centre of preasure/lift for bi-plane wings 1

[1917Alby]

Hi from NZ.Could some kind knowledgeable member calculate for me where the centre of lift should be for unequal cords on a bi-plane. Measurements: Top wing span 7100 cord 1230 millimeters, Bottom 6600 x770 not including fuse. Gap 1140. Stagger 525 measured at leading edges. MAC 15 degrees at 1/3 cord.all measurments are in millimeters. Also could it be explained to me in simple terms how this figure is arrived at,My algebra is non existent.Thanks in anticipation. 1917 alby.

 

[rb1957]

"MAC 15 degrees at 1/3 chord" ?

 

[1917Alby]

Correction. It is STAGGER that is 15 deg.
Wings are parallel of equal span.
Thanks

 

[rb1957]

how about relative incidence ?

the catch with the bi-plane configuration is what is the CL of the other wing, 'cause the wings interact (as i'm sure you know or suspect). Hoerner "fluid dynamic lift" would consider this.

 

[rb1957]

go to the NASAS NTRS site ...
NTRS.NASA.gov

search for "biplane"

about the 2nd hit was a paper on a large cargo biplane by Zyskowski, Michael K. ... very interesting. and a fist full of NACA references (like [33] TR 362) with more equations than you can shake a stick at !)

Hoerner (or von Doenhoff) are more likely to give you generalised expressions. these NACA refs might just allow you to calc more exact numbers for your particular situation.

is this for a home built one-off, or a prototype for a commercial project ?

 

[rb1957]

from the same site, NACA TR-458 "relative loading on biplane wings" looks good ... particularly as there's a worked example at the back !

 

[1917Alby]

Sorry to mess you about but further correction to stagger now 19 degrees after more accurate measuring of aircraft already rigged, and redrawing to scale.
Thank you all for you for your further notes, I had looked to some of these sites but the formula are far beyond my capabilities.
I have constructed a 1917 Albatros D5 replica homebuilt to spar with my SE5A replica(built to plans 35 years ago) I am test flying the Albatros at present. Flights have been short, 3-5 sec, as my strip is only 340 metres long, so have wait for a favourable breeze.
The C/L and CofG relationship have been calculated by a knowledgeable but not qualified person (who has just designed and built a gyrocopter)
The aircraft is controllable in flight but drops the nose when the throttle is closed.
I suspect the CofG is too far forward as I run out of elevator even at 50 degrees up. Tail plane is on a jack but does not help. Wings are set at zero incidence.
Before I start shifting the CofG I need to know for certain where the C/L is,calculated by a qualified person for biplanes.
I understand these persons are scarce as sesquiplane design went out of fashion a long time ago.
Another reference that I have viewed is "relative loading on Biplane wings of unequal chords" Diehl, Walter S. This was written about 1934 but is beyond me.
In anticipation of happier landings!
Thanks

 

[1917Alby]

Just to give the complete Diehl reference details
"Relative Loading on Biplane Wings"
"Relative Loading on Biplane Wings of Uneaqual Chords" Diehl, Walter S., NACA Report 458 & 501 Both NACA reports can be found at

http://NTRS.NASA.gov/search.jsp.

 

[thruthefence]

Decalage

In the biplane, the difference in the angle of incidence of the upper & lower wing panels.

http://www.culpsspecialties.com/

I think this guy has also built some SE5 replicas

 

[rb1957]

your project explains your handle !

building a scaled plane is much easier than starting from scratch ... it least you know the thing should fly (tho' WW1 fighters were somewhat notorious for getting close to -ve stability margin).

if 50deg elevator doesn't control it, then i agree that you've go cg problems. a couple of thoughts ... how easy is it to rig an oversized elevator ? this would only be temporary but would confirm the design problem (lack of elevator power), but the real fix is going to be different (so why go down a dead-end?). can you move the pilot position ? i figure that the pilot is about 1/3 the weight of the plane, moving him 3" fwd would move the cg of the plane 1" forward. this is probably difficult to do (rerigging the flight controls) ... do you have power enough for ballast, again as a temporary measure, but at least easy to do and tells you pretty directly what to do to fix the problem ? you've got the h.stab. with variable incldence and this doesn't help ? ... that's surprising, i'd've thought that a degree or two of incidence would have created tons of lift ??

sounds like a great project, good luck with it

 

[1917Alby]

Hi RB1957
Thanks for your suggestions. As I want to recalculate the static margin I need to establish the centre of lift as a start.
Thanks again

 

[JQCF1]

1917ALBY

By any chance was your SE5A replica built in beautiful, downtown Twickenham??

JQC

 

[1917Alby]

jqcf1

Negative NZ as shown in my greeting

 

[sheppo]

Gday Alby

Im currently building a full scale Dva in NSW and have a solid pile of data for the full size machine.

What are your angles of incidence for the wing and tailplane set up to?
The wood and fabric horizontal stabs should be angled 1.5 deg above the C/L at the forward attachment point.

Do you have an image (tells a 1000 words and all that) of the project I could see.

It does sound like a CoG issue at first reading as the Dva was originally designed to be slightly tail heavy.
What are you running for a powerplant?

chris

 

[sheppo]

Oops I missed the note about being set to 0 deg incidence. The original design called for significantly higher Angle of attack than 0 (relative to the C/L or roughly speaking, the middle longeron).
Not knowing for sure how yours is built etc I cannot state that this is a contributing issue however the angle of attack (upper wing) was noted as 5 deg by Idfleig (between the V struts for clarities sake ) but only 1 to 1.5 deg at the tips due to washout in the upper wing & ailerons.
Does your machine have the washout built in as per the original?
This is going to result in a fairly nose down attitude in flight.

It does seem that you may have a C/G issue though.
Note that the original was in fact tail heavy...


cheers
sheppo

 

[n222cd]

n222cd
Hi, There is a problem in determining a wing center of lift. The position of the center of lift depends on the angle of attack ( based on "Low Power Laminar Aircraft Design by Alex Strojnik ). A higher angle of attack moves the Center of lift moves forward, and conversly, a lower angle of attack moves the center of lift toward the trailing edge. In fact, Strojnik states that the center of lift may be aft of the trailing edge (I'm not sure about that myself). Have Fun, Dave

 

[rb1957]

"the center of lift may be aft of the trailing edge" ... could happen if the airfoil is creating a large pitch moment

 

[n222cd]

n222cd

Strojnik's opinion "Don't be surprised if you find that for some very low angle of attack ( and, therefore very low lift coefficient ) the lift moves even behind the trailing edge !" It seems to me that any aircraft would enter a dive under those conditions. Other than that I don't have an original thought on the subject.

 

[shebly]

"It can be seen from the foregoing discussion that the
biplane has no useful counterpart of the monoplane
aerodynamic center. For this reason, biplane problems
are best solved by proceeding directly to a solution of
the forces and moments."

NATIONAL ADVISORY COMMITTEE
FOR AERONAUTICS REPORT No. 631

AIRFOIL SECTION CHARACTERISTICS AS APPLIED TO
THE PREDICTION OF AIR FORCES AND THEIR
DISTRIBUTION ON WINGS
By EASTMAN N. JACOBS and R. V. RHODF