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Old Sep 20, 2012, 02:02 PM
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Joel K. Scholz's Avatar
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What won't my Kestrel fly?

I have designed this epp foam falcon and did initial test flights this morning, with no success. Any suggestions as to what problems and solutions there may be will be appreciated.
Final Kestrel test flights (2 min 58 sec)

Build thread is here.http://www.rcgroups.com/forums/showthread.php?t=1727632
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Old Sep 20, 2012, 02:02 PM
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Old Sep 20, 2012, 02:35 PM
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Do you have an coordinate file for the Eppler 128 airfoil? It seems to be pretty rare...

That airfoil looks like it has a lot of camber for such a close-coupled tail, You may run into tail stall trying to counteract the airfoil pitching moment.

How did you locate the CG? From the flight attempts, it looks to be too far aft, especially with the large negative incidence of the tail causing a large pitch -up force. If you send me the Corel Draw file, I can calculate the required CG position, assuming the model matches the drawing for wing sweep, etc. Given the forward sweep of the wing centre section, and the very short nose on the body, I think it will be very difficult to get the CG forward far enough. You will have to learn to fly with an unstable CG position, like the real one!

PM me for an e-mail address.

Kevin
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Old Sep 20, 2012, 02:56 PM
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Definitely looks unstable in pitch, and possibly in yaw. I would increase the tail area and/or moment arm and move the CG as far forward as possible. As Kevin said, though, you might not be able to get the CG far enough forward without extending the nose.

Out of curiosity, could you put a mark on your top view picture for where the CG is currently?
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Old Sep 20, 2012, 03:19 PM
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I ran a quick CG calc using dimensions scaled off the drawing you included above (so rough at this point) using Dan's spread sheet. It will be difficult to get the CG forward far enough but here is a top view with a 10% static margin and neutral point shown. Although 10% SM would be nice, the close coupled tail will likly only be able to deal with a smaller static margin, maybe halfway between the neutral point and the 10% shown to give 5%SM. But then the pitch will be quite sensitive.

I agree with Dan that the yaw stability will be very low, and with the dihedral you will likely have dutch roll instability. An autopilot might be able to cope with that. But the CG definitely looks too far aft, and until you get some pitch stability the behaviour will be hard to judge. I took a normal sailplane and moved the CG far behind the neutral point, and it was interesting to fly even with a 3-axis autopilot.

Kevin
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Old Sep 20, 2012, 03:35 PM
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Maybe the tail is not big enough and adding dihedral wont be enough. A lot of these bird models also have a vertical ventral on the bottom. Looks great though. The kestral is the prettiest little falcon, like a minature Peregrine with some extra bling.
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Old Sep 20, 2012, 04:07 PM
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Here is a screen shot of the apprx furthest forward cg I optained. It is about 2 inches from LE at fuselage . Chord is 8 inches. I may have discovered the culprit. It appears I inadvertantly built in some up incidence on the tail .estimate 1 to 1 1/2 degrees. Large tail surface area probably makes this significant. I have cut off the tail and will try to correct to zero incidence.
Kevin, icould not find a pm address to email you . here is a screen shot.
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Old Sep 20, 2012, 04:47 PM
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Another discussion came up today while testing this model. I had set up the radio to alow for flap adjustment both above and below nuetral. The outer panel of the wings has 2 degrees of up in the trailing edge, My thinking was it would be flying much like a flying wing. When the plane was continually pitching up and stalling I suggested lower the wingtip incidence to give more downward pitch. Mike , My test pilot believed I would get less pitch up by raising the ailerons. His theory is this would diminish lift. Who is right?
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Old Sep 20, 2012, 05:41 PM
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The pitch effect of the ailerons depends! They will be less effective than the tail in any case, because they are close to the CG. Putting them down is also equivalent to wash-in, which will likely create more tip stall problems. You will already have tip stall issues with those pointy tips, so trying to use outboard ailerons for pitch control will not work very well.

I would put the ailerons up a bit to give some additional washout, at least initially. The pitch effect should be small, and easily overcome by the tail elevators.

The tail incidence isn't super critical at this point. The elevators should provide adequate pitch trim to get it flying. If you end up with a bunch of elevator offset one way or the other for trim, then you could change the tail incidence.

