Gremlin maiden flight problem

Gremlin maiden flight problem
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Gremlin maiden flight problem - 9/6/2004 3:01:20 AM

artandscience

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Joined: 8/2/2004
From: Camano Island, WA, USA
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Ok.. so I had my instructor take (or try to take up) my Gremlin today. It balances properly at 1.7" back from the LE (upside down, tank empty). I've got a Fox .29RC with a 9x4 Master Airscrew prop.

I launched it underhand, it travelled three or so meters and then nosed down into the grass at about a 30-40 degree angle. Twice. Breaking props both times.

I'm at a loss as to why. I set the elevons to 3/8" travel up and down. With this sort of behavior, is it likely that the CoG is too far forward? Or maybe I don't have enough elevator travel. Or maybe my motor is too puny?

thanks for any help,
Stefan

Post #: 1

RE: Gremlin maiden flight problem - 9/6/2004 3:13:49 PM

wind junkie

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Joined: 11/16/2002
From: N. Syracuse, NY, USA
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Stefan,

It sounds like your motor is a bit weak. What is your Fox turning with that 9x4 prop? Also, what prop is it? How heavy is your plane?

Be sure your elevator trim is level! (just checking, I'm sure you know this already). When I say "level", I actually mean just slightly up (1/16 to 1/8"

from absolute neutral. It's scary how little elevator travel is actually needed to make the plane turn, so setting up your elevator trim won't be easy until it's airborne.

From my experience, it really helps to run when throwing a Gremlin. Have a helper do this for the first flight and tell him to throw it AWAY and UP SLIGHTLY. A 30 degree angle is good. Once released, the plane will still need a little space to gain speed, but it will be about a second until you can start flying it.

With higher power setups, engines turn a 9x4 prop anywhere from 18K to 21K on the ground, and launching a 2.25 lb plane is simply a matter of letting go. With less power and / or more weight, you should run to launch, and be extra careful to manage that prescious altitude until speed is gained.

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Post #: 2

RE: Gremlin maiden flight problem - 9/6/2004 3:44:21 PM

stdun

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From: Fort Bragg, CA, USA
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Have you checked your servo travel direction? Be sure up elevator is really up.

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Post #: 3

RE: Gremlin maiden flight problem - 9/8/2004 6:25:35 AM

artandscience

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From: Camano Island, WA, USA
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I tried several different props this evening. The best turns out to be the one recommended by Fox -- a 9x6. It turns at between 11100 and 11500rpm. (Lord knows how you folks get better than 18,000rpm out of a .25). The motor is still in the break-in period (probably only a dozen heat cycles so far) but I don't see it
gaining five or six thousand rpm without a replacement of crankcase and crank. :>

The plane weighs in at about 3lb 2oz (empty tank). Balances between 1.5 and 1.75" back from the leading edge (using the string method).

I have a TT GP .25 and an HB .25 as well. I suspect that if I cannot launch it tomorrow with the new prop I'll have to change motors.

And, yes, the control surfaces moved in the right direction (up is up like a regular plane).

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Post #: 4

RE: Gremlin maiden flight problem - 9/8/2004 12:29:07 PM

Which_way_is_up

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From: Carrollton, TX, USA
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A couple of observations, a 9X4 Master Air Screw prop gives me between 16k and 16.5k on an OS 25FX or Mag 25XL. With a mousse can muffler or commercial tuned muffler like a Nelson or Jett, I get 18k to 19k. At 11k you're going to have really shuck it. "We need more power Scotty"!

Next, you must be more precise on your CG, with a flying wing an 1/8" can make a big difference. What is the CG suppose to be? Mark the bottom of the wing and use the edge of a metal ruler to balance the plane. You can either hold the ruler or put in in a vise. Can't just get the CG kind of close.

Next, the eleavons need about 1/16" of up or reflex. Depending on the airfoil that may mean that the control surface is flush with the bottom of the wing or slightly raised. Lack of just a little reflex can make a big difference.

Good luck,

Roy

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Post #: 5

RE: Gremlin maiden flight problem - 9/8/2004 2:03:47 PM

Cajun

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From: Nacogdoches, Tx
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Roy hit it on the head. A MA 9x4 spinning 11K is no power for a 3+ lb. ship.

Put some serious power on the nose and reflex the elevons up more than needed until you get it trimmed and you'll be happy with the Gremlin.

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Post #: 6

RE: Gremlin maiden flight problem - 9/8/2004 6:55:34 PM

artandscience

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From: Camano Island, WA, USA
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I was "afeared" of that (that the motor was too weak).

