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Author Topic: Help with Trimming an Ebinezer  (Read 517 times)
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piecost
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« on: April 30, 2013, 07:07:20 PM »

I am looking for some advice, I know the real answer is to build a more sensible model but I will appreciate any constructive ideas.

I have built a Starr Bumble bee II for the Ebinezer event at Old Wardon (in the UK) taking place on 13 May. The subject was intended to be unconventional and colourful, but has the side effect of not flying

Some details

Span 11"
Mass 15 oz
Motor PAW55
Propeller 6x4
The fuselage is a flat plate 1/4" thick, the wings and tailplane are flat plates 1/8" thick with rounded leading edges.

The CG is about an inch aft of the top wing leading edge (after trimming)
Declage is 3º and down-thrust is 7º, there is no side-thrust

The glide test confirmed the model to be a brick and susceptible to pitch-up on stalling. This was attributed to the CG being too aft and/or the tail being blanked by the wings at high incidence. More lead was added which caused bunting until about 1/4" of packing was added to the lower wing (increasing its incidence). I could not add much incidence to the top wing and this didn't seem to help. Now the glide was straight I decided to go for broke  and tried powered flight - first of all with the propeller reversed, then the correct way round since it needed all  the thrust it could get!

Powered flights exhibited a torque roll to the left, sometimes causing a negative flick. In desperation I added 1/4" thick Gurney strips under the Left hand trailing edge of both wings, but the roll remained. To me, this model looks like it lacks any real stability in any axis and such a far forward CG may be required that it would be impossible to trim. I am considering Expoxying a ply plate to the nose with, say, 7º sidethust to counter the torque. What about a smaller propeller; would this reduce the torque? Do you think that this may help? Any other ideas?
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piecost
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« Reply #1 on: April 30, 2013, 07:09:39 PM »

Forgot to add arrows to picture
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FreeFlightModeller
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« Reply #2 on: April 30, 2013, 07:24:50 PM »

I'll bring my helmet this year!  Wink

I fear that even a hefty dollop of dihedral would not be enough to overcome the torque roll? ... but the model itself is definitely in the spirit of Ebeneezers  Smiley
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« Reply #3 on: April 30, 2013, 07:38:44 PM »

It's a long while since I had anything to do with IC (though I have had plenty of experience with electric) .... but would it be worth trying a 6x3 prop?
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« Reply #4 on: April 30, 2013, 07:40:47 PM »

.... and I just noticed that you have no right thrust. A couple of degrees maybe?
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OZPAF
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« Reply #5 on: April 30, 2013, 09:40:30 PM »

You may be able to get more effective dihedral by replacing the wing end plates with much bigger ones as close to a chord width as possible.
John
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piecost
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« Reply #6 on: May 01, 2013, 02:53:05 AM »

Thanks guys, I was thinking of upping the engine size to 1cc since it appears to be underpowered; I will add some sidethrust as well. Unfortunately the dihedral came out less than intended. The tip fence idea I will keep in my back pocket for now.
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billdennis747
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« Reply #7 on: May 01, 2013, 03:08:05 AM »

11" span, 15oz and rising, no apparent stability, 1cc. I think the most important thing would be not to throw it in a mass-launch crowd at Old Warden.
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piecost
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« Reply #8 on: May 01, 2013, 12:58:52 PM »

Good point Bill; best kept away from bystanders. I think that with the larger engine the best I can aim for is a spiral climb and steep spiral descent at high speed.
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OZPAF
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« Reply #9 on: May 01, 2013, 07:37:44 PM »

If you scare them all off - you may win by default Grin
John
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piecost
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« Reply #10 on: May 05, 2013, 03:38:09 PM »

Its flies !

So, I obtained a 1cc PAW from a friend and was about to fit it and remembered the Cardinal rule of trimming; that is to make one change at a time. Since the change  from the 55 to the larger engine would be permanent I thought I had better have one last go with the original motor. So, I added as many washers as I could under the front bolts and went back to the trimming field. Unusually I did not invite my friends since I was convinced that I would quickly prove that the model was underpowered and untrimmable.

I couldn't get a good enough flick to start the motor with a 5" nylon prop I changed back to the Kavan 6x4 (it even matches the paint scheme). My starting of IC engines is truly painful to behold. Success; the model didn't fly - but it rolled to the right. So I removed those nasty Gurny flaps on both LH wings. Later, I removed the crude packing under the trailing edge of the lower wing and added more lead - if in doubt more nose weight being a good axiom. I tried 45º upward launches - the model stalled, more gentle angles did the same. Eventually I found that a level launch with the motor at max revs to be the best. Launching the model with 1/2 a fuel-line of duel produced a perfect straight and level flight with the model climbing to a giddy 10ft! Worried that it was heading out to the river it eventually turned to the right before landing downwind. Success!!! It seemed very happy and stable. A second flight produced a similar flight, but to the left. Worried that I would loose the model I decided to quite whilst ahead.

I may tempt fate and have another trimming session tomorrow night since the weather is perfect - this time with witnesses. I am uncomfortable with the model not having a definite turn since that is unpredictable - dangerous in public places. I may give it a gurny on the trailing edge of the fuselage/fin. A am concerned since the body/fin is deep compared to the span that it may well cause a roll rather than the intended yaw. But, we will see.


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piecost
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« Reply #11 on: May 05, 2013, 03:58:34 PM »

Some more statistics

It weighs 15.9oz all up
It has 2.5oz of lead noseweight

There is 13º of right thrust and the CG is 1/2 inch aft of the leading edge of the top wing.
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OZPAF
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« Reply #12 on: May 05, 2013, 09:44:43 PM »

Great no Ebinezer will beat this man Grin  Good on you PC
I hadn't noticed before but if the CGis i/2 " behind the LE of the top wing it would still be a fairly aft CG looking at your last photo.
Good luck in the big mass fly.
John
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piecost
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« Reply #13 on: May 06, 2013, 10:53:02 PM »

Thanks for your kind words John. Just when I become overconfident the free flight model plane cuts me down to size. A further trimming session tonight, with witnesses, and taking advantage of perhaps the last of the good weather - the thing didn't fly. Lots of problems in starting the motor and it seemed to be developing less power. The Gurney added to induce left yaw caused terminal left roll as well. So it was removed. I experimented with slightly less nose weight but this caused problems as well, so perhaps your observation on the CG position was correct. I had a short flight with the model underpowered and over elevated and the model displayed gentle wing rock. It seemed under elevated with the original weight back on so some up Gurney was added. After the session I repaired the damage and added the undercarrage. This is expected to cure the slight nose up trim resulting from the last change. I may yet get a final trim session before Sunday and may try a smaller propeller again - if I can start the engine.
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