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Old Jan 03, 2013, 07:58 AM
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richard hanson's Avatar
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Theory and actual practice sometimes don't agree .
But I think yoursetup will work - just needs some tuning .
When in doubt -add more power or reduce weight
either setup always fixes it.
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Old Jan 03, 2013, 11:25 AM
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Richard, with the demonstrated reluctance to aviate, I'm gonna go to the original ZAGI elevons, with the vertical thingy on a 3-position switch. Play with that when it gets up, and then try again with only the vertical and ailerons.
But... I did increase the AOA on the ground cart... maybe.....
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Old Jan 03, 2013, 11:46 AM
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If you look at that newsreel one, it had the split drag surface about half a wing span above the CG. It was quite a large surface, and looks like it opens to 45 degrees or so. The pitch moment will be proportional to the moment arm of the drag force. Split surfaces aren't particularly good at creating drag, unless you get the opening angle near 90 degrees.

With the much smaller moment arm your fin has, and the fairly small opening angle of your split rudder not really creating much drag, the pitching moment will be quite small. I wouldn't expect it to be able to overcome even a small out of trim condition of the elevons. Unless the stability margin is quite low, the split rudder will be pretty ineffective as a pitch device.

Kevin
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Old Jan 03, 2013, 02:13 PM
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Quote:
Originally Posted by kcaldwel View Post
If you look at that newsreel one, it had the split drag surface about half a wing span above the CG. It was quite a large surface, and looks like it opens to 45 degrees or so. The pitch moment will be proportional to the moment arm of the drag force. Split surfaces aren't particularly good at creating drag, unless you get the opening angle near 90 degrees.

With the much smaller moment arm your fin has, and the fairly small opening angle of your split rudder not really creating much drag, the pitching moment will be quite small. I wouldn't expect it to be able to overcome even a small out of trim condition of the elevons. Unless the stability margin is quite low, the split rudder will be pretty ineffective as a pitch device.

Kevin
From one who has actually done it - the split drag is very effective -even at moderate opening- of course I had a much longer moment arm BUT it DOES work effectively - I could loop the model.
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Old Jan 03, 2013, 02:39 PM
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.....
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Old Jan 03, 2013, 04:13 PM
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Congrats!
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Old Jan 03, 2013, 04:18 PM
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A discussion about improving the performance of flying wings mentioned use of a vertical surface with split moving parts for control in the pitch axis. This ZAGI was modified to have such a surface, and flown. There are 3 positions... closed, 1/2 open and fully open, selectable on the transmitter. In essence, the surface affects the pitch angle to an observable extent, but insufficiently to take the place of the elevators. It is more of a speed brake than anything else. Takeoff was made using a droppable cart to ease the first moments of flight. The scheme would need a longer moment arm with the surface well behind the wing to be of any practical use, for which there is no real need, the aerodynamics of flying wings being well known.
.
Drag-Elevator-3rdcut (6 min 19 sec)
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Old Jan 04, 2013, 12:28 PM
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Thanks for trying it Paul, the results are about what I expected.

Just a couple of notes:

I don't think anyone, especially not me, ever mentioned that adding drag surfaces for pitch control was ever going to improve the performance of a flying wing. It was just a strange concept I remembered seeing. The one benefit it does have, is that when you add drag for the landing aproach with a high drag centre device, you get to add some camber to the wing to compensate. This is a good thing for a flying wing, that usually ends up with it's lowest camber during the landing approach and flare, exactly opposite of what you would want.

To make the drag surface more effective for pitch control, the surface would have to go higher above the CG, not aft behind it. Moving it back would not make it more effective at all. It is the vertical distance between the centre of the drag force and CG that matters. That is why the newsreel one had the drag surface so high above the CG.

Kevin


Quote:
Originally Posted by Sparky Paul View Post
A discussion about improving the performance of flying wings mentioned use of a vertical surface with split moving parts for control in the pitch axis....

