure region below the upper wing from interfering with the low pressure region above the lower wing, rather than anything to do with wake interference.  Surely, if the foreplane on the Avanti, which is separated from the mainplane more than sufficiently for pressure region interference to be a non event, is producing lift, then the wash will be deflected downward, passing under the mainplane, this wash position being first determined by the low position of the cannard, which means that a downward tending airflow starts out in a low position.  I can understand the analysis results if the cannard is close to the mainplane, and pressure region interference is a possibility, but with the separation on the Avanti, I just don't see how a high cannard would be superior to the position chosen.

If analyses and tests indicate otherwise, all I can say is "I'm gobsmacked!"  I also find it hard to envision how the foreplane wake can interfere with the mainplane on the Avanti.  Still, I've just managed to make a circuit, which was perfect on paper and in simulation, stop oscillating when realized as a prototype.  Nothing to do with the design, but the printed circuit board added some "phantom" components which managed to add TWO positive feedback loops to the circuit, so I shouldn't be surprised if, in aerodynamics, a subject about which I know far less than I know of electronics, "Things are not always what they seem, skimmed milk masquerades as cream!" (G&S).

One aircraft which has a cannard above the mainwing is the EF2000 Typhoon, which uses a cannard/delta configuration for extreme maneuverability.  But comparing the configuration of a supersonic, high maneuverability fighter with a nine place executive turbo-prop is probably an exercise in frustration.  I do notice, however, that the cannard on the Typhoon is far closer to the mainplane than it is onthe Avanti.  Perhaps there is a preffered vertical position for the foreplane for every separation, for which a simple formula/graph could be derived.  Just a thought.

John



John

RE: Three-Lifting Surface

(OP)
An acquaintance just pointed me to a website,

www.aceair.ch

showing a flying prototype 3-wing aircraft.  I realized that I had seen this thing on the web a few years ago when it was just a mock-up and moulds doing some airshow tours.  Looks like the paper airplane has become a reality in this case.  It literally looks like a mini-Avanti!

STF

RE: Three-Lifting Surface

It does look like a mini Avanti, down to the anhedral on the cannard amd the swept back vertical stabilizer and stabilator.  I also notice that it uses a rotary, presumably wankel, engine, which will make it almost as vibration free as a turbo.  Performance is pretty fair for just over 100 HP.

I would say that Mr. rutan has some very viable competition.

Anybody noticing a common thread about these aircraft - that thread being Europe?

John

RE: Three-Lifting Surface

John,

Your aerodynamic insights are quite good, actually; much better than my electrical insights, of which there are few to none.

I looked around for something to clarify the placement issue for biplanes and canards, but haven’t found the right box of reports yet. (Remodeling a house can make many things disappear under stacks of boxes.)  I will admit to being a little sloppy in saying that the canard and biplane interference characteristics are similar.  I meant that the result was similar – it’s best to put the forward wing up high (or put the high wing forward, depending on the point of view).

You have given the usual explanation for the advantage of positive stagger, an explanation based on near-field pressures.  It’s interesting that Munk’s biplane theory doesn’t show any difference for positive vs. negative stagger; but that’s because it’s based on Prandtl’s lifting line theory.  Munk’s result, therefore, wouldn’t have anything to say about the interference of local airfoil pressures.  Anyway, canard interference effects would be more of a far-field thing.  

It is important to keep in mind that, in subsonic flight, the effects of a wing are felt at quite a distance from the wing, even forward of it. So far-field effects can be extensive.  (Wing vortex and source-sink methods use mathematical forms for the aerodynamic force fields that are identical to the form for the field of a charged particle.)  Interestingly, what makes supersonic flow different is that the air ahead of the shock wave is unaware of the airplane coming because pressure effects propagate at the speed of sound, which is slower than a supersonic airplane.

A typical value for main wing downwash angle for a business turboprop in cruise is about 1.5 –3 degrees, depending on cruise speed, altitude, and wing loading.  At 20 ft behind a wing, then, the center of the wing vortex sheet wake would be only about 6 – 13 in below the wing.  A highly loaded canard might have roughly twice as much downwash angle.

The interference between a canard and the main wing is a far-field effect, meaning that we should consider the vortex wake patterns. Tip vortices tend to be on and above the plane of the wing as they are shed off the tips.  Something like this.

    Tip                        Tip
   Vortex                     Vortex
    _ _                        _ _
   /   \    Vortex Sheet      /   \
   \___/______________________\___/

What we see is that for a low canard, while the center of the canard vortex sheet (trailing edge wake) does not impinge on the main wing in cruise, the canard tip vortices almost always do.  So it is entirely possible that the reason that it is better to put the canard high is that the tip vortices of a high canard are less likely to impinge on the wing.