I have the plane balanced precisely at 1.75" back from the leading edge - what the designer recommends.

But I will check the elevons are "up" a bit.

Maybe I should just stick a .40 in it (I have a .40 FSR lying about).

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Post #: 7

RE: Gremlin maiden flight problem - 9/8/2004 10:36:06 PM

Demon

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From: Ledyard, CT, USA
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quote:

ORIGINAL: artandscience

Maybe I should just stick a .40 in it (I have a .40 FSR lying about).

That .40FSR should do the trick. With a 10x4 prop, it will give you all the pull you need.

Also, with the CG at 1.75", it will be very stable about the pitch axis. Set the elevons to be level with the bottom of the wing (see below), and it should fly right out of your hand.

HTH.
D

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Post #: 8

RE: Gremlin maiden flight problem - 9/8/2004 10:45:43 PM

Which_way_is_up

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From: Carrollton, TX, USA
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Good visual of what I was trying to explain.

< Message edited by Demon -- 9/9/2004 9:54:55 PM >

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Post #: 9

RE: Gremlin maiden flight problem - 9/9/2004 12:08:38 AM

Montague

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From: Laurel, MD,
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You said you have a 9x6 prop on there now. Before you go with another engine, try a 9x4 on the same engine. You may not get the power we get out of the more common combat motors, but you should turn more than what you are getting with the 9x6, and the lower pitch will help with the launch because you'll get much better acceleration. With a 6" pitch, you're going to need a hard throw to get enough airspeed, even with a hotter motor.

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Post #: 10

RE: Gremlin maiden flight problem - 9/9/2004 10:00:05 PM

Demon

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Roy,
That "edited by" message was my attempt to get your signiture pic to show (unsuccessfuly). Hope ya don't mind.

D

< Message edited by Demon -- 9/9/2004 10:01:44 PM >

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Post #: 11

RE: Gremlin maiden flight problem - 9/10/2004 1:53:02 AM

Which_way_is_up

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From: Carrollton, TX, USA
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quote:

ORIGINAL: Demon

Roy,
That "edited by" message was my attempt to get your signiture pic to show (unsuccessfuly). Hope ya don't mind.

D

Not a problem, in fact I'm glad you tried! At least I know that the problem wasn't with me not understanding the setup!

Is RCU impotent, can't get that graphic up!

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Post #: 12

RE: Gremlin maiden flight problem - 9/10/2004 2:02:12 AM

Which_way_is_up

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From: Carrollton, TX, USA
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-deleted-

< Message edited by Which_way_is_up -- 9/10/2004 2:04:25 AM >

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Post #: 13

RE: Gremlin maiden flight problem - 9/10/2004 3:07:42 PM

1Jimbo

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From: Sulphur, LA, USA
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I've been flying Gremlins for 10 years and I have found that a .40 or .46 bearing engine is too much for that plane. They are too heavy and have too much power. These are the engines I've had the best luck with. A strong .25, a .32 or .36 and a plain bearing .40 The Gremlin will not turn like it's should with a heavy engine. If you are not using it for combat I would recommend the TT.40GP with a 10x4 prop.

(in reply to Which_way_is_up)

Post #: 14

RE: Gremlin maiden flight problem - 9/10/2004 5:10:25 PM

artandscience

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From: Camano Island, WA, USA
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I wasn't going to use it for combat until I was a better flier. (I can land my trainer pretty nicely but that doesn't yet
make me a good flier). So you would recommend the 10x4 on an OS .40 FSR as well?

I'm going to make up a second fuse with the .40 (to take to the field) but try the Fox .29 one more time. Looking at
Demon's picture, I don't have sufficient "up" in my elevators at rest. Perhaps its very sensitive. Perhaps I just need
to chuck it harder.

It turns fastest with the 9x6 MAS. This was the prop recommended by Fox (or a 10x5). Should I go back and try the
9x4 MAS again??

(in reply to 1Jimbo)

Post #: 15

RE: Gremlin maiden flight problem - 9/10/2004 9:56:50 PM

Demon

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From: Ledyard, CT, USA
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LOL Roy!

quote:

ORIGINAL: artandscience
So you would recommend the 10x4 on an OS .40 FSR as well?

Yes. If you feel it is winding up too much (RPMs too high) with a 10x4, switch to an 11x4.

quote:

ORIGINAL: artandscience
Looking at Demon's picture, I don't have sufficient "u