The scheme would need a longer moment arm with the surface well behind the wing to be of any practical use, for which there is no real need, the aerodynamics of flying wings being well known.
.
https://vimeo.com/56720370
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Old Jan 04, 2013, 12:35 PM
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Some of my planes that would pitch with rudder as mentioned, the Stearman in particular, I attributed that to the longitudinal axis of the inertias passing below the rudder due to the weight distribution of the plane, giving the rudder a moment arm for the input to the pitch axis as well as the yaw axis.
The patent claim that a surface on the body of the plane could be used as the elevator is really not defendable.
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Old Jan 04, 2013, 12:46 PM
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I think there is something else going on in the case of the Stearman having rudder coupling to the pitch axis. There could be blanking of the stab from the big fuse when it yaws, or some other change in the stab effectiveness changing the pitch trim.

The vertical distance between the rudder centre and the CG is too small on most aircraft to cause a pitch change from the sideways lift force, and small drag change of the fin, from a rudder application.

Even a swept rudder hinge line has basically no effect on a conventional layout airplane. The stab is just too powerful in comparison. It is just styling. I just did some analysis on a swept rudder hinge line on a plank, and it looks like it could have some effect there, with the very small static margins typically used on planks.

I have no idea what that patent said, I never really looked at it.

Kevin
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Old Jan 04, 2013, 12:58 PM
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This is the way I conceived of the rudder effect on the Stearman..
The inertial axis would be determined by the masses and areas of the parts, and not necessarily be along the aircraft longitudinal datum...
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Old Jan 04, 2013, 01:23 PM
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You are arbitrarily angling the datum, and the aero centre of the fin/rudder, to make a big distance between the CG and the rudder force.

The effective point of the fin will be more or less the mean aero centre of the fin and rudder. The effective height of the aero centre of the fin/rudder will be the vertical distance from the CG to the aero centre of the fin/rudder, measured perpendicular to the aircraft datum. Most airplanes are designed so the fuselage is aligned with the free stream at cruise, for minimum drag. I expect the Stearman is close.

Airplanes rotate around the CG in response to aero forces. By convention, and the way the 3 axis controls are usually arranged, there are three perpendicular axis aligned with the aircraft datum. There can be occasions where using the free-stream direction results in a simpler analysis. Using the datum as you have drawn it will make everything very complicated.

Kevin
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Old Jan 04, 2013, 01:50 PM
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My CAP 21s were really awful "pitch to the belly with rudder" airplanes, especially when landing. Made landings the most "interesting" part of the flight.
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Old Jan 04, 2013, 03:33 PM
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My CAP 21s were really awful "pitch to the belly with rudder" airplanes, especially when landing. Made landings the most "interesting" part of the flight.
The high location of the prop and the low location of the wing have a lot to do with this
also a stock scale Cap 21 has a rather smallish horizontal stabilizer
Some guys got spooked by the Cap tending to zoom when throttle was chopped
best bet was to always land em with power on and stabilized till the wheels touched.
Some call this dragging em on but it works well on th e CAP 21.
We designed & built some of these in 40% sizes for IMAC contest flyers -ourdesign had clipped wings otherwise stock layout throughout . The smaller versions flew the same - We also redesigned th old Midwest Cap again clipped wing - thesewer e really great aerobaticsetups - done lighetr than the eriginal setup Mike MCConville did.
The Horizon Hobby ones wer pretty darn good - we also did othe r size CAP2322 -really liked the Cap setup because th force arrangment permited excellent tumbling characteristics here are some pix.
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Old Jan 04, 2013, 04:10 PM
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Quote:
Originally Posted by kcaldwel View Post
...................

......................

Even a swept rudder hinge line has basically no effect on a conventional layout airplane. The stab is just too powerful in comparison. It is just styling. I just did some analysis on a swept rudder hinge line on a plank, and it looks like it could have some effect there, with the very small static margins typically used on planks.

I have no idea what that patent said, I never really looked at it.

Kevin
Have you flown some of the vintage aerobatic models with the swept rudder hinge line? It does seem to help with conventional layout models. I actually did a litttle old style 2 channel aerobatic flying, rudder and throttle only and the swept hinge line does work. I will say that it does seem to depend somewhat on the position of the stab as to how much effect the swept hingeline has on adding a bit of pitchup with rudder application.
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