What effect on a bi-plane would the wing incidense be if say both wings (upper and lower) are set the same . I know that it is desirable to have the upper wing stall first so it should have a bit of negative in it. But if both wings were set the same what effect would it tend to have on the flight of the plane??? Lets say both have 1.5 to 2 degrees of positive.

Thanks for any thoughts .

Depends on how far each wing is from the center of drag of the whole airplane for one thing. Ignoring that (and we don't have enough room to do otherwise), if you change both equally and compare the change that'd give..............

The AOI basically decides where the fuselage gets pitched. Most of us won't see the difference with the airplane in the air and not close to us. But since the h.tail is greatly influenced, it's going to cause an effect. What will that be? Depends on what it was doing before the change.

The wing(s) airfoils and the gross weight of the airplane and it's speed decide the AOA. The pilot sets the throttle and elevator trim. The wing decides the rest. The fuselage goes along for the ride. Actually. Not literally, but actually.

Change the AOI, go back up and set the same throttle and elevator thrim and you get a different speed. Until you change the elevator trim. Do that and you wind up with the same flight as before (ignoring the difference in efficiency of the h.tail because of it's new "shape" ) because the lift is going to be the same as before. Because the AOA is back to what it was before, and the h.tail is going to be working in agreement as before. It'll just have a different elevator trim.

But the fuselage will have a couple of degrees difference in it's pitch.

Let's look at it with less theory.

You've been flying your bipe flat out and trimmed to fly level. You change the wings' AOI by a couple of degrees equally. You take off to fly flat out and you discover you need a click or two to fly level. You do that. The bipe is now going flat out and level. It now has a click of elevator trim and the sucker looks like it's flying with the tail a bit in the air.

O.K. Thanks for the help. What I have is a 33% Stearman at 46 pounds with a RCS215 for power. This is a Balsa USA kit. When I checked the AOI it shows that both the upper and lower wings at the same AOI. The bottom wing would be very hard to make a mistake on the AOI due to the way the kit is is designed, but the upper wing is a different story. The instructions that come with the kit give no numerical values as to what the AOI should be. It just explains on how to set up the center section of the upper wing. I did my best to stick to those values. I'm a very experianced builder so I don't think I messed up there. But both wings being at 1.5 positive AOI just seems strange to me. My 33% Waco YMF5 calls for "0" AOI on the bottom and 1.5/2 negitive on the upper wing.

I just wanted to find out if the thing would do something really bad on its maiden flight due to the AOI being what it is.

Again thank you for your time.

Hi SoCalSal
In the second sentence of your first post you state that if the upper wing has a negative incidence (and the low wing is at zero) the top wing will stall first. I see it as stalling last. Am I mixed up?

Usually, with model biplanes, the upper wing has an angle of incidence set about 1 degree or so less than the lower wing with respect to the fuselage datum line. This helps the model "track" better at higher airspeeds, such as in downlines and the backside of loops. As far as one wing stalling before the other, theory would tell you that you want the more-forward wing to stall first so that there will be more of a nose-down pitching force at the stall. In practice, it really doesn't much matter with models, and with real biplanes, the optimum angles for the wings were found in flight test. What works for full-size aircraft doesn't always translate to models because while models get smaller, the air doesn't and has different effects with smaller objects.

Hi "O" AOI is still more than negitive AOI that is why the top wing would stall first if it is set at a AOI value less then the bottom.

Thanks Bax, I do understand the theory of this. But I don't think on my Stearman I will be doing a lot of hard core 3d type of flying. Heck I can't even fly a 3D type aircraft other then to just do the very basic stunts. I am more concerned with what type of handling I will have with both wings set at the same AOI. This model is just for some lazy weekend flights and such. I enjoy building much more than flying.....I'm a much better builder than a pilot. Need to change that though.

SoCal,
BalsaUSA support should be able to recommend a starting point for your incidence settings. The settings have a lot to do with the relationship with the horiz. stab as Rock mentioned. They also have a lot to do with the airfoil type. My 1/5 Waco with Clark Y airfoils makes a ton of lift at -2 AOA, whereas a fully symmetrical would generate no lift at 0.

Assuming the Stearman has symmetrical airfoils, setting everything (upper,lower,stab) at 0,0,0 would mean the plane will cruise at some positive AoA to generate suffcient lift to maintain altitude. Setting the plane at 0,1,0 will mean the plane can generate some lift at 0 AoA keeping the fuselage more level than 0,0,0 reducing profile drag. Setting the plane at 1,1,0 will produce twice the lift as 0,1,0 at 0 AoA and may be required for a draggy biplane with exposed wheels, bracing wires, etc just to maintain altitude at cruise.

