mscic-RCU

My Feedback: (6)

it is fairly common practice to balance all low wing planes upside down. It is easier to "hang" the weight of the airplane to make it easier to balance.

michael wood

My Feedback: (1)

I flew it this weekend and it flew good but it needed about 1/4 trim of down elevator for hands off so iam going to add some more weight to the nose to try and level it out I can not adjust the incidence on the wing anymore nor can I adjust the rear stab I also thought about adjusting the bottom ailerons down a little to see what that would do about the trim

mscic-RCU

My Feedback: (6)

Instead of adding nose weight, you might try some down incidence in the motor. Try adding 1 or 2 washers in between the motor mount and fire wall. This might help in counteracting the extra lift from the wing incidence being so positive.

michael wood

My Feedback: (1)

but would not adding more down thrust cause the airplane to balloon up when the throttle was closed for landing?

mscic-RCU

My Feedback: (6)

adding down thrust will counteract the ballooning of the excess wing incidence. It will not really affect the plane as you land as much as adding weight will. By adding weight you are just making the airplane heavier not really correcting or counteracting the real problem which is the wings sitting at too much incidence.

buzzard bait

Just stick with the down trim. Adding nose weight has other consequences. It would be best to adjust the stab, but since you can't, elevator trim is the way to go. Jim

mscic-RCU

My Feedback: (6)

I also agree with this. If you can live with the down trim, fly it and have fun.

jester_s1

Engine thrust should be set so that the application and removal of power doesn't cause the plane to pitch up or down. You don't use engine thrust to fix a stab or wing incidence problem. You should be able to get the plane flying at full throttle straight and level, and chop the throttle with no immediate pitch change. The nose will drop as the plane loses airspeed of course, but that will take a couple of seconds. And then when cruising at part throttle, you should be able to open the throttle and not see any immediate pitch change. You might see the plane gain altitude if your wing incidence or CG is off, but again, that should take a couple of seconds to start happening.

BWooster

If the forward wing is set to negative incidence, doesn't that mean the stall will be delayed and the lower wing will stall first? I understood the negative incidence was to account for a different relative angle of attack due to airflow effects in a biplane configuration. That is, the air tends to be moving slightly upwards above and before the LE of the lower wing. This gives the upper wing a higher angle of attack. The negative incidence counteracts this.

Rodney

+1 jester, my experience exactly and the way I set mine up.

dbacque

My Feedback: (3)

Wooster, I think you're right. Going back and re-reading my post, I think I flipped my logic. Looks like my explanation of the reasons are correct but my summary of incidence is 180 degrees out. I'll make a quick e-mail and report back.

Dave

dbacque

My Feedback: (3)

Well, I was surprised by the reply from my Aero PhD buddy. Looks like my post #4 had the logic twisted but the answer correct. Mods, can we delete post 4? It contains incorrect information.

So the answer of upper wing having a degree or so of negative incidence compared to the lower wing is correct. But the reason is wrong.

It's not about a stall causing a CG change, it's about a stall causing drag. It turns out that it doesn't even relate to forward or backward stagger. You always want the lower wing to stall first. Here's the reasoning.

When an airfoil stalls, the drag increases greatly. If the upper wing of a biplane stalls first, the drag on the upper wing will cause much more of a nose up pitching moment than any CG change ever could. This drag induced pitch would then stall the lower wing. Think of the upper wing being at about 45 degrees positive incidence. Imagine the drag it'll cause and the resulting nose up pitching moment. No amount of nose heaviness on the bottom wing will offset that. So the result is the plane pitches up, slows down and the lower wing stalls too.

However if the top wing is at a lower incidence, the lower wing will stall first, increasing drag at the bottom of the plane and tending to pitch down. This puts the still flying top wing at a better angle of attack and gives it a chance to soften the stall.

I knew the incidence on all of my bipes have been set this way but I got twisted around in thinking about why.

Sorry if I've added confusion to this subject. Top wing negative incidence gives less top drag in a stall and less increase in pitch so it softens the stall.

Dave

AA5BY

One of the problems is that after a biplane is commissioned, it is often more difficult to decrease the incidence of the lower wing because it usually sits in a saddle. Consequently to reduce climb, the upper wing gets simple shims to reduce its incidence.

Also, the foil as presented is not symmetrical so there will be lift even at zero incidence, which will increase with speed and often require greater amounts of down trim. When power is reduced, the landing glide slope then is too steep and needs trimmed back up. This problem has traditionally been resolved by down thrust but can now be done by a throttle trim mix.

Non symmetrical air foils generally require a bit of positive stab to counter the lift of the asymmetrical wings. Such a configuration provided biplanes with a lot of lift because both the wings and stab were lifting, all in all a very good thing for slower flying planes but not so good for fast planes or aerobatic planes that when inverting, would fall like a rock.

Another problem exist when a lot of down elevator trim is used. When upright, it is holding the tail plane up so that the wing doesn't get too much angle of attack. When inverting, it is holding the tail down and the gravity difference between the two comes into play and the tail is pushed down and the plane wants to out tuck at the top of a loop.