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Old Apr 19, 2004, 09:26 AM
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Jacques Flambeau's Avatar
Backwoods Alabama
Joined May 2000
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Self-portrait and a question

Last weekend the winds died down and I got comfortable enough with the SS to put on the Pencam and try some AP. Attached is the Obligatory Self Portrait.

I did have one minor problem that I can't figure out. The plane is trimmed so that it has a nice, level power-off glide. But at full throttle it has the tendency to "raise it's nose", up 20, 30 or more degrees; I have to be careful and keep giving it down elevator to keep it from getting into a stall attitude. This tendency is intermittent and seems to be related to wind speed: if the wind gusts and picks up, the nose raises. I'm talking 5-10mph winds, rather gentle.
The motor is a GWS EPS 400c and the motor mount has the suggested three degrees of down- and right thrust. CG is set at 4-1/8" behind the LE and the wing incidence is "stock".

Not a big problem, just a minor annoyance. Suggestions on what to look at?

--Bill
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Old Apr 19, 2004, 09:43 AM
Bit-Twiddler and Flyer
USA, VA, Chantilly
Joined Apr 2002
1,665 Posts
Bill,

Here's a couple ideas.

1) Move the CG forward.
2) Put a small wedge under the back of the wing to tilt it up at the back a bit. That'll change the angle of attack to be a bit more flat.
3) If you have a computer radio, mix throttle with negative elevator. This will compensate automatically for what you're doing manually.

-- ggunners
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Old Apr 19, 2004, 10:51 AM
Veni, Vidi, Fragoris
djslack's Avatar
Shreveport, LA
Joined Mar 2003
384 Posts
One widely used mod for the SS is to take a piece of the extra foam from around the tail pieces (or something of similar thickness) and place it on the rear wing saddle under the wing, similar to what Ggunners said. This will decrease the angle of incidence of the wing.

The other thing you can do in combination with this is to add a little bit of down thrust by pointing the motor mount down a few degrees. You may also add right thrust with this as well.

By combining those you should be able to get your SS reasonably trimmed for flight.

Chris
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Old Apr 19, 2004, 10:55 AM
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BAR-STOOL's Avatar
Lakeville Minnesota USA
Joined Jul 2002
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Quote:
Originally Posted by Bill Harris
Last weekend the winds died down and I got comfortable enough with the SS to put on the Pencam and try some AP. Attached is the Obligatory Self Portrait.

I did have one minor problem that I can't figure out. The plane is trimmed so that it has a nice, level power-off glide. But at full throttle it has the tendency to "raise it's nose", up 20, 30 or more degrees; I have to be careful and keep giving it down elevator to keep it from getting into a stall attitude. This tendency is intermittent and seems to be related to wind speed: if the wind gusts and picks up, the nose raises. I'm talking 5-10mph winds, rather gentle.
The motor is a GWS EPS 400c and the motor mount has the suggested three degrees of down- and right thrust. CG is set at 4-1/8" behind the LE and the wing incidence is "stock".

Not a big problem, just a minor annoyance. Suggestions on what to look at?

--Bill
Hay Bill

As the planes speed increases so does the lift. When you are glideing the plane is slower and less lift. The more powerfull the motor the faster the plane gos & the more lift and the higher the nose

What you need is more down thrust.

BAR-STOOL
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Old Apr 19, 2004, 11:21 AM
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Jacques Flambeau's Avatar
Backwoods Alabama
Joined May 2000
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Thanks folks, this is a good starting point. I thought that 3 degrees would have been enough downthrust, but evidently I was too conservative. I'll add more on the next incarnation of this plane, but for the time being I'll try to mix some down elevator with the throttle. I tried shimming the rear of the wing up 1/8" to decrease the incidence, but that didn't seem to do much; I'll fiddle with it more. And once I get it trimmed out properly I can twiddle with the balance.

One major problem that I corrected was a warp in the wing: when mounted on the plane, the right wing TE was 1/2" lower than the left TE, which caused a strong left-turn tendency. I corrected that by putting a 1/16" shim under the right edge of the rear wing mount, which counter-twisted the wing back straight whne the wing was rubber-banded on. That was a ham-fisted approach, but I didn't see a way to "twist and shrink" the foam wing straight.

Thanks,

--Bill
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Old Apr 19, 2004, 11:29 AM
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BAR-STOOL's Avatar
Lakeville Minnesota USA
Joined Jul 2002
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Quote:
Originally Posted by Bill Harris
Thanks folks, this is a good starting point. I thought that 3 degrees would have been enough downthrust, but evidently I was too conservative. I'll add more on the next incarnation of this plane, but for the time being I'll try to mix some down elevator with the throttle. I tried shimming the rear of the wing up 1/8" to decrease the incidence, but that didn't seem to do much; I'll fiddle with it more. And once I get it trimmed out properly I can twiddle with the balance.

One major problem that I corrected was a warp in the wing: when mounted on the plane, the right wing TE was 1/2" lower than the left TE, which caused a strong left-turn tendency. I corrected that by putting a 1/16" shim under the right edge of the rear wing mount, which counter-twisted the wing back straight whne the wing was rubber-banded on. That was a ham-fisted approach, but I didn't see a way to "twist and shrink" the foam wing straight.

Thanks,

--Bill
On one of my SS with a hacker B-15 I have 8 degrees down thrust

BAR-STOOL
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Old Apr 19, 2004, 11:44 AM
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Harm Botman's Avatar
Hillegom, Netherlands
Joined Sep 2001
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and nice portrait by the way.....
HