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Old Sep 24, 2012, 12:13 AM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,618 Posts
Tractor propellers are destabilizing in pitch and yaw. It looks to be marginally stable in yaw without the motor running, but perhaps unstable with it on. The motor torque and destabilizing effect of the prop could well be overcoming the little yaw stability it had while gliding, and then adverse yaw from the aileron deflection adds on top of that.

Some slope soaring without the prop would perhaps let you trim it out - any slopes around? Perhaps not in Texas. A light balsa fin might not look great, but perhaps it would help get it flying and then hopefully it could be removed.

Short coupled airplanes usually have more - ah - interesting - flying characteristics! They have a lot less damping, and lower stability, in pitch and yaw. They can be trickier to get flying decently, as you are discovering.

Kevin
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Old Sep 24, 2012, 08:39 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,253 Posts
I still go back to how easy and gentle my Eagle flies, and with no additional vertical stab.

The Kestrel's glide test video seemed nice and straight, though a little fast, (but then it is a Kestrel). But under power it's speed is so much higher than any small twist, difference between wings, will show up immediately.

Djacob7's Hawk video seemed to me to be flown too fast to sort out any trims. If a model glides nice, it only needs a little power to fly and start climbing nice.

A reflexed flying wing, (just the same as many models with cambered wing sections), only needs a little speed change to require an elevator trim change to compensate for that increase in lift.

My Eagle is released on quite low throttle, and the speed gently increased to climb. Full throttle will have it going vertical with me holding full down elevator to stop it looping.

I don't see anything particularly wrong with the Kestrel and the Hawk other than too much throttle.
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Old Sep 24, 2012, 09:33 AM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,618 Posts
Quote:
Originally Posted by eflightray View Post

A reflexed flying wing, (just the same as many models with cambered wing sections), only needs a little speed change to require an elevator trim change to compensate for that increase in lift.

My Eagle is released on quite low throttle, and the speed gently increased to climb. Full throttle will have it going vertical with me holding full down elevator to stop it looping.

I don't see anything particularly wrong with the Kestrel and the Hawk other than too much throttle.
Climb with throttle is 90% of the time having the CG far forward. High pitch stability will cause a strong pitch up with speed increase. If you like flying it like that, then don't change it, but moving the CG back closer to the neutral point will reduce the pitch up tendency with speed increases.

Of course the elevator will get more sensitive as the CG goes back, so throw will have to be reduced and down trim will have to be added. It is quite possible to fly conventional layout airplanes with the CG right at the neutral point, so there is zero pitch trim with speed changes, assuming the thrust angle is correct. Most DLGs, and many pattern airplanes and sailplanes are flown like that.

The lower pitch damping of short-tailed bird-like models means it is pretty hard for most of us to keep up with the pitch on them with low pitch stability, although Biber flew his Mulibumm with a neutral CG quite successfully:

http://www.rcgroups.com/forums/showthread.php?t=534401

I still think the yaw stability of the Kestrel is pretty marginal. It might be possible to make it work without an added fin, but it will take some fine tuning of aileron differential and thrust line to get it there. Less throttle is a good idea!

Kevin
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Old Sep 24, 2012, 01:10 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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I agree with what you have posted Kevin, the problems comes from those first couple of flight with a CG in the recommended 'safe' position, (usually slightly forward). Once you can achieve some successful flights, then you can determine the models reaction to the throttle, and trim the CG to suit how you want it to handle.

As for my Eagle, basically she is over powered, but on very low throttle and the CG I have set, she will almost hover. I did try the CG further back, but she didn't 'feel' right.
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Old Sep 24, 2012, 02:18 PM
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peterangus's Avatar
Blackpool, Great Britain (UK)
Joined Dec 2003
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Quote:
Originally Posted by Joel K. Scholz View Post
I plan on taking photos tomorrow of the two wing halves and compare to see if I can find a warp or other anomaly.
Your photos will not reveal the problem "warps".

The problem warps are transient and variable. They occur during flight, caused by structural loads.

The unwanted effects are speed-related. They might disappear if you could fly slower.

Early aviation [pre 1930] was bedevilled by similar perplexing behaviour.

Read pages 1,2,&3 of this link. http://catdir.loc.gov/catdir/samples...2001052552.pdf
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Old Sep 24, 2012, 02:20 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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I'm in accord with the evidence from this latest video that it's likely that the yaw stabilty is lacking.

Joel, all ailerons will produce some adverse yaw when used. It's simply locked into the nature of how displacing the ailerons alters the local airfoil camber and angle of attack over the area of the ailerons. And since this area is always out at the tips the difference in drag they cause has more leverage to work with. On a normal model it's simply not an issue since the vertical tail is well back and overpowers the drag differences. It does show up on longer span sailplanes to some extent and folks tend to use differential throw to lessen the effect. On plank style wings and similarly short models such as your Kestral it becomes a very noticable effect as Kevin has indicated. The only way around this is to have a fairly large vertical fin area to work against it.

Another way would be to model the wings with the washout needed to generate a Horten like lift distribution. With that the ailerons actually produce a PROverse drag couple.

There is also two other options. One is to go with spoilerons located out at the tips. Another is to use split surfaces at the tips similar to the drag rudders found on the B2 bomber but where the lower flat stays in place and only the upper flap extends upward to produce both rolling and PROverse drag.

You could model this last option fairly readily. Glue on lower surface aileron "floors" made from 1/32 ply. Then, assuming you have the ailerons set up on separate servos and separate Tx channels, you set the control throw volumes so that the up travel on each side is 100% and the down travel volume is 0%. That produces ailerons that only travel up on the side commanded for roll.

