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U-control thrust-offset

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U-control thrust-offset

Post  roddie on Fri Jun 15, 2018 8:01 pm

I've been building a U-control 1/2"-scale (1/2A) P-38 Lightning. The model will be powered by identically-configured Cox .049 reed-valve engines, equipped with horseshoe backplates/carbs. The engines are basically stock product-engines which draw fuel from external tanks.

The model is my own design.. and has not been test-flown yet. There are no offset-angles built-into the fins/rudders.. and the radial-mount engine-firewalls were built with zero-offset.

There "is" a slight amount of positive-offset in the "yaw-axis" via the lead-out guide-placement.. but I wanted a "trimming-tool".. just in case the model exhibits any tendencies to roll-in if it gets breezy..  

I had some wedges of composite-material..  so I cut two of them to match my model's firewall-size. The angle is very slight.. maybe 1 or 2 degrees.

I drilled an oversize pattern of the horseshoe backplate's mounting-holes.. which should easily facilitate installing the wedges.




Packs of wedges (wood or composite) can be found in the lumber-departments of home-centers. They are quite handy when you need them...
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Re: U-control thrust-offset

Post  batjac on Fri Jun 15, 2018 9:54 pm

Roddie,

When I scratch build my twin a couple of years ago, I didn't put any right thrust on the inboard engine, and I had about 3 degrees out thrust on the outboard engine.  With this setup, the plane flew very well when the inboard engine quit.  You can see that at about the 1 minute mark on the below video:



These were two Bee engines, but I can't see that it would be that different with product engines.

The Near-Sighted Mark
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Re: U-control thrust-offset

Post  getback on Sat Jun 16, 2018 10:26 am

That Bad Boy was carrying the mail nicely Mark!! Flying
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Re: U-control thrust-offset

Post  roddie on Sat Jun 16, 2018 4:45 pm

Thanks for the tip and posting the vid Mark. I don't remember seeing that vid before. Do you remember the line-length you used?

The photos I have of the BTC 4T show two R/H propellers (safety-yellow) installed. Are they what was used in the flight video?
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Re: U-control thrust-offset

Post  batjac on Sat Jun 16, 2018 9:15 pm

roddie wrote:Thanks for the tip and posting the vid Mark. I don't remember seeing that vid before. Do you remember the line-length you used?

The photos I have of the BTC 4T show two R/H propellers (safety-yellow) installed. Are they what was used in the flight video?

I'm pretty sure it was on 35' of 10 pound test Spectra.  It was indeed running the yellow 5x3 rubber duckies (https://www.youtube.com/watch?v=f5d8pVg3Qtg).  I really love those props.  I thought about using a left and a right hand prop, but just stayed with the standard props.


getback wrote:That Bad Boy was carrying the mail nicely Mark!! Flying

Eric, I was quite surprised at the time how fast the plane flew.  It's a neat plane, and I continually think about going back to do some mods to it to make it a pseudo-German WWII bomber.

The Roomy Mark
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Re: U-control thrust-offset

Post  getback on Mon Jun 18, 2018 10:54 am

I think if i were to build one i would put it on 40' lines to slow down the excitement Very Happy Babe Bee .049 Babe Bee .049
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Re: U-control thrust-offset

Post  RknRusty on Mon Jun 18, 2018 11:15 am

That was an impressive flight with two good engine runs. It went from 3 to 4 second laps when the first engine quit.
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Re: U-control thrust-offset

Post  batjac on Mon Jun 18, 2018 8:03 pm

RknRusty wrote:That was an impressive flight with two good engine runs. It went from 3 to 4 second laps when the first engine quit.
Rusty

Yeah, It would have been a longer flight, but I found out that if I don't get the engines synced just right, the harmonics cause the elevator to flutter horribly. I thought the plane was going to shake itself apart. I spent a good minute-minute and a half getting the engines synced. I think the next time that I fly it I'll put some kind of clip between the elevator and rudder to hold the elevator still until I release the stooge pin. That's what I get for making such a frictionless control system.

Shaky Mark
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Re: U-control thrust-offset

Post  roddie on Mon Jun 18, 2018 8:48 pm

batjac wrote:
RknRusty wrote:That was an impressive flight with two good engine runs. It went from 3 to 4 second laps when the first engine quit.
Rusty

Yeah,  It would have been a longer flight, but I found out that if I don't get the engines synced just right, the harmonics cause the elevator to flutter horribly.  I thought the plane was going to shake itself apart.  I spent a good minute-minute and a half getting the engines synced.  I think the next time that I fly it I'll put some kind of clip between the elevator and rudder to hold the elevator still until I release the stooge pin.  That's what I get for making such a frictionless control system.

Shaky Mark

I wonder if having a twin-boom/tail might lessen that phenomenon? I planned on running L/H propellers on both engines. I have 5" x 4" left-hand propellers planned for running.. and can start the engines "left-hand" with my electric-starter.

I think about thrust-offset vs. rudder-offset.. and don't really understand how one is that-much different than the other...
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