RE: Three-Lifting Surface

Miper,

Thanks for the explanation.  I can see how tip vortices could impinge on the mainplane, especially at higher angles of attack.  It would appear that this is one explanation for the anhedral on the foreplane, since, as I understand it, this will reduce tip vortices.  

If tip vortices are a problem, however, I would have expected Piaggio to have attached some cool looking, swept back tip fences to the foreplane!  Maybe I'm totally wrong here, but I was underthe impressionthat these significantly reduce tip vortices (Not the cool sweep back, just the fences)

A rather wierd thought, but is it possible that Piaggio are actually using the foreplane tip vortices to increase lift on the mainplane.  As I understand it, the tip vortex spirals off the wing tim and spreads as it is left behind.  If the vortex is actually striking the lower surface of the maniplane, increasing local pressure, then this might contribute to lift.  Such an effect would depend on airspeed but, at cruise speeds, might be reliable enough to be useful.

RE: Three-Lifting Surface

The strength of a tip vortex depends on a wing's total lift and its spanwise lift distribution.  I don't know how much anhedral affects it, but it wouldn't be a lot (2% ?).  Tip plates or winglets effectively add span, slightly reducing (by ~4-5%)the vortex at the tip, itself.

I've mentioned before that the Avanti canard tips line up with the nacelles.  This may be merely an artifact of the original layout, when it may have been judged that vortex impingement on the nacelle would be better than on the wing.  I doubt that canard anhedral was in the early layouts.  

Wakes on an airplane with high wing loadings are formidable.   Sometimes quite a bit of work is required to eliminate wakes that hit the tail as the airplane approaches stall.  Pilots call this buffeting; but in flight test, until we get it controlled, the wake effects might be better called "bone jarring rattling and shaking".  This effect is exacerbated by wake that enters an aft propeller disk.  The turbulence is then multiplied by the horsepower.  Pilots report nothing pleasant about the experience, especially in go-around tests.  Nor is it safe.  Buffet loads are among the worst that the tail of an airplane experiences; and prolonged buffet loads on an aft turboprop installation would do severe damage.  Even if you could make it structurally safe, customers won't accept it.  The airplane shakes so much that everything in it is a blur -- not the kind of thing you want to do each time you flare for landing.

So I suspect that the canard anhedral may very well be there to lower the vertical position of its tip vortex.  There are other possible reasons, of course; but you would have to talk to their wind tunnel and flight test people to get more details.

RE: Three-Lifting Surface

Guys:

I want to thank you for this very interesting discussion. Based on the discussion, for my jyrodyne, I'm lowering the aspect ratio of the canard to help keep it from stalling before the main lower forward biplane wing, and plan to add drooped tapers to the ends a la Steve Wittman's on his Tailwind. This should reduce and lower the tip vortices. It's real low below the fuselage anyway, so they should be below the lower wing far enough to miss it during regular cruise. I'm going to be looking at the vortex interactions at high angles of attack to estimate the onset of buffeting of the canard vortices on the lower wing.

Chasbo

RE: Three-Lifting Surface

This is a really interesting thread.  To add a little, I just came across this Ilan Kroo paper; it's somewhat germane to this topic, but far too general to be of real use in an Avanti discussion.  Good fodder for configuration trade studies though:

http://aero.stanford.edu/Reports/MultOp/multop.html

Anyway, back to that foreplane.  I'd tend to agree with you, Miper--anhedral in the canard looks suspiciously like a late-stage change.  I'm sure the original 'napkin drawing' was perfectly straight.

What puzzles me is that if the tips were dropped to eliminate some undesirable wake/vortex impingment, why such a small amount?  I can only guess (so take this for what it's worth), but if Piaggio found that, say, the canard tip vortices were impinging on the propeller disk, the vertical distance they've dropped probably wouldn't be enough to solve the problem consistently.  Or am I full of it?

Of course, it could well be that the canard tips were dropped as far as possible allowing for tip over clearance, and the change was just enough to sort things out.  Who knows?

Something I can see happening though is that perhaps due to structural/packaging issues, the canard mounting had to be raised.  The aero guys, however, weren't confident in raising the tip vortices, for the same reasons Miper and John have listed previously.  So, the compromise--tips stay where they are, and the roots move up.  Just being the devil's advocate:)

After all this great discussion, it sure would be nice to get a hold of a Piaggio engineer for a few hours!

RE: Three-Lifting Surface

1278,
Thanks for the reference.  The discussions about various configurations are very useful.  The author does note that his aero model has some simplifications, so there will be some flight conditions and configurations not covered by these results.  Still, they are generally applicable.

Your thoughts about anhedral on the Avanti are good ones. If they used the anhedral to avoid wake interference, it would only have been a useful fix if the wake impingement had occured only at extreme high angles of attack.  Again, we would have to hear from the designers to know exactly why they did it.

