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Old Dec 04, 2012, 01:52 PM
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Hi Chris

I've watched the video again several times and realised I have seen just such a "flight pattern " before, and on that occasion I was holding the transmitter! The model in question was my RBC kits Spitfire II, powered by a 2.5:1 geared Speed 600. The first couple of flights were fine, but on flight three, as I turned finals, it did just what your model did and I too just avoided the ground. This, of course, is an aileron model. I decided I had just slowed it down too much and continued to fly it, but over the next month the same thing happened several times and it just became too nerve racking, culminating when it actually did hit the ground, albeit with only minor damage. The model was fine when flying at full, or close to full, power, but turns at reduced throttle on the landing approach were always an accident waiting to happen.

The reason I mention this is that the wing shape is, of course, very similar at the tips to your model. I eventually concluded that the CG just had to come forward and added some nose ballast, but also raised both ailerons by 1/8" to introduce a little effective aerodynamic washout. The effect of these changes was dramatic and almost twenty years later I still have the model and it is an absolute pussy cat to fly and can be dragged around low and slow turns with absolute confidence. In view of your CG issues, I hesitate to suggest even more weight up front, but maybe it might need it. Or perhaps you could try steaming a bit of washout into the tips?

Despite the model having a 12" x 10" (folding) prop (lots of torque) I never experienced any funny behaviour on opening the throttle, in fact rather like yours that was what saved it on several occasions.

May all be irrelevant, but your model's gyrations certainly looked very similar! Best of luck with finding a solution.
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Old Dec 04, 2012, 03:41 PM
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Thanks George, I welcome the thoughts.

Chris
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Old Dec 04, 2012, 04:30 PM
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Just some thoughts.

To add to what George has said, i had a glider years ago that would turn nice one way and if you tried to go the other way it would stall every time, the problem was washout in one wing and washin on the other.Looking at the video the model seemed to pitch up just before the first stall( at 57s on the video) then followed by some wing rocking,with a nose high attitude as if it was about to stall again, maybe some down thrust on the motor might be helpfull. Also I've heard people talking about a dive test to sort out the C of G , I've never tryed it but think its worth looking at. Lastly what ever you do make sure you'r at least two mistakes high.

Colin.
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Old Dec 04, 2012, 10:00 PM
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Given your increased weight, you'll need to fly faster than an original Falcon. So the decalage angle on the plans is probably not optimal for your bird. The rubber banded wing could easily accommodate shims to play with that. Beautiful plane and I wish you the best with it. Patrick
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Old Dec 05, 2012, 01:32 AM
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Thanks guys, I am sitting back and taking all these ideas in before I actually try something.

Hey Colin. I started at around 6 mistakes. I was on my last mistake when I just missed terra firma!

Did anyone notice how I cringed just before it looked like the Falcon was going in!

Chris
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Old Dec 18, 2012, 06:53 PM
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Good question Colin. Higher power and the Falcon seemed fine as you could see in the takeoff. The glide seemed fine and controllable. It was the the transition from glide when I added power. I have watched the video numerous times to try and figure out what happened. I was wondering if I was stalling the wing and needed more power when the climb began. I was running on instinct and noticed I kept reducing power to settle it down. When I punched the throttle before it almost hit the first time it seemed controllable but again I reduced the throttle and the wing started to rock again. I can't explain that last wing drop when it landed. I am also wondering about the torque from the prop.
If anyone wants to share an opinion, I welcome it.

-------------
Hi George, we fly from a sod farm. You are looking at the back quarter. We move around as the sod is taken off.

Chris
Hi Chris - After watching your video a few times, the most likely culprit is wash-in in the wings. At higher speeds, the wing tips are not stalling so it handles well. But as you bring power back and reduce flight speed, your wing tips are stalling. Putting the plane into a turn enhances the likelihood of one tip stalling worse than the other, resulting in the plane corkscrewing into the ground.

