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Old Feb 27, 2002, 07:55 AM
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HardLander's Avatar
Minneapolis, USA
Joined Oct 2001
245 Posts
HELP! My Low-wing Skimmer 400 won't fly!

I built it stock with a speed 400 motor and folding 6x3 prop. I used an eight cell 500 mAH AR NiCad pack. On my first flights the CG was a bit back of the recommendation and the thing roller-coastered and crashed. I put a big steel washer in front of the motor mount and now CG is a little in front of the recommendation. There's no roller-coastering but there is just about zero rudder response. My rudder travel is almost far enough to hit the elevator in either direction. There's plenty of elevator response. I hand launched this morning and climbed slowly at full throttle. The plane was moving fast so I don't think it's an air speed issue.
The wings look straight. A buddy has the high wing Skimmer 400, which flies beautifully. Its wing is no straighter than mine.
Today's flight was very hairy. The plane almost went into a power line but finally inched its way into a very gradual turn and just missed the line. While trying to bring it down it started thrashing side to side so the wing would be vertical one way and then the other. Somehow I managed to set it down on its wheels on the street and came to work.
It flies steadily enough when going straight but it refuses to turn.
Any ideas on what could be wrong?
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Last edited by HardLander; Feb 27, 2002 at 07:59 AM.
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Old Feb 27, 2002, 08:30 AM
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Alan W's Avatar
NY
Joined May 2000
2,233 Posts
Did you build it with the proper amount of Dihedral in the wing ??

Rudder-only (no aileron) planes usually require a good amount of dihedral for the rudder to be effctive and for rudder response to not just Yaw the plane but also bank it.

If the Dihedral is correct you could try to build another rudder with 50% more area.....increasing the amount behind the vertical stabilizer.


Alan W
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Old Feb 27, 2002, 08:37 AM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,717 Posts
Smells like lack of washout to me. I think what's going on is that the airplane has a nasty stall with the cg back, so you're flying the airplane nose heavy and it doesn't want to turn. (Make sure that you seal the gap between fin and rudder with tape as well- too much gap will decrease control effectiveness incredibly!) Make sure that viewed from the back, the tips of the wing curve upward. This airplane has a long wing- at least 1/8" up relative to the center of the wing should help. Then put the c.g. in the right spot and see what happens.

Sam
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Old Feb 27, 2002, 08:50 AM
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Joined Feb 2002
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I agree with Sam. Check your CG again. Move it forward a little more than it is right now, preferrably by shifting the battery forward. If the plane is stalling (wings going from side to side as you described) the rudder will not work, nor will the ailerons if it has them.

Bernie


Quote:
Originally posted by Megowcoupe
. . . "the airplane has a nasty stall with the cg back, so you're flying the airplane nose heavy and it doesn't want to turn" . . .

Sam
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Old Feb 27, 2002, 09:21 AM
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HardLander's Avatar
Minneapolis, USA
Joined Oct 2001
245 Posts
I'm judging CG by the measurement given in the instructions. The battery is as far forward as possible already, that's why I added the washer in the front. Dihedral is exactly as the plan specified.
The point about gap between vertical stabilizer and rudder is interesting because I used regular plastic pinned hinges instead of the tape hinges recommended and left a good sized gap between stab and rudder. I'll try taking some tape that is partially doubled over on itself to cover the gap. I'll do it so the tape sticks to the stab but slides over the rudder.
I'll also look into the washout.

Thanks for the help!
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Old Feb 27, 2002, 11:34 AM
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Dick Huang's Avatar
Dallas,TX, USA
Joined Mar 1999
834 Posts
Hardlander,
Alan is correct in my opinion. Did your plans tell you to use more dihedral for the low wing? I have been flying elevator/rudder for many years and use 9 to 10 degrees per panel for mid to high wings. Also, your plane is still tail heavy; it was unstable on your first flight. With the aft CG location you should be getting plenty of rudder control. You may need to increase the rudder area as well as the dihedral!
Dick Huang
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Old Feb 27, 2002, 12:34 PM
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Joined Apr 2001
120 Posts
Is there any slop in the control wire

Just a thought, is there any slop in your control rod set-up...i.e. loose conections to the servo arm or to the control surface, or thin guage wire flexing under power loads?
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Old Feb 27, 2002, 12:45 PM
pushing both extremes
Zoom's Avatar
San Jose, California, United States
Joined Oct 2001
567 Posts
Since your plane is a low wing I would expect it should have a few more degrees of dihedral than your friends high wing model. I also think tape on the rudder gap is a good idea, and also look to see that your pushrod isn't flexing or binding under load. You ought to be able to gently hold the rudder in the neutral position, move the stick, and feel the rudder move your fingers back and forth. Next time you fly, whatch what happens when you quickly move the stick from centered to full left or right. If the plane yaws, but does not roll, then you lack sufficient yaw / roll coupling (more dihedral can help this). If it doesn't even yaw, then the rudder is not effective for one reason or another.
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Old Feb 27, 2002, 01:40 PM
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tekochip's Avatar
West Dundee, IL, USA
Joined Jul 2001
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I've got a low wing skimmer too. My vote is for the dihedral and maybe the rudder gap. Mine needs the full rudder travel in order to turn well, but it does turn. I had neglected to build enough dihedral in my Aventura, and it acted just as you described. The effect was more pronounced in a tail wind.

I've had the CG very nose heavy on mine (big cells) and this didn't hurt the handling at all, but it did make landings a little long, so I don't think you have a CG problem.

The stall speed is very sneaky on this plane. Level flight speed is very close to stall speed, so when you slow it way down it seems to stall quite suddenly. This can give you that snapping back and forth that you saw when landing.
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Old Feb 27, 2002, 02:13 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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A sudden stall with a nasty break can often be cured with more washout. Try it, you'll like it.

Sam
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Old Feb 27, 2002, 03:09 PM
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Mr Benn's Avatar
San Francisco Bay Area
Joined Dec 2001
24 Posts
I'd test all the suggestions here. I have a LW Skimmer, and failed to put in the washout for my first flight - it was quite a ride. I put in 3/16 wash out, and it now flies great. Have to keep the speed up for the rudder control, and I have the CG a little further forward than on the plans.
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Old Feb 27, 2002, 04:07 PM
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HardLander's Avatar
Minneapolis, USA
Joined Oct 2001
245 Posts
Has anyone tried putting ailerons on this ship? It sounds like it's unhappy with rudder only.
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Old Feb 27, 2002, 04:22 PM
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Danny Troy's Avatar
Westchester County, NY, USA
Joined Jun 2000
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Ailerons sound like a good idea, but some washout and moving the CG still farther forward should be tried first.
I install ailerons in everything, even if it means adding an untapered strip to the trailing edge of the wing. I'm doing just that now on the new Hobby Lobby Aventura II amphibious.
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Old Feb 27, 2002, 04:47 PM
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San Francisco Bay Area
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Ailerons

Hmmmm.... I sense another modelling project. Ailerons on the LW Skimmer....
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Old Feb 27, 2002, 07:15 PM
pushing both extremes
Zoom's Avatar
San Jose, California, United States
Joined Oct 2001
567 Posts
Like you guys, I love ailerons, but ailerons on this long narrow wing (almost 60 inch span) will add a lot of weight, and wont be very effective. A long time ago I put ailerons on a glider and because of the huge roll inertia, they were very mushy and very draggy. In fact when it looked like I was going to overshoot my landings, I just waggled my ailerons a bit and 'poof' - speed brakes!

- Zoom -
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