With the CG 2" from the fuse wing intersection, it will be extremely pitch sensitive. I think you need to get it 1/2" further forward than that, at least initially. I'd also recommend unpowered hand launch glides over tall grass until you get some sort of controlled glide going.

Where did you find an Eppler 128 airfoil? I'd like to see what the pitch coefficient looks like. It looks like a high camber, high Cm airfoil, which will be a problem on a short coupled airplane like this. A plank style reflexed airfoil would have been a better choice.

I PM'd you my e-mail address. I'll refine my CG calc with the bigger drawing you posted.

Kevin

Edit: Attached new CG calc (V2) based on the newer drawing. At 2" back from the wing root, the CG would be behind the neutral point. A 7% static margin would have the CG at about 1.3" from the fuse/wing intersection. Pitch control will still be pretty sensitive, and the damping will be low in pitch.

What is the dihedral angle on the tail?
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Old Sep 20, 2012, 09:59 PM
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In the first attempt your launching buddy simply tossed it up at too high an angle. Or it pivoted in his hands to end up leaving more or less vertically. So let's simply discount that test flight as bad from the get go at no fault of the model.

In the second one he pushed it down towards the grass and you responded with a quick stab of up elevator which again made life suddenly very interesting. But indications are from that one that there's a decided lack of yaw stability and possibly the wing tips stalled and that began a nasty snap roll.

The third one I simply can't tell much from other than again it does not have enough yaw damping. It could be that it suffered from yet another tip stall and snap roll.

Part of the issues could well be due to using an airfoil with a negative pitching moment and a super short tail moment. While our CG prediction software does not include a spot to input a pitching moment value for the airfoil in extreme cases such as your flying bird it really needs to have such a factor added. The tail moment is made even shorter by the fact that the wing tips are swept back so the MAC of the overall wing is shifted back to where the tail is all but sitting within the effective planform of the wing itself. So that even further reduces the tail moment distance to darn close to zero.

All of which suggests that your best bet is to treat your design as a plank style flying wing that happens to have a slightly remote elevator surface. As such I would suggest that the model would benifit from using a reflexed airfoil with a postive pitching moment. And to go with that you should use a CG location that starts at 15 to 17% of the MAC and shift it back as needed until you reach a flat glide with no up trim needed.

Granted this means building a new model. In the meantime you can try a few things with this one. Namely make your airfoil into a reflexed format that will have a zero to positive pitching moment by reflexing the flaps and ailerons above the zero line by some amount. At the same time shift the CG ahead to around 17% of the MAC, whereever and whatever it turns out to be.

I'd also suggest simply removing the prop and test gliding it over that lovely tall grass a lot before your next powered attempt. In reading the other thread I see that your wing loading came out to a little over 10oz/sqft. For a "glider" like a kestral should be that's a little on the heavy side for a model of this size. So you may want to wait for a breezy day so you have some wind to do the test glide launches into so you don't need to run too fast or throw it so hard that you lack directional control over the launch.

To learn much from these test glides you need to launch the model so that it is pointed very closely in alignment with the launch direction. Otherwise you can be stalled when it's still in and leaving your hands. That's where the head wind comes in since you can devote more of your attention to a good launch attitude and less worrying about throwing hard enough to reach flying speed.

Watch the descent for signs of issues. You're looking for it to settle in flat but not in a nose high mushing sort of way. If your CG is well forward it'll tend to want to arc down. Shift the CG back until there's no sign of an arcing down but no sign either of wanting to lift the nose up into a stall.

If there seems to be little or no happy zone and it goes from arcing down and in to wanting to balloon up and stall it means you still don't have enough reflex in the airfoil. So raise the flaps and ailerons some more and try again with the CG back to the forward position. What you're looking to do is get to where you add on enough reflex on the wing control surfaces to shift from a negative to a positive pitching moment. Because you're doing this with an airfoil that wasn't intended to be shaped this way the final result may be flyable but it likely won't have the slowest and most bird like glide.

Along the way if it seems to show signs of precious little yaw damping and you find it often flying sideways and producing a sudden and violent snap roll as a result then add a ventral fin of stiff clear plastic such as .060 lexan. Again you simply do not have a great deal of tail length here to work with due to the swept back tips shifting the MAC back on you. So err on the side of too big for now and you can consider cutting it down later if it seems like it would tolerate such trimming.
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Old Sep 20, 2012, 10:57 PM
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Bruce,

No flaps I can see, so reflexing the centre section will only be possible by bending the foam. That could work. Would the EPP take a set if it is heated and held in a reflexed position?

