Design > Stunt design
Stunt Plane Parameter Calculator
Howard Rush:
--- Quote from: Brett Buck on January 17, 2021, 02:08:58 PM --- It's worse than that, it's a differential equation (or series of them) with trig functions as some of the factors, so there is almost certainly no closed-form solution.
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It's easier than it looks. One can linearize it about an operating point. Stunt planes don't have that many operating points, and fly at pretty constant speed and at constant altitude.
--- Quote from: Chuck_Smith on January 18, 2021, 07:44:25 AM ---Igor, I'm not certain I agree with the changing AoA in your illustration. (I admit I maybe interpreting it wrong though.) The curved streamline you drew is not representative of what's going on around the plane. There will be significant downwash behind the wing so the streamlines will deflect rather than stay constant with the flight path. I suppose there may be some superposition of the velocity vectors but my experience lends me to believe the downwash effect will greater. It's pretty easy to quantify the downwash.
If you look at modern sailplane you'll see that the nose is low and the aft fuselage curves downward. This is done to align the fuselage with the incoming upwash and outgoing downwash of the wings at flying speed. Given that sailplanes have really high aspect ratios and thus have much lower induced drag than other aircraft, the fact that this effect is visible in the fuselage design shows how strong the effect can be.
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I worried about downwash a lot as a kid, so I put long tails on my combat planes. The benefit I got was also due to pitching moment due to pitch rate (Igor's picture). Low-aspect-ratio combat planes have lots of downwash, but they have q out the wazoo. It's different from putting the flaps down on a Cessna and retrimming to keep flying level. There, one has downwash, but no pitch rate at all.
I think that the sort of question that a stunt plane designer asks is, "what happens if I take a given design and make the tail longer?" The pertinent parameters may be rate of change of downwash or Cmq with tail length. As Igor said, downwash decreases and Cmq increases (absolute values), both of which call for more elevator deflection. A curious consequence of this stuff is that elevator required for a given loop size changes with air density. This effect may increase with tail length (TBD (by me, anyhow)). Those of us with long-tail combat planes had to cut way down on elevator travel at the hot-and-high 1984 Reno Nats. I blamed Reynolds number at the time, but now I think most of it was Cmq. Was downwash part of it, too?
Interesting tidbit about sailplanes. That figures if they are optimized to fly level, but they are probably optimized to fly in a turn with pretty high pitch rate. I'm too lazy to calculate anything, but I can ask guys who did.
--- Quote from: Ken Culbertson on January 18, 2021, 01:43:14 PM ---One thing that has always bothered me is the comment that comes up all of the time "but we are flying in a circle, not a straight line". I wonder just how much that really matters when it comes to the basics of airfoil and movements ...
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Me, too.
--- Quote from: Ken Culbertson on January 18, 2021, 01:43:14 PM ---Are there any wind tunnel tests in a 120' donut?
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Yes, sorta. Early aeronautical guys like Langley tested wings on whirly things. Data were confounded by the wake from the previous lap, so folks didn't pay much attention to it, not realizing the important application to control line airplanes.
--- Quote from: Ken Culbertson on January 18, 2021, 01:43:14 PM ---Should wind speed be considered in formulas?
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Wind velocity should, specifically the sideslip component, as Ken said. You can see the effect from old NACA reports.
Then there's the issue of control line airplanes being part kite.
--- Quote from: Chuck_Smith on January 18, 2021, 01:59:40 PM ---Ken, it matters because our planes are (hopefully) continually in a sideslip. This means that the dihedral effect is always there. Most CLPA ships have the wing pretty much centered in the fuselage so that limits the effect.
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Centering the wing vertically makes the dihedral effect more symmetrical in inside and outside loops, but it's still there, and I think it's important. Modern combat planes have swept-forward wings to reduce or eliminate dihedral effect.