Sight down each wing panel or lay it on a flat bench and measure tip LE and TE heights above the bench to know for sure. Let us know what you find

John
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Old Dec 19, 2012, 12:54 AM
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Hi Chris - After watching your video a few times, the most likely culprit is wash-in in the wings. At higher speeds, the wing tips are not stalling so it handles well. But as you bring power back and reduce flight speed, your wing tips are stalling. Putting the plane into a turn enhances the likelihood of one tip stalling worse than the other, resulting in the plane corkscrewing into the ground.

Sight down each wing panel or lay it on a flat bench and measure tip LE and TE heights above the bench to know for sure. Let us know what you find

John
Hi John, Yes I will be rechecking the wings to be sure they are straight among other things.
I am taking a breather from the Falcon at the moment and working on a few builds I had set aside during the Falcon construction. Rest assured I will be looking at it in the new year.

Thanks,
Chris
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Old Jan 15, 2013, 12:56 AM
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I guess I should bump this thread back up to the top. I have revisited the Falcon and made some modifications. She is ready to maiden again. Well, maiden when spring returns here.

I checked the wing and couldn't find any signs of a twist in it. All nice and flat.
I was able to reduce the length of the motor mount by about 3/4" and the cg is now spot on. I also checked the lateral balance and noticed the wing was out a little. About 6 - 1 1/4" finishing nails tapped into the wing tip corrected it.
Added about 2 degrees of right thrust and after reading an old article by Frank Zaic on balancing for powered free flight, I measured and then added about 4 degrees of downthrust.
I am still going to wait to finish the cowl as I might have to add more downthrust.

Chris
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Old Jun 13, 2013, 02:15 AM
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Revisiting the Falcon this evening

Well it is time for an update.

With calm air this evening I headed out to the field with the Falcon to see if the modifications I made last year would make this Falcon soar.
I am happy to report that scary wing drop has disappeared. The wing dropping is nothing more than torque from the 12 inch prop.
I added some right thrust on the motor to clear it up. The reason behind wing drop being so severe is the twin rudders. A normal tail has the rudder in the prop blast but the twin rudders are out of the artificial airflow and I need to gain speed for the twin rudders to become effective to correct the drop. A normal tail setup is easy to correct wing drop with just a blip of rudder at any speed.

I had three 10 min flights this evening with about 6 landings. I will say that one wheel gear is interesting to land on. Haven't quite got the knack of it yet as no landing was smooth. At least to my liking. Nothing broken yet. I really worry about snapping one of the fins off in a less than stellar landing.

More flight testing is needed as the cg needs some adjustment and I need much more downthrust on the motor.

It was nice to be able to relax while soaring with the Falcon and enjoy the shape of it in the air.

Chris
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Old Jun 13, 2013, 04:53 AM
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Hi Chris .. That's great news, sounds like your on the right track. Keep us informed on your progress.

Colin.
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Old Jun 13, 2013, 12:00 PM
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Glad to hear you've got it sorted out and could relax/enjoy seeing it soar.

I really enjoy tweaking OT models since fairly simple changes can take a bad or so-so bird and turn it into a real performer. Often the guys in my club who attempt OT projects after only a few initial flights conclude it just doesn't fly well and give up when all that's needed are some easily implemented adjustments.

For example, a good friend of mine built a beautiful 65" Sparky from a short kit he picked up at our club swap meet last fall. It was an older electric design intended for 400 brushed motor and NiCads. We updated it to brushless power on a 3S 2200 Lipo and it came out super light at 2 1/2lbs. The motor we selected was a 750kv hextronik bell motor. He started off with a 9x6 prop and it wouldn't even get off the ground!! Problem was the prop size; we moved it up to a 14x7 to get the ~100W of output we were shooting for. With that change, it could now get off the ground at a little over 1/2 throttle. Now that we had it in the air, initial handling was quite poor (tip stalling, severe throttle/pitch coupling, weak elevator control). After getting some washout in the wingtips, lowering the wing incidence, and moving the CG back to 65% of root chord, the plane now flies beautifully and is one of his favorites. He typically gets 45mins on the 3S 2200mah pack since the low rpm results in a super efficent prop setup. In addition, it looks, sounds (makes absolutely no noise), and flies like a rubber powered model with that giant prop on the front. All of these tweaks costed nothing except for a $3 prop.

