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Old Dec 09, 2009, 08:12 AM
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Si Goodchild's Avatar
Lincolnshire, UK
Joined Oct 2009
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Any Twin-Engine Gotchas?

Guys,

I've started the build on my first ever scratch-built jet: an SR71. I intend to post some pics later, but before I go too far down this road can you advise me if there are any particular cautions I should take with a twin-engin model of this nature? Having looked at previous threads, I've designed the power system with both batteries working together in parallel and then running the 2 ESCs from that single power source, thus giving power to both motors if one battery quits. The ESCs don't have onboard BECs so I'll be using an external one with a separate 2s 850mah lipo rx battery for the early flights.

Are there any other aerodynamic, structural, stability or electrical issues I should be aware of on this, my first ever twin engine project?

Any advise greatfully appreciated.

Si G

PS - I'm not planing to fit working rudders to the SR71, but am a little concerned about any assymetry in the delivery of thrust. Perhaps I should run the 2 motors off separate rx channels and use a mix in the rudder channel on my DX7 to generate a small differential in thrust?
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Old Dec 09, 2009, 08:24 AM
Checking CG is for NERDS!
Smokescreen38's Avatar
Joined Jun 2005
5,347 Posts
Si,

Test your low voltage cutoffs! With most twins, it's really important to land well before you hit it in flight. No matter how close they are on the bench, they will almost certainly hit LVCs at different points in flight, which will probably result in a spin...which could be the death of your model if you're not high enough to recover.

Good luck! Let us know how you make out!

Also, just about every SR-71 that I have seen rocks side to side a little (in the roll axis)...but I'm not smart enough to know why. Let us know if yours does this... it would be interesting if you figure a way to avoid it. If yours does rock, I suspect a gyro might be a band-aid solution, though it would probably give your servos a work-out.
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Old Dec 09, 2009, 08:34 AM
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Si Goodchild's Avatar
Lincolnshire, UK
Joined Oct 2009
522 Posts
Cheers for the LVC tip - exactly the sort of info I'm after. As for the roll axis: I've already got the Gyros sat of the shelf ready to go! I'll put one in the roll axis as a minimum (assuming ailerons run off a y-lead) and I have a second gyro ready to go if I choose to split out the ailerons and employ a more complex mixing of the 4 rear control surfaces.

The inspiration for the gyro came from rcsuperpowers.com when Dave Powers was experimenting with an 70mm edf F18 (distributed by CML here in the UK) in high alpha configuration. He put a gyro on each of the 3 axes (if I remember correctly) and the model behaved extremely well at high alpha: it looked rock solid in the sky and the control surfaces were working really hard and fast to maintain position.

Si G
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Old Dec 09, 2009, 08:45 AM
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fh64_lucky's Avatar
United States, TX, Garland
Joined May 2005
3,333 Posts
I dissable low voltage cutoffs in the twin setup,I would rather take chance of over draining a battery than crash a jet over it.You can most of the time tell when your batteries are getting low on power and land before they are hurt,or just fly with a timer,working your way up to limit of battery life little by little.After you recharge,you can keep track of how much you put back into them.I have never hurt a battery myself doing this,but I do not fly while nitro planes fly,I like to listen to my stuff as I fly it.
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Old Dec 09, 2009, 08:50 AM
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Si Goodchild's Avatar
Lincolnshire, UK
Joined Oct 2009
522 Posts
Agree 100%: this is exactly what I do. I've never hit LVC in flight yet. Overtemp definitely, but never LVC
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Old Dec 10, 2009, 04:36 AM
dro56595@bigpo's Avatar
Australia, NSW, Williamtown RAAF
Joined Aug 2008
2,404 Posts
I lost a fan blade from my twin 90mm MiG-29 a few weeks ago. Not a lot of assymetry there as the motors are reasonably close together. I kept it at partial power to reduce the thrust differential (and yawing moment) - just enough to get it back on the ground. No rudder.

Just a warning that the electrics are not the only things that can fail.

I'll also be building a HET Me-262 over the holidays, so I'm all ears for tips too!!

Have fun with the build.
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