I have an Andrews Aeromaster with fully symmetrical airfoils. I have it set at -1,0,.3 because I (and many other Aeromaster owners over the years) have discovered the plane is too pitch sensitive at 0,0,0. Having the upper wing negative creates a more gradual progression in total lift (U.Wing + L.Wing Lift vs. AoA has a shallower slope).

I know from reading many of your posts that you are a highly experienced pilot and won't get into trouble with any reasonable settings. I would encourage you to start with BUSA's recommendation but experiment with the three numbers until you find the combination that works best with your kind of flying. Just change one number at a time and take some notes to share with the rest of us!
Crate.

Airfoils always stall at some positive AoA. For example lets say 16 degrees. If the bottom wing is at 16 degrees it is in stall. If the upper wing is set at -1 AOI relative to the lower wing it is only at 15 degrees and has NOT stalled yet.

Don't worry about stalling a model biplane!
Setting one up for the stall regime, which is an insignificant part of the flight envelope will degrade all the other areas where the plane is going to be spending all its time anyway.
The difference in angles between top and bottom wings was done for full-scales.
We exceed all the normal parameters of full-scales with our models.
If you like the way it flies with 0-0, leave it there!

Thanks, CrateCruncher
That is how I see it. My ears are still open if there is a contrary argument. I am unable to understand the explaination given by SoCalSal. Sorry.

Having both wings at the same rigged angle will not affect the model. It is the difference between the tail plane and the wing that will, in effect, set the cruise speed. The rigged angle of all these wings to the fuselage datum will determine what angle the fuselage appears to fly at. And that depends on the completed models weight. Simple, eh! Hmm...
Evan, WB #12.

Thanks for all the help. I like getting some input for others on these matters. As much as I try. I just don't know everything and I' not afraid to admit it.
Again thanks to all.

Not true I have built and flown at least 7 Aeromasters, three Lazy Aces (one large and two 40 size), several Moths plus many more. All my bipes flew best when the upper wing was at about 1.5 degrees less angle of attack than the lower one. I did many tests on the Aeromasters and found this to be true on all of them. Things got especially bad when the upper wing had a higher angle of attack than the lower one, on some you could not fly level as the plane would oscillate like it was searching for a neutral position. If you have the time, do a little experimenting, you will find that there is a sweet spot for your type of flying.

And all of my biplanes fly well enough with the decalage (rigging difference between the wings of a biplane) of 0 degrees. In other words, both at the same angle. I have to point out that my biplanes are all around 1/4 scale, so there will be an aerodynamic difference there (seven foot wings and up). Just goes to show that different models, cg's pilots, requirements etc will determine what you consider a satisfactory setup. Setting both the same for the initial flights will not be a problem. You can, of course, experiment as you wish, providing that the adjustment is easy enough to do and you can accurately determine the angular difference. Fly it where you like it the most.
Evan, WB #12.

Both of my planes are 33% in size. So I will just play with it (Stearman) and see what we come up with. My Waco is set at "0" and neg. 1.5 and it seems to fly just fine. It is a floater on landing so you have to set up right or it will just float on by while the runway gets shorter then the dreaded "Rock wall" at the other end.

Thanks for your feedback.

My latest is the Giant Aeromaster. I have found that it flys best with the upper wing minus 1 degree relative to the lower wing. On all my quarter scale bipes (four of them), the upper wing has always given best performance with the negative (relative to the lower wing) incidence. Yes, it does sometimes take a little ingenuity to try the different setups but, so far, it has always been worth it due to the improved flight characteristics.

Ever looked hard at the airflow diagrams in the "how does lift happen" texts?

There is a significant upflow ahead of a wing. (Just as there is a significant downwash aft.) It has been theorised that biplane wing most certainly have upflows and that the upflow of the lower wing influences the flow that goes to the upper wing.

It would follow that since the upflow of any wing affects the airflow, artificially influencing the upper wing's performance might result in conditions where rigging that upper wing differently from the lower one would have beneficial results. Or not-beneficial ones. And rigging with no decalage might have unwanted consequences or beneficial ones.

It is already well understood that the distance between those two wings really matters. It's just not usually stated what those things that matter are.

HUH!!

SoCal, my poor Waco is only 25% but exhibits the same float. I'm using 0 decalage, and cause it took so long to set everything up to get them the same I cannot bring myself to pull it all apart and start again...I have no idea how it stacks up against other similar models, as there aint any around here to try. Other pilots who have had a stir are quite complimentary, but as I said, I have no other similar models to fly and compare it with. I do have other bipes, but again, they are from a different period and I set them up with 0 decalage...
Evan, WB #12.

SoCalSal:

You can find good practical information in this book, also available at many public libraries:

http://www.amazon.com/Rubber-Powered...281045-6883141

Regards!