Murphey

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I'm in the process of re-building an old biplane. This plane has a 70 & 55 inch wingspan and flat bottom airfoil. In checking the wing incidence, it currently is +2.5 degrees for upper and lower wings. Should this be less...or is this OK? This is not an high performance aerobatic bipe like an Ultimate, just a sport flyer

Murphey (As in Murphey's Law)

Edwin

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You're gonna get a lot of different answeres on this one. On my Phaeton 90 I use 0deg on the lower wing and -1.5deg on the upper. When I bought this plane it was setup for 0-0 deg on both wings. It flew alright but was a real problem to land, it just floated on by. When I changed the upper wing to -1.5deg it was a whole different plane. Flys like a trainer and settles down nice and predictably. The idea is to set it up so that one wing stalls before the other, just my opinion. Doesnt seem to matter which way you do it.
Edwin

Murphey

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Thanks Edwin. This plane should fly more like a trainer. I've flown it in the past and it did seem to float on landing, so I'll heed your advice.
Murphey (As in Murphey's Law)

lawsonaero

and I've been advised to set the top wing at +1 degree for a bipe that needs help floating.

what's the difference between the top wing stalling first vs. the bottom wing stalling first?

Edwin

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I dont really know there is a difference. Someone smarter than me might know but I've seen both setups and both seem to work equally well.
Edwin

dbacque

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It depends on the stagger of the wings (which wing is more towards the nose).

Having the forward wing stall first leaves you with a nose heavy airplane. The nose drops a little, the airspeed increases and the forward wing regains flying speed.

Having the rear wing stall first leaves you with a tail heavy airplane, not a good thing when you're on the verge of a stall. The tail drops a little, the airspeed increases, the forward wing gets even closer to a stall than it was and it sets you up for a vicious snap roll.

So for a bipe with normal wing stagger (top wing forward of the bottom wing), the top wing stalling first makes for a more forgiving plane that has some inherent recovery built in.

But stagger reverses when inverted. So on a plane intended for aerobatics, you want things closer to 0/0.

Dave

BWooster

When I put -1 or -2 degrees on the upper wing of my Tiger Moth, I could finally get it to spin. I believe this is because I could get both wings to stall at the same time.

The negative upper wing incidence gives the upper wing the same effective incidence as the lower wing.

I would expect that the airfoil has a role to play as well. Symmetrical airfoils probably want 0-0 (I'm guessing here). A cambered airfoil needs the upper wing to be set at a negative incidence.

Multimaniac

Hi all!

First I have to say that I have not much experience in biplanes. I have built three from plans, but I have also thought a lot about how they got the incidences of the wings.
On all the three planes they had more incidence on the upper wing than on the lower wing (e.g. +2,5° on the upper one, +1,5 on the lower one), and the lower wing was smaller and more aft than the upper one. One thing is in general that the wing with the higher incidence stalls first (given that they use the same profile!). And the second thing is, like Dave explained, that it will be a good idea to make the forward wing stalling first, leading to a nose heavy plane which can be controlled easily.

I think when you design such a plane you have to choose between the advantages of a making the forward wing stalling first and the advantages of making the smaller (usually lower and more aft) wing stalling first.... The great art in designing biplanes!

regards

Chris

dmcmike

My Feedback: (14)

I've been building and flying bipes for years. They'll do ok with 0-0-0 on the surfaces, but some turn out to be a big 'twitchy' in pitch set up like this. If you find that's the case, set the upper wing at approx -1 degrees, while leaving everything else at 0. You'll find that the airplane smooths right out, I've been using -1 on the upper right out of the box for the last few i've built, (Hog Bipe, Super Aeromaster, etc.) and they fly great!

Rodney

I have built and flown a large number of bipes, 2 Phaeton 90's, 2 Phaeton 40's, 7 Airomasters, a couple of Moths plus many more. All performed best with the upper wing about -1.5 degrees, lower wing zero, stab zero, engine 5 degrees down and 3+ degrees right thrust. All varied somewhat, two, do not remember the model, worked okay with zero, zero, zero. If possible, try to make some minor variations in the upper wing incidense and try it out. You will find that there is what I call a sweet spot where the airplane behaves like you want it to.

Edwin

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Hey Rodney,
I take it you like the aeromaster? Seems like that says a lot for the design.
Edwin<g>

Eindecker_pilot

Doesn't it matter how one measures incidence? You have to establish the orientation of the wing relative to the datum, but what I'm wondering is how one does that with a flat bottom airfoil (as opposed to a symmetric airfoil). On a flat bottom airfoil, is it more correct to measuring the incidence of the wing by placing the meter on the flat bottom of the wing? Or should one still locate the meter by clasping the leading & trailing edges? One might have a wing that reads 0 degrees if measuring the angle between the flat bottom & the thrust line, but that same wing might read +2 degrees incidence if you clasp the meter to the leading & trailing edges of the airfoil (since on most flat bottom airfoils there is a generous rounded leading edge, sometimes even raised up a little).

