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CRAZY YAK 100

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Old 07-09-2012, 07:19 AM
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DMENACE
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Default CRAZY YAK 100

Hi Guys,

We have been breaking our heads how to make Jimmy┬┤s Yak flight like a normal plane so help in needed from the experts. This is the panorama:

Jimmy bought a 100CC YAK from Nitro Planes, its balance according to CG on the manual, we check the incidence of the wing and stab and looks ok, has a 100cc RCGF for power.

The problem is that in order to flight level at cruise speed it needs a considerable trim up elevator, as soonest you throttle back the plane goes nose up like crazy. Any ideas before making a campfire with the yak.

Thanks in advance,

DMENACE
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Old 07-09-2012, 09:29 AM
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Default RE: CRAZY YAK 100

Check your engine thrust line. If it has too much (any?) down thrust it can act like that. This would be your best bet, however not the only possibility.
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Old 07-09-2012, 10:16 AM
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Default RE: CRAZY YAK 100

If you have checked the incidence of the wing and horiz. stab and those seem to be okay, then I would guess that the engine has too much down thrust / needs more up thrust, or is extremely nose heavy.

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Old 07-09-2012, 12:31 PM
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Default RE: CRAZY YAK 100

Thank u both,

Karol yes we check the incidence of the wings and stab putting the plane level and with the incidence meter check both wing and stab and everything looks ok. 0 deg.
The first flight on the plane had 0 deg thrust on the engine, on the take off run the yak tried to nose dive touching the tip of the prop on the ground. After this and considering that we have the right balance of the plane we decide to raise the engine thrust to 1deg up, and the result are the conditions I describe on my first forum. That┬┤s what is driving us nuts because the elevator is trimmed up to flight level, u cut the engine the bird goes nose up; wouldnt the natural behavior to drop the nose if no power or speed??

I┬┤ll guess we should try increasing the thrust up,

Regards,
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Old 07-09-2012, 01:00 PM
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Default RE: CRAZY YAK 100

The way I always describe thrust line trim problems is as follows.

Imagine that you have the engine of a plane tilted way down. You will get some forward thrust but also get lots of force pulling the nose down. To counteract this you would need lots of up elevator to hold the nose up in level flight (sounding familiar yet?)

So you finally get enough up elevator trim that the plane flies level while under power. What happens when you chop the throttle? The engine is no longer pulling the nose down and all that up trim pulls the nose up.

I'm adjusting a plane with that very problem right now.

Dave
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Old 07-09-2012, 02:46 PM
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Default RE: CRAZY YAK 100


Got it Dave,

We┬┤ll do as you all recommend we are going to adjust the engine thrust up 2 deg and let you guys know the results of the next flight.

Thank you guys

Regards,

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Old 07-10-2012, 05:37 AM
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Default RE: CRAZY YAK 100

That a real cool way of explaining it

Karol
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Old 07-10-2012, 07:20 AM
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Default RE: CRAZY YAK 100

From your original  description of the problem, it sounds like the CG is too far aft. I wouldn't deviate from the manufacturer's specs for incidence and thrust line until the CG is located correctly. The first thing I'd try is to use a very reliable way to make sure the MAC is the correct length and it is located correctly.  Next, locate the CG at 20% of the M.A.C with the incidences and thrust line set to specifications. It should fly a little nose-heavy but be very stable. 

Next, check to see what % of MAC that the Mfgr recommended for a CG location. They or maybe you, have made a mistake there.

With the CG at 20% of MAC, see how it flies. If it is nose heavy, change the CG to 23 or 25 % of MAC and retest.  Above 25%, move the CG aft in 1% increments until you get the performance you want. Caution: be sure to find the neutral point location before you go aft of the 25% of MAC point. For a better description of this, look at www.rcaeronauts.com.
Good luck.
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