And after I built that plank with a Clark Y airfoil just to show you that wing Cm doesn't effect stability, just the trim Cl. Imagine this Kestrel without the tail - it would be the same as the Clark Y plank. If the CG is ahead of the neutral point, the airplane is statically pitch stable. The trim Cl would be negative (inverted), the same as the Clark Y plank. Having the CG ahead of the neutral point is all that is required for static pitch stability. No need to include wing Cm in the stability equation, since it does not effect stability. It is of course important for the pitch trim equation, where it certainly appears.

Do you see why the Clark Y plank demonstrates that wing Cm does not effect static pitch stability, just trim Cl?

Adding the short coupled tail doesn't change much, except to move the neutral point back a bit, and provide a more powerful means to adjust the trim speed with an elevator. It still just needs the CG ahead of the neutral point to be pitch stable. Whether the short coupled tail is powerful enough to trim at a the desired upright Cl without stalling is another question. I suspect it will be a problem unless the CG is very close to the neutral point (low to zero static margin). The low pitch damping will make it very challenging to fly with the CG back that far.

We agree on the solution: the wing airfoil should be a low or positive Cm airfoil, so that the tail load with the CG ahead of the neutral point, at a reasonable static margin, is low.

And some more fin area at least until it is trimmed out would certainly be a good idea.

Kevin
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Old Sep 20, 2012, 11:50 PM
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Gentleman , I appreciate all your imput. I have already redone the rear stabilizer to a nuetral from a positive incidence. I believe you are correct in that I neec the cg further forward. I will have to add lead as I am out of battery room.
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Old Sep 21, 2012, 07:24 AM
Herk
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Joel - you are a real artist - that is a pretty piece of work.

I've been flying a lot of rather unusual models lately and I can tell you that a bungee gives a way better launch than just tossing it. At least the model gets a decent start before it has a chance to get in trouble. The last time I didn't use one, the launch looked just like yours. (Except I was tossing and holding the transmitter.) I went back to the bungee real quick.

It would also let you get on the sticks - instead of just being the launcher.

Kind regards and best wishes ---- Herk
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Old Sep 21, 2012, 08:37 AM
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Quote:
Originally Posted by kcaldwel View Post
Bruce,

No flaps I can see, so reflexing the centre section will only be possible by bending the foam. That could work. Would the EPP take a set if it is heated and held in a reflexed position?

And after I built that plank with a Clark Y airfoil just to show you that wing Cm doesn't effect stability, just the trim Cl. Imagine this Kestrel without the tail - it would be the same as the Clark Y plank. If the CG is ahead of the neutral point, the airplane is statically pitch stable. The trim Cl would be negative (inverted), the same as the Clark Y plank. Having the CG ahead of the neutral point is all that is required for static pitch stability. No need to include wing Cm in the stability equation, since it does not effect stability. It is of course important for the pitch trim equation, where it certainly appears.

Do you see why the Clark Y plank demonstrates that wing Cm does not effect static pitch stability, just trim Cl?

Adding the short coupled tail doesn't change much, except to move the neutral point back a bit, and provide a more powerful means to adjust the trim speed with an elevator. It still just needs the CG ahead of the neutral point to be pitch stable. Whether the short coupled tail is powerful enough to trim at a the desired upright Cl without stalling is another question. I suspect it will be a problem unless the CG is very close to the neutral point (low to zero static margin). The low pitch damping will make it very challenging to fly with the CG back that far.

We agree on the solution: the wing airfoil should be a low or positive Cm airfoil, so that the tail load with the CG ahead of the neutral point, at a reasonable static margin, is low.

And some more fin area at least until it is trimmed out would certainly be a good idea.

Kevin
While this is true, if your Cm is such that your trim angle of attack is negative, it will make for a pretty "exciting" maiden flight even if the CG is far enough forward. That said, from the video it looks like the problem is static instability. The Cm of the wing could be too high as well, but until the CG is fixed it will be hard to tell if that's the case.
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Old Sep 21, 2012, 09:19 AM
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Each test flight had at least one violent pitch up. That would suggest that it doesn't have any trouble producing a pitch up moment. Of course, that could change when the CG is moved forward.
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