I know of one business jet with a large amount of tail dihedral that was added after flight tests that showed the airplane needed just a little more dihedral. (Dihedral is dihedral.  It matters little where you put it, but you will need more degrees of dihedral on the tail than on the wing to get the same overall effect, because the tail is smaller.)  It was cheaper to put it in the tail than in the wing, because the tooling was already built and the tail tooling was much less complex and easier to modify.  I have always thought the tail was awkward looking, but it's the best selling jet in the world, so looks aren't everything.  There are two reasons for relating this story --1. Many design tweaks can happen in order to obtain desired flying and handling qualities, and 2. It isn't always obvious why some tweaks were done.

RE: Three-Lifting Surface

I have always been mystified by the old F4 phantom layout; Dihedral on the outer wings and anhedral on the tailplane.

I supposse one possibility is that the tail has anhedral to avoid the wing downwash and the wings have dihedral to compensate.  

Again, sometimes its not the "normal" reason why things are done, it's some reason you won't think of unless you can read the designers mind.  

The Aceair Aericks also has anhedral on the foreplane and, from the front elevation supplied in the web site, the tip vortices should pass well outside the propellor arc or anything else, except it it's so nose high as to be stalled.  The anhedral, in this case, is unlikely to be to increase maneuvarability, since the large ventral fin will ensure adequate roll rate.

Anyway, I've broken down sufficiently to contact Aceair and ask them why their design has foreplane anhedral.  If they answer, I'll share it with you guys.  Whether their reason will also apply to the Avanti is another moot point.

Regards

John

RE: Three-Lifting Surface

John...

I'm not sure, but I've heard that the anhedral was added to the Phantom tail to eliminate high-mach-number instability--although what specifically was the problem and how the fix worked I don't know.  The dihedral was added to the wingtips, I assume, to maintain the plane's level of roll stability with the minimum tooling impact.  (Existing tooling drives a shocking number of late-stage design changes, as Miper aptly pointed out)

The same anhedral on the Harrier, however, is there for an entirely different reason:  it, along with anhedral on the wing, reduces unwanted roll due to sideslip.  So, reinforcing Miper's comments, looking simply at the fix needn't give any insight into what the fix is fixing:)

Thanks for asking Aceair; it'll be interesting to see if they actually give you an answer. And, if they do, I hope it's a little more than 'because the Avanti does it that way!'

Regards

RE: Three-Lifting Surface

Hi all,

I receioved this answer from Aceair:

"Dear Mr.Fortier,
Thank for your interest for our Aeriks 200.
First let me answer you to your second question:
The Foreplane anhedral is for improve pilot outside visibility and increased gap between the foreplane wake and the main wing surface."

Which sems simple enough and just about what the discussion here had surmised.

Regards

John

RE: Three-Lifting Surface

I'm a bit late to this thread, which seems to have stagnated...
but my understanding is that anhedral is applied to the
forward wing to reduce dutch roll tendencies.  Anhedral
can also be applied to the rear stab for the same reason,
and if you notice on the Avanti both the forward wing and
rear stab have anhedral.

Given all the CFD and wind tunnel optimization that the
Avanti went through, the vortex control aspect of anhedral
that this thread has pursued was likely noticed and
exploited, but I'm wondering if anyone could comment on
this stability aspect of 3SLC anhedral.

RE: Three-Lifting Surface

(OP)
I wouldn't expect that the lateral stability contribution of the nose stab would be very great.  Its span is small compared to the main wing: about 1/4.

STF

RE: Three-Lifting Surface

The high horizontral stabilizer, mounted on the tip of the vertical stabilizer, will provide a great deal of lateral stability, for the same reason that a high wing monoplane is stable.  Any, undesired, roll will cause side slip which in turn builds preasure under the stabilizer (or wing) on the side toward which the aircraft is slipping.  This in turn tends to reduce and straighten out the roll.  

the Avanti also has a pair of rather large strakes under the rear fuselage.  I initially thought that these were to protect the propellors turning excessive take off rotation, but they may also ensure adequate roll rate, given the aircraft's inherent stability due to the high horizontal stabilizer.

Anhedral on the horizontal stabilizer is probably there to reduce the stabilizing effect of the design, again allowing adequate roll rate.

Anhedral on the forward surface is probably there to ensure that foreplane wake misses the wing and, as mentioned above, to improve pilot visibility.  Its affect on stability, compared to the other forces acting on the airframe, will probably be minimal.


Regards

John

RE: Three-Lifting Surface

The delta shaped ventral strakes on the Avanti are most likely stall prevention devices, though they do provide some needed yaw stability.  Such strakes were first used on the Lear Jet.  At low angles of attack, the drooped strakes provide no lift; but at high angles of attack, they develop lift and raise the tail.  The delta shape is key to this behavior.

