artandscience

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

wind junkie

My Feedback: (1)

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.

stdun

Have you checked your servo travel direction? Be sure up elevator is really up.

artandscience

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).

Which_way_is_up

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

Cajun

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.

artandscience

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).

Demon

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

Which_way_is_up

Good visual of what I was trying to explain.

Montague

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.

Demon

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

D

Which_way_is_up

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! [X(]

Which_way_is_up

-deleted-

1Jimbo

My Feedback: (2)

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.

artandscience

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??

Demon

LOL Roy!

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

The elevon angle is very critical. If your's doesn't match the diagram, I highly recommend that you adjust it so it does.

Perhaps. With the plane set up as recommended here in this thread, it should glide about 20' on a mild toss without the engine running at all.

I would. Since the 9x6 has a steeper pitch than the 9x4, the 9x6 WILL turn slower RPMs. If the 9x4 ran at slower RPMs, something else was wrong at the time.

HTH,
D

Eric.Henderson

A correctly trimed Gremlin will hand launch with an easy forward push.

I just stumbled on this topic and was surprized to see the new dates on the posts.

The first Gremlin flew in 1990 and was published in December 1991 RCM...

I stil have four of five of then operational... Glad you are all having fun with them.

Regards,

Evil-Eric.

ysrcflyer

Eric:
I still keep the original copy of RCM on my workbench. We were going to get into combat at our club at one time and one of our members cut about 20 sets of cores. There was only 3 planes that eventually made it in the air. I am the only one that still flies them. I love the rock solid feel that you get from flying it. By the way I built one with a YS 45 for power. It sure is fun coming in low across the runway at full tilt. Just like the article says, a ducted fan jet!

Thank you for the fun that I have had for the last 10 yrs!

yankee samurai

My Feedback: (175)

ysrcflyer, The Gremlin in the picture, what is the wing covered with?[8D]

ysrcflyer

I sheeted the wing with 1/16 balsa and then covered it with Monokote. This one is only a Sunday flyer.

yankee samurai

My Feedback: (175)

Thanks for the info, it looks great.

NightFlight

Beautiful plane ysrcflyer!


My first gremlin I just finished last week. My plan is to learn on it until the combat season starts up again next year. I only have a .46 which I pulled from my trainer, so it's a bit fast ;-) Jumping from to a trainer to a combat gremlin was a hoot! I took it out for it's maiden flight by myself too... That was one wild ride! New radio, new plane and it was the first time I "crissend" a plane myself. There were 30kph winds, but what the hell I thought. It initially wanted to dive as well. I was so out of sorts I could not even glance at the new radio long enough to trim it out. I manged to compensate for the diving and the wind pushing it all around. Came in with a nice gentle flop. I found after that first flight I was all jitters for a bit. I can tell you, that was the most fun I'd ever had. It was like learning to swim by just jumping in.

I called it a day. The day after I took it out after making adjustments. I managed to get really comfortable with it, doing low passes at top speed coming out of a dive. The fuse gave way at one of the bolts and I lost control. Cut the throttle and managed to keep it in the air long enough to loose some speed before doing a nose plant. I guess the PVC pipe already broken; took the brunt of the impact, as that's all that appears to have broken.

Now I just can't find 2.5" replacement PVC pipe. I've looked everywhere.
It has to be a common question.

- NightFlight.

MikeSell

My Feedback: (2)

I bought my first Gremlin at a swap for $30 with RTF with a slight dent in the wing tip. The OS.40 H ran fine but I pulled the old slider elevon system and programmed my radio. I worked as a spotter for a friend for an event demo then started piloting it for myself. It can fly so well, even hands off but put in some throw and its lotsa fun.
One evening I was flying with a 10x6 and a new carb. Performance was up and it was flying quite a bit faster than normal. I thrashed it unmercifully and then I heard it sort of a buzz. Not sure it was my plane or what the source was I slowed and didn't hear it any more. I flew for a while longer put it into a 720 degree @ sec roll and lost all control. The plane was not badly damaged considering, but I saw the problem. I had snapped off the elevons at the control horns. My new elevons and the next two I'm building will have substantial fiberglass reinforcing to hold the wood together.

Eric.Henderson

Glad to see that you are all having fun.

I'll post some pictures of the "surviors" when I get a moment.

artandscience

Well, at the suggestion of my instructor I put a set of landing gear on the Gremlin and with a long ROG finally got it up.

It flies quite well when in the air (low rates). Very stable and predictable. (So I guess I got the CG right!).

But the motor seems woefully underpowered, even with a 9x4 prop. So I'm planning now on putting a newer motor
in it. My choices seem to be: Thunder Tiger, Magnum, or Norvel.

As far as these go (all in my price range), I am soliciting your input as to:

1) which is most powerful out of the box,
2) which can be modified with easily available pipes to perform better,
3) which has the best longevity

Thanks much for the input guys. With your luck, I'll actually manage to fly a combat before the winter
monsoon season starts!