Rodney

Even on a flat bottom wing, you measure the incidence angle by the line between the front of the leading edge and the center of the back side of the trailing edge, not along the bottom surface. As for what you measure it against (what is the reference?) it is not important. What is important is the relative angles of the airfoils on the wings, stab and the thrust lines. Of course it is nice to have the fuselage track with minimum drag which is usually when it is about lined up with the lower wings incidence or at least close to it.

Murphey

My Feedback: (11)

The previous 2 comments on wing incidence are very interesting. Since I started this thread, I have found a set of plans for this particular airplane, a "Golden Oldie". The plans show 0 degrees on the stab, 1 degree positive incidence on the top wing and 0 degrees on the bottom. Using my incidence meter clamped around the LE and TE showed 2.5 degrees and 2.5 respecitvely. So I set up the wings using the bottom of the wing as a reference. This method agrees with the plans in that if I line up the incidence meter on the BOTTOM of the airfoil on the drawings, they are +1 and 0.

Can it be that Eindecker_pilot is correct for a flat botton airfoil? I can easily see that a line between the TE and apex of the LE would divide a symetrical airfoil into equal halves. But a line drawn from the TE to the same relative point in a flat bottom airfoil does not. In fact, the line is not parlell to the botton of the wing.

Now I'm REALLY curious...

Murphey...as in Murphey's Law

laryboy

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where is the balance point on your phaeton 90 ? i have a evolution 25 cc sitting out at 7 inches (to propeller drive ) to achieve balance as per plans

Edwin

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lazyboy,
I balance my P90 about at the leading edge of the bottom wing. (I'm pretty sure thats it) I should probably go back and check the planes. Been awhile. I'm at work right now, I'll confirm it tonight when I'm back at home.
Edwin

Eindecker_pilot

Murphey, I'm glad I'm not the only crazy one!

I had the same experience you did with plans for a Nieuport 17. The plans indicated 0 incidence but that only matched if you measured it using the flat bottom side of the airfoil for the wings.

BWooster

If you look at airfoil books, flat bottom airfoils like the Clark Y are plotted and measured relative to the flat bottom, not the line between the leading edge and trailing edge. (This is not true for symetrical or semi-symetrical airfoils.) So incidence should be measured from the same reference point, I think.

tom orcutt

I am currently scratch building a 1/3 scale fly baby bi-plane. The plans are by Ronald J Bush. Would anyone know what the correct incidence would be on both the top and lower wings should be? There is nothing on the plans to help. I am thinking of 0 degrees on the bottom wing and a positive 1 degrees on the top wing. Also would one or two degrees of down thrust on the motor help?
Thanks Tom

dmcmike

My Feedback: (14)

I've always had great results in using 0 degrees on the bottom wing, -1 on the upper. Let's the lower wing stall first, keeping everything nice and stable.

Rodney

Tom Orcut, I think you will find the model will fly much better with the upper wing at -1 to -1.5 degrees relative to the lower wing. Yes, you will benefit greatly by having about 3 degrees downthrust and also about the same in right thrust.
BWooster, I'm afraid you are in error on the belief that most flat bottom wings measure incidence by referencing the flat bottom of the wing. Check any good aeronautical manual for the correct measurement.

BMatthews

I've seen flat bottom airfoil data given both using the correct leading edge to trailing edge centerlines and the wrong way using the flat bottom. It's all relative since it pushes the air around the same way but for the sake of discussions like this you have to know or at least mention which baseline you're reffering to.

Angles are all fine and dandy but don't forget that it's all relative. If you move the CG around the effective stabilizer angle changes to regain the correct neutral trim. This neutral trim will have a lot more to do with floating on landing than the wing angle. Also keep in mind that biplanes with more wing area will have a lighter loading. This can lead to floating along for a lot longer distance if you bring it down with excess flying speed. Learning to get the nose up so the wing is operating at a higher lift coefficient and a higher drag coefficient will steepen the glide angle and drop the model in with a steeper and more predictable glide slope. The key is that you'll be closer to the stall angle but don't get too close to it.

zbo2

good thread...............i have a super aeromaster and on the great planes site it shows +1 on the top wing and +1 on the stab...has anybody used these numbers on this kit. this is all new to me, i have never checked the incidence before but now that i'm about ready to go to larger planes i want to start checking what i have and what i build.

dmcmike

My Feedback: (14)

use the +1 on the stab, but I think you'll find the -1 on the top wing will make the plane much more fun!

zbo2

thanks for the input......i'll give that a try.