Incidence difference on bottom & Top Wing  
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All Forums >> RC Airplanes >> Aerodynamics >> Incidence difference on bottom & Top Wing
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Incidence difference on bottom & Top Wing - 4/12/2005 9:30 PM   
Dobbis



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From: Stockholm, SWEDEN
Status: offline 		I´ve just finished a Protech Christen Eagle ARF

Due to pretty poor fit and quality of the pieces combined with the construction, the angle of attack (incidence) of the top wing is not well defined. Of cause, the ARF manual does not mention anything about recommended AoA.

Now, I have asked around on the Swedish forums and got recommendations that top wing incidence should be equal to bottomwing or less, about 1 degree. This recommendation is probably right, my problem is that I can not understand why.

If the top incidence is less than the bottom wing, the bottom wing will stall first and loose its lift and thus, the plane should appear like a really tailheavy top winged plane ( since top wing is considerably in front of the bottom wing ) and in a stall situation nose up, stop and fall over either wing. Not really what one would like to see.

On the other hand, if top wing incidence is higher than the bottom wing, top wing would stall first and make the plane appear like a noseheavy low winged plane and therefor it would nose down and regain speed, which is the way you would like it to handle.

Now, according to recommendations, this is obviously not the case. Why not ?

Can someone explain to me why one would want bottom wing to stall first and if so, why wouldn´t that scenario result in a undesired nose-up ?

Regards

Dobbis

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RE: Incidence difference on bottom & Top Wing - 4/12/2005 10:27 PM   
LouW



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From: Moreland, GA, USA
Status: offline 		You are not seeing the whole picture. When two wings are operating in close proximity they affect the flow. During the biplane era many studies were done regarding this interference between wings in a biplane configuration. The actual angle of attack of the lower wing is generally reduced by this interference. For this reason the incidence of the upper wing is usually decreased a little so that the wings both lift their share of the load. This doesn’t mean that the lower wing will stall first. In fact with a staggered configuration, the lower wing is effectively operating in the downwash of the upper one and its actual angle of attack can be several degrees less. The flow interference is complex and the amount of difference between the upper and lower wing is usually determined by flight testing. It isn’t often critical but can make a noticeable difference.

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RE: Incidence difference on bottom & Top Wing - 4/13/2005 12:27 AM   
dick Hanson



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From: slc, UT, USA
Status: offline 		If the bipe is reasonably light - the downwash -is very small and interference is very small .
If both wings are same area - then changing incidence -simply adds drag and the wings will try to balance the load between them
You CANNOT simply change one wing and expect the other to fly at it's original AOA
change one --they both change.
I presume the model has some scale looking airfoil- - so - just set em both the same
The old rule of thumb was one chord seperation betwen lower and upper panels -
It is just a rule of thumb - speed /stagger position etc., change this .
Full scale bipes were for the most part slow as hell -except for the Schneider racers -so the whole picture is different than your model .

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RE: Incidence difference on bottom & Top Wing - 4/13/2005 3:35 AM   
Rotaryphile


 

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From: Fredericton, NB, CANADA
Status: offline 		Very extensive biplane wind tunnel tests conducted in the late 1920s and early 1930s showed that the incidence difference between the two wings of a biplane is not at all critical in its effect on either induced drag or stability. (Remember that biplanes were the dominant airplane species in that era, so tweaking their performance intrigued a lot of experimenters.)

Induced drag was found to be very slightly reduced by giving the upper wing a degree or two more incidence than the lower. The commonly-held belief that giving the upper wing more incidence, particularly if it has positive stagger, will result in a gentler stall, was largely found to be false. In reality, when the upper wing stalls, it loses some lift, but its drag increases far more dramatically, which raises the center of drag of the airplane, tending to cause more pitch-up than the pitch-down caused by the loss of lift.

The fact that induced drag was slightly reduced by slight positive decalage surprised the theoreticians, who had expected the opposite result. The upper wing of an equal-wing bipe tends to develop more lift, leading the aerodynamicists to believe that giving each wing equal lift would minimize induced drag. The explanation for this discrepancy was argued for some time, and never, to my knowledge, satisfactorily resolved. Bipes can be hard to figure.

Interestingly, the lower wing was found to remain unstalled up to, and beyond, 30 degrees angle of attack, long after the upper wing was in deep stall. This appeared to be caused by the downwash of the upper wing tending to keep the flow attached to the upper surface of the lower wing.

I have found that using between one-half and one degree less incidence on the upper wing seems to produce better aerobatic line-holding, and the increase in induced drag, according to those wind tunnel tests, would be almost unmeasurable.

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RE: Incidence difference on bottom & Top Wing - 4/13/2005 6:59 AM   
Dobbis



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From: Stockholm, SWEDEN
Status: offline 		thx for the first reasonable explanation I´ve seen !

Dobbis

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RE: Incidence difference on bottom & Top Wing - 4/14/2005 12:39 AM   
dick Hanson



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Status: offline 		If you want you model to work as well as it can - keep both wings as close as possible to exactly same aoa
The best improvement you could make is to reduce wing loading -but that is likely out of the question .

Adding in difference in AOA simply makes a mess of things.
The Christen Eagle is not a lumbering turkey from the past --it was developed as a fast high performance machine.
Having done high performance bipes for $$$$ competition- I have been thru this ----

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RE: Incidence difference on bottom & Top Wing - 4/14/2005 2:43 PM   
Rodney



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From: FL
Status: offline 		From considerable experience with RC bipes, 7 Aeromasters, 2 Phaeton 40's, 2 large Lazy Aces, 1 mid sized Lazy Ace, 1 quarter scale Moth, 2 40 sized Moths, ALL required about 1.5 degrees less incidence in the upper wing relative to the lower wing to fly well. I did considerable experimenting with them (especially the Aeromasters) and, when the upper wing incidence was made equal to or more than the lower wing, severe hunting occured when trying to fly level: i.e. plane tended to gallop. I'm sure that some designs will work better than others without the difference in incidence but none of the above fell into that catagory.

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