The idea of the lower side fixed flap over the ailerons is that when the surfaces deflect up they produce a tip down roll but the drag from this split surface also pulls that side back at the same time. So now you have the roll and yaw forces working in the same desireable direction.

In the meantime though I would concur with the folks above and suggest that you install a rather big subfin for now in an attempt to get the model flying first and then cut it away in small steps and perhaps install and configure my suggestion for the "dragerons" split aileron setup. A subfin made from something like 1/16 ply is not going to add much tail weight at all. And it appears from that last flying video that you sorely need it. I'd suggest that the sub fin be sized to be the same size as the current stabilizer and elevator area and shape. Start big like this so you KNOW it's enough.

Once you install all this crud and get the model to fly THEN you can begin slowly removing it in small amounts per flight and work towards seeing just how much it actually needs.
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Old Sep 24, 2012, 03:54 PM
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Kingsland, Texas USA
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Managed to find a suitable slope today and had about a 10 mph breeze. This is the first breeze since I started test flying.
I removed the motor to save the shaft and added lead in its place.I cut a balsa vertical rudder and jammed it up you know where. I added differential to the ailerons and experimented with different flap settings. I found I got my longest flights with aileron trim just a fraction above nuetral. The wing tips have 2 degrees of washout built in.
I also took some measurements on the wings prior to flying. The left wing seems to have 1/2 centimeter more dihedral over the span. As I repaired the wing last night , I am not sure how they compared in previous flights. I have found the weak spot in the wings as hard impacts produce a tear starting at the trailing edge an running along the root. I have taken advantage of this today to repair the dihedral. \
At one point I was actually able to slope soar briefly until I encountered a tree. This prompty broke off the vertical rudder. Further flights after this produced no discernable difference in Yaw. The bird responds to both left and right aileron even at these slow speeds. The last flight the plane banked and turned left and when I picked up the plane the left rear elevator was jammed in the down position. I was a litte surprised as how much it effected the flight. I may go back to the V-tail aileron setup. Final trim reguired just a bit of up elevator. I plan on reinstalling the motor with 3 degrees of right thrust.
The poor bird is beginning to look like its been shot at and hit.
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Old Sep 24, 2012, 04:24 PM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
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Maybe the left elevator was stuck on the powered attempt as well? It looks like only the right elevator is up in the dive to the ground?

Kevin
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Old Sep 24, 2012, 05:35 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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Sounds like a lot was learned from the gliding sessions. You're a lot better equipped now to deal with power related trim issues.

Keep in mind though what Kevin mentioned about the prop running up front being a de-stabilizing force. If you go back and look at the old Northrop XB-35 prop driven flying wing bomber when they converted to jets they had to add fins to make up for the loss of the rearward fin like effect of the prop discs being gone. So having a moving prop up front is sort of like having a forward fin acting to destabilize the craft. You may want to re-fit that sub fin for at least the first powered flights and again trim it down in partial amounts after each successful flight. It may well be that you need that little bit of added help.
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Old Sep 24, 2012, 07:37 PM
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Kevin I cant be sure. I cannot duplicate the anomaly on the ground. I remember when I first installed this servo it seemed to have less travel than the other. I thought it might be a radio issue. Another thing I noticed today when preparing for glide test. The left elevator had reduced throws programmed in. This may have occured on the first test flight. Mike, set up the radio the way he thought would work best and he may have tried to match throws via the radio. When we switched from V-tail configuration to pure elevator the trim factor was not accounted for. I just don't know for sure, but I may change the servo. You can bet I will check thoroughly. I would hate to think all this is about a malfunctioning servo. I doubt it though. You had mentioned you would have chosen another airfoil for this bird. I am curious. What would you have used?
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Old Sep 24, 2012, 08:42 PM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,618 Posts
I've looked at your powered attempt video a few times, and it sure looks like only the right elevator is up in the final dive.

I have my own bird-like planform 2m sailplane model half done. It has Dr. Drela's AG 44 to 46 series, with the flap in the 2 degree reflexed position. My e-glider has quite a bit lower wing loading than yours but bigger wing chords, so it's Re range might be a bit different. The AG 4* airfoils have excellent low Re performance, and have very low Cm (moment coefficient) with the flap reflexed. This reduces the tail load on a short coupled airplane, and makes it more efficient because the tail doesn't have to lift downward so much with a stable CG position. My model's low wing loading should help with slow flight, because the max Cl is a bit lower with the reflexed flap. It is a bit of an experiment, so I'm not planning on movable flaps at least initially.

Your Eppler 197 isn't very good below Re = 200k, so it wouldn't be the best glider airfoil for smaller chords and slow flight. For powered flight, if the stall is OK, the airfoil isn't too critical unless you are trying to optimize the design. I think a lower Cm airfoil with better low Re performance would be a better choice for your Kestrel (Mk2 ?).

http://www.charlesriverrc.org/articl...t-airfoils.htm

Kevin
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Old Sep 25, 2012, 03:28 PM
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We had a 15mph gusty winds today, so I tried a bit more slope launching. I was actually able to gain about 10 ft in altitude. The stalls are straight foward, with no tendency to tipstall. Pitch is stable. I did find that the vertical fin made for a smoother glide. Without it the plane seemed to hunt side to side a bit. I reprogrammed the V-tail function in, and it seems more effective for turning than ailerons. I have reinstalled the motor with some right thrust and hope for a powered flight this evening or tomorrow if conditions allow it.
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Old Sep 28, 2012, 08:58 PM
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Kingsland, Texas USA
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Here is a video of the first successful powered flight. Plane is flying well. Thanks guys.
Utube Power Kestrel (14 min 0 sec)
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