Why does the Avanti need this?
3LS aircraft are supposedly stall resistant.  True, but
this depends upon the balance between the fore and aft wing moments.

An inherent problem with 3LS pusher aircraft is that the loaded CG is farther forward of the landing gear than on conventional aircraft, which are placed to avoid tail sitting when empty.  At rotation, the aircraft
rotates around its landing gear; to keep rotation speed low,
the elevator must have a lot of power.  More than it needs in cruising flight, enough to bring the main and
fore wing very close to stall during approach,
when they are working hard.  The ventral strakes prevent
the nose from lifting too high while extracting the maximum lift coefficient out of the main wing.

The aerodynamic advantage of 3LS (and canard) aircraft is that they need less wing area in cruise than conventional designs since there's less trim drag (to overcome the additional intersection and wing interaction drag of the 3LS configuration, 3LS designers must minimize total wing area).  This advantage disappears
at takeoff and landing. More so than canards, 3LS aircraft can have nasty unrecoverable stall behavior, and can't be allowed to stall. They tend to have longer takeoff and landing distances than conventional aircraft of similar
wing loading because of this.  The game 3LS designs play is "how close can we get to
the edge", and the Avanti uses every trick in the book.

That's my 2 cents.  Please correct me if I'm wrong.

RE: Three-Lifting Surface

I wuld disagree about the stall behaviour of 3LS aircraft, since the stalled surface, assuming this to be the foreplane, is not the control surface, which is the big problem with cannards during stall.  

It's probable that the strakes perform more than one function on the design - prop protection, roll rate assist and stall prevention.  Certainly, as the angle of attack increases, the affect of the strakes on lonitudinal stability will increase, but I haven't heard any reports of the Avanti needing a long and careful glide path.  The takeoff run, at under 900 meters, is pretty typical for aircraft of this size and performance and doesn't indicate any difficulty in getting airborne.

It's going to take a lot of 3LS aircraft in the air before everyone is convinced that the design is the next logical step for subsonic aircraft, now that materials, control systems and manufacturing methods can actually produce an aircraft which can demonstrate the real advantages of this design.

I remain convinced that this is a brilliant design and still intend to buy one.

Regards

John

RE: Three-Lifting Surface

Good point about the separation of trim and control surfaces.  The Avanti doesn't have a bad takeoff run, but
it also has gobs of power...

The Avanti is a brilliant design.  If I had 4.3mil I
might buy one as well.  To me it serves to illustrate that
3LS designs must be very carefully engineered to realize
the benefits of the configuration.  This translates into
increased development costs that may be borne better by
the executive jet/prop market than the light/sport aircraft
market.  If the configuration proves generally advantageous,
the development premium will shrivel.

It appears that 3LS + single engine tractor propellors are
not a good combination, if the Eagle 150 is any example.
This design tried to marry the practical operating and maintainance advantages of a tractor engine/prop with the
aero advantages of 3LS, but didn't wind up with anything
better than a simpler, cheaper conventional design.

The A-200 has heeded the lessons of the Avanti, and has
carefully optimized the aerodynamic advantages of 3LS in
an unusually roomy single-engine configuration.
The price is an unconventional drivetrain,
a rather long takeoff relative to other designs with similar wing and power loading owing to the tradeoffs I posted
previously, and an arrangement that may not be scalable to
a 4-place cabin ( but could be scalable , curiously enough, to a 6-place ).

Any 3LS designers out there?



RE: Three-Lifting Surface

(OP)
There are many "paper airplane" 3LS designs that I've seen on the 'net.  If you want to see a site full of bizarre, and some not so crazy, airplane designs:

http://www.the-wildings.com/aircraft/

That should get you guys going...

STF

RE: Three-Lifting Surface

Have just completed a project at uni, the brief being to desgin a 1000 seater aircraft still within the JAR regs.

We used 3 LS on our desgin, prooved very helpful in the CofG calcs as was able to put more weight up front which helped with the CofG margin, and we planned to use fuel triming durning flight for stablilty

Quite a good project but all a bit of a pipe dream, 1000 people in one plane, maybe someday

Ritchie

RE: Three-Lifting Surface

Actually, Ritchie, while I hadn't considered a thousand seater to be the next logical design step for TLS, I had thought about a possible replacement for the C130 Hercules.  

I leave the rest up to your imagination, but it seems something of a natural development.

Regards

John

RE: Three-Lifting Surface

Yea this thing was HUGE.

The cannards at the front had a span of 40m, with 80m span wings, and 80m long, 700 ton flying in the air, now i am all for natural progression and all that but hm kinda worrying

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close