John
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Old Jun 13, 2013, 01:22 PM
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Thanks Colin!

Hey John
I am hoping it is just some minor corrections now. I really wanted to solve that wing drop first as that was going to put me in the ground. Not really happy with the transition from power flight to gliding and Falcon drops faster than I like while gliding. Landings at this time are basically ugly, but hopefully the down thrust I will add will stop the pitching up. Landing too fast or stalling the wing from the pitching up is not nice. I realize there are some quirks to this plane that I might just have to live with and be aware of while flying.
I should also mention the pitching up slowed the Falcon's airspeed and was also causing the stalling of the wing. This loss of airspeed allowed the wing to drop from the torque of the prop that I couldn't easily correct because I didn't have sufficient airflow over the rudders.
At least till it started to dive towards the ground.
The pitching up is still there when power is added but more manageable and not as severe at lower throttle settings. I am hoping more downthrust will cure this issue.

Of course 50 km wind gusts today, so we are grounded from anymore testing at the moment.

Chris
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Old Jun 13, 2013, 06:08 PM
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Thanks Colin!

Hey John
I am hoping it is just some minor corrections now. I really wanted to solve that wing drop first as that was going to put me in the ground. Not really happy with the transition from power flight to gliding and Falcon drops faster than I like while gliding. Landings at this time are basically ugly, but hopefully the down thrust I will add will stop the pitching up. Landing too fast or stalling the wing from the pitching up is not nice. I realize there are some quirks to this plane that I might just have to live with and be aware of while flying.
I should also mention the pitching up slowed the Falcon's airspeed and was also causing the stalling of the wing. This loss of airspeed allowed the wing to drop from the torque of the prop that I couldn't easily correct because I didn't have sufficient airflow over the rudders.
At least till it started to dive towards the ground.
The pitching up is still there when power is added but more manageable and not as severe at lower throttle settings. I am hoping more downthrust will cure this issue.

Of course 50 km wind gusts today, so we are grounded from anymore testing at the moment.

Chris
You've figured out a great deal in just a few flights so keep going on the tweaks!! Secret is to do a little at a time and not too many changes at once.

For example, if you find the downthrust isn't as effective as you like; try moving the CG back and/or lowering the wing incidence a little at a time. Also, if you haven't already, a little washout in each wing tip always makes for a better flier. Keep us posted as I'm sure you'll have it dialed in with a handful of additional flights

John
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Old Jun 14, 2013, 01:33 AM
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You've figured out a great deal in just a few flights so keep going on the tweaks!! Secret is to do a little at a time and not too many changes at once.

For example, if you find the downthrust isn't as effective as you like; try moving the CG back and/or lowering the wing incidence a little at a time. Also, if you haven't already, a little washout in each wing tip always makes for a better flier. Keep us posted as I'm sure you'll have it dialed in with a handful of additional flights

John
Thanks John, I appreciate all the tidbits of info to think about. Right now it is all a learning experience for me.
That is the way I am heading now with the tweaks. Only little changes and see how it flies.
I was thinking, instead of adding washout, maybe add stall strips to the inner portion of the wing. They force the wing to stall at the root before the tips. Easy to do and see what happens.
Chris
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Old Jun 17, 2013, 08:00 AM
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Originally Posted by Tomahawk View Post
Thanks John, I appreciate all the tidbits of info to think about. Right now it is all a learning experience for me.
That is the way I am heading now with the tweaks. Only little changes and see how it flies.
I was thinking, instead of adding washout, maybe add stall strips to the inner portion of the wing. They force the wing to stall at the root before the tips. Easy to do and see what happens.
Chris
Haven't ever tried stall strips but, in principle, should do the same thing as washout. Would look good on a plane like this. If you find a layout that works, could you share with the group via pics, etc? I'd like to know how many, what size/placement you find worked, etc

BTW, you built an absolutely gorgeous model!! Love the colors, graphics and lines; just oozes personality from every angle.

Thanks and good luck, John
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