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Increasing wing span

Increasing wing span

Increasing wing span

I am looking into various plans built airplane designs in the experimental category.

One airplane in particular has a lot of the right numbers for me as far as time to build, complexity and cost, but the stall speed is a bit higher than I like and the climb rate is lower than I like.

What I am considering is increasing the wingspan to solve both problems AND to boost its useful load slightly.  The span is currently 18 1/2 feet (short) and I would stretch that to 20 1/2.  The current wing loading is 13.7 lbs/ft^2 (1150 gross, 84 ft^2).  At 93 ft^2 and 1225 gross, wing load would be 13.2lbs/ft^2

A) Would increasing the span work address the stall speed and rate of climb?  (W/o getting quantitative)

B) What would be the structural considerations for the spar and wing structure to ably handle the additional weight of the wing and the load it carries in flight?  Basically, is increasing the span as easy as adding two more ribs on each side, or would the spar need to be lengthened and the skin thickness need to be changed?

RE: Increasing wing span

I think that increasing the span would probably work... and this is my reasoning: Larger span -> larger area -> more lift. Now assuming that the increase in lift is greater than the increase in load (weight due to the extra wing structure) then to achieve the needed lift at a given speed, you would fly at a slightly smaller angle of attack. Therefore, the speed at which you reach the stall angle of attack would be higher than before. Don't forget you would increase drag a little and therefore you have to make sure that your engine is strong enough. Also, and I haven't given this enough thought yet, your tail might have to get a little bigger in order to provide enough force (unless there was enough spare to begin with).

As far as the structure goes, I would imagine that the thickness of the skin would not have to change. But you might want to have to look at increasing the thickness of the spars which would have to be a little stronger (higher bending moments)

Hope this helps a little (if it is still relevant).


RE: Increasing wing span

Have you looked at slotted flaps or even a custom wing - there are some for various kit planes like the dedalius wing for the zenair etc that add slotted flaps, air pressure activated slats which drop stall dramatically, but the cruise is not real high - but then you didnt say what your criteria are interms of flight envelope. Simplistically what you suggest sounds good, but like most I know enough to be only dangerous and suggest you model changes properly when messing with aspect ratio and the like. Whether you need a thicker wing skin depends on how it was engineered in the first place. Composite wings may use lots of uni fiber for bending loads and biaxials for twist/tortional etc so yes its relevant, whereas for ali it is unlikely cause its isotropic and a standard product so you build loads mostly into spar structures etc- but again find out first. THere used to be phenomona called wing divergence - it was around when materials lacked stiffness and still occurs where structures just aint strong enough.

RE: Increasing wing span

A) Theoretically an increased span, resulting in increased area would help stall speed, and maybe would help rate of climb, though there are alot of other factor affecting it.

B) If you were to just add more ribs to increases the span, can you please tell me the tail number, so that I never hop on that aircraft.

Increasing the span will increase the lift, and move the moment arm further from the wing root, hence giving a much larger bending moment. If there is not sufficient strength in the spar, and its connection to the fuselage, then it may fail.

Increasing the length of the wing will also decrease it stiffness, both in bending an torsion. This may reult in unwanted aeroeleastic effects (flutter, among others) if properly excited.

As already stated, the tail/rudder area may be needed. Due to the lower stall speed, the tail may need to be increased to maintain the same level of control at lower speeds.

Will increased wing span will probably mean decreased angle of clearance, which may be an issue if not landing perfectly wings level.

These are just a few of the issues that I can think of immediately, but I am sure that there are many others. So NO, it isn't "as easy as adding two more ribs on each side"

RE: Increasing wing span

All these comments are spot on, but there is another which you might want to consider, especially on experimental planes.
You might have a swept wing (forward or back), in which case, increasing the span of the wing will change the position of the centre of lift in relation to the centre of gravity.
You might need to (slightly) reposition your CoG accordingly (forward or backwards) to maintain the right static margin.

The best way to increase climb rate is with a bigger engine !
The best way to decrease stall speed is through flaps.

RE: Increasing wing span

Here's what I recommend, go to www.zenithair.com and get a STOL 701 and be done with it.

WIll changing the spand, chord, airfoils etc, be handled in the existing airframe or are you going to beef those up to handle the new loads?

Joseph Moylan

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