--- Quote from: Brett Buck on January 18, 2021, 09:36:13 PM --- A large factor in the latest "best airfoil" thread was how it worked in the wind. In the good old days, how it would work in the wind dominated most of the aspects of how the airplane was designed, from Aldrich and his limited power/thin wing/giant flap, to Al's thick wing, giant flaps, and far forward CG, then the west coast long tails and small flaps and then radically aft CGs. Only when we got overkill power did flying in the wind stop being the primary design driver.
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Did I tell you my Brett Buck flying-in-the-wind story? At the October Golden State Stunt Championships, the West's biggest stunt contest, we have a banquet Saturday evening. Traditionally this includes the Address of the National Champion. Said Champion was away at work or something at the last GSSC banquet, so Brett was drafted to speak. He talked about how to fly in the wind. It was a good talk. He stressed the need to keep the airplane from blowing down after the the first loop of the clover. The next day, it was breezy for my second flight. The clover will be no sweat, I told myself. Not only had Brett advised me what to do in his lecture, but the wind was blowing at the sun, and the sun elevation app on my telephone told me that the sun would be at about 42 degrees during my flight. I did the first loop, then observed my airplane flying the level part. "What's it doing way down there?" I asked myself.
Brett Buck:
--- Quote from: Howard Rush on January 19, 2021, 02:14:00 PM ---Did I tell you my Brett Buck flying-in-the-wind story? At the October Golden State Stunt Championships, the West's biggest stunt contest, we have a banquet Saturday evening. Traditionally this includes the Address of the National Champion. Said Champion was away at work or something at the last GSSC banquet, so Brett was drafted to speak. He talked about how to fly in the wind. It was a good talk. He stressed the need to keep the airplane from blowing down after the the first loop of the clover. The next day, it was breezy for my second flight. The clover will be no sweat, I told myself. Not only had Brett advised me what to do in his lecture, but the wind was blowing at the sun, and the sun elevation app on my telephone told me that the sun would be at about 42 degrees during my flight. I did the first loop, then observed my airplane flying the level part. "What's it doing way down there?" I asked myself.
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I am glad people got something out of that. They hit me up to do it at about 11 in the morning, and I came up with that entirely off the top of my head, and spent most of the afternoon looking around for supplies at various art or office supply stores. It wasn't totally terrible, but preparation was not to my usual standards.
Brett
Chuck_Smith:
Cool comments Howard.
About Cmq, pitch rate moment is the one stability coefficient that is hard to predict and to measure - at least for me on slow flying, light airframes.
Once you get to the point where you're past thinking in the classic 2D, infinite wing mode (which isn't reality on an airplane) you start thinking more in terms of circulation and Prandtl. That's where it gets really iffy and the predicted results start to be less than the theoretical predictions. BTW, this is all for slow flying planes. Once they go fast life gets easier.
On something like your Nemesis the wing shape is fixed so the Cla and Cma are fairly easy to predict.
Compare that, however; to a CLPA ship with large flaps and large deflections. The change in circulation after the vortex is shed because of the flap movement is comparatively quite large because of the non-trivial change in camber and AoA due to the flap's deflection. That means a larger vortex shed, and hence a large change in circulation. The wildcard now becomes viscosity and how far away from wing the circulation propagates. And, since air has mass it
1) takes (ultimately) thrust to replace the airplane's momentum lost to the air movement
2) needs to be tucked away in our analysis that Sir Isaac still applies, and any change in the angular momentum of the air needs to be balanced out to net zero plus thrust impulse during the turn. [And that's the big wet blanket that gets thrown over the infinite wing model.]
Now add in that the same effect is happening in the reverse direction at the tail, and that the shed vortex of the wing gets superimposed upon that of the horizontal tail and well, I warned you leaving infinite wing 2D analysis can be fun!
Lucky for us, we have mountains of empirical results. And with an understanding of circulation and lift lines, we can all appreciate why canards suck at subsonic speeds.
Peace,
Chuck
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