stunthanger.com
Design => Engineering board => Topic started by: Peter Germann on July 02, 2014, 07:58:13 AM
-
Assuming level flight with elevators slightly up and flaps slightly down, how much (order of magnitude) is the actual AoA of a typical, flapped stunt wing?
Peter Germann
-
Assuming level flight with elevators slightly up and flaps slightly down, how much (order of magnitude) is the actual AoA of a typical, flapped stunt wing?
Peter Germann
Figure the Cl is about .1 per degree, solve the lift equation for Cl, should get you pretty close.
f=1/2*Cl*rho*A*v^2
2*f/(rho*A*v^2)=Cl (f/a= wing loading)
putting in my airplane as an example
f=4 lb, a=4.61 sq feet, rhos = 0.0023 lb/ft^3, v=80 ft/sec
Cl=0.12 degrees, so about 1.2 degrees. Some of that is taken out with flap deflection, so, maybe the fuselage is about 0.5-0.6 degrees or so .
Brett
-
I'm flying a sorta-Genesis with a dead-straight fuselage top. My calibrated eyeball tells me that the difference in fuselage angle between upright and inverted flight is maybe 5 degrees.
I can't vouch for how well calibrated my eyeball is.
Depending on how the flaps are rigged, this could be just about anything -- I think theory needs to be put aside, and direct measurements taken. I'd be tempted to look at as many contest photos as possible, and try to infer the fuselage angle from that.
-
Cl=0.12 degrees, so about 1.2 degrees. Some of that is taken out with flap deflection, so, maybe the fuselage is about 0.5-0.6 degrees or so .
How my folks would look at it is that flaps shift the lift curve, hence decrease the angle of attack for a given Cl. So besides wing loading, speed, and aspect ratio, angle of attack would be a function of CG and flap/elevator ratio around neutral. My current airplane, looking at the stripe on the fuselage relative to the fence at our field, is close to zero. It was negative rightside-up until I corrected the elevator bias.
-
I'm flying a sorta-Genesis with a dead-straight fuselage top. My calibrated eyeball tells me that the difference in fuselage angle between upright and inverted flight is maybe 5 degrees.
I can't vouch for how well calibrated my eyeball is.
Depending on how the flaps are rigged, this could be just about anything -- I think theory needs to be put aside, and direct measurements taken. I'd be tempted to look at as many contest photos as possible, and try to infer the fuselage angle from that.
Why didn't the rest of us think of that decades ago?
Near as I can tell, it works out pretty close to what we always calculated. And you can also see how far the flaps are deflected. If you try this again, what will shock you is the difference between upright and inverted for some airplanes, and some of the absolutely crazy trim rigging. The study I did at the 2003 TT was remarkable, there were a few airplanes from competitive pilots that had everything on them crooked. Some of them were rigged differentially
It's very easy to do, get a DSLR, set it to ISO 800 or so, and bang away. Take multiple shots of the same airplane in level flight and average it out to ensure that you are not catching it in a transient.
Brett
-
If you try this again, what will shock you is the difference between upright and inverted for some airplanes, and some of the absolutely crazy trim rigging.
What shocked me recently was when I put in a correction with the wrong sign. Bad dog!
-
What shocked me recently was when I put in a correction with the wrong sign. Bad dog!
uh,, this is rather cryptic Howard,,
-
What shocked me recently was when I put in a correction with the wrong sign. Bad dog!
uh,, this is rather cryptic Howard,,
What? What's unclear about that?
I'll bet that you saw the "correction with the wrong sign" and thought that Howard was referring to, like, plus vs. minus, instead of the obvious-in-hindsight Cancer vs. Gemini vs. Leo.
-
What? What's unclear about that?
I'll bet that you saw the "correction with the wrong sign" and thought that Howard was referring to, like, plus vs. minus, instead of the obvious-in-hindsight Cancer vs. Gemini vs. Leo.
uh well the "sign" I had not thought of was the cancer gemini,,
I was thinking since the discussion of inverted versus upright,, and he input the wrong sign as in up versus down,,
-
uh well the "sign" I had not thought of was the cancer gemini,,
I was thinking since the discussion of inverted versus upright,, and he input the wrong sign as in up versus down,,
I read it as engineer-speak for biasing the elevator the wrong way the first time around. I'm sure he would have done better if he'd built an Ares instead of an Impact.
-
Depends on several factors like wing load, flap : elevator ratio, CG position, lap time etc, my calculator says (for data of models which I really had (first 3) and TP for some expected data 1700g heavy and CG at 22%) :
model ------ elevator deflection -------- AoA
max ------------ 1.65 -------------------0.31
cardinal--------- 1.44 -------------------0.52
dreadnought --- 1.64 -------------------0.39
trivial pursuit --- 1.36 ------------------0.53
-
Once more you guys leave me deeply impressed. Hats off, this is what I call international cooperation for the common cause.
Thank you very much,
Peter Germann
H^^
-
What shocked me recently was when I put in a correction with the wrong sign. Bad dog!
What this up elevator instead of down? Which way was the prop turning?
Getting it the wrong way around can have alarming effects on stability, even in tiny amounts.
Brett
-
What this up elevator instead of down? Which way was the prop turning?
Getting it the wrong way around can have alarming effects on stability, even in tiny amounts.
Sorry to take so long answering. I was preoccupied with a couple of contests. I forget and negative about the X axis, respectfully.
Curiously, it didn't affect stability, but something cruder. It was hard to turn insides, and the airplane was stalling in outsides. I biased the elevator to put more flap in outsides. The airplane was still messed up some, in particular having the tendency to do lomcevaks on the top of the hourglass. Changing the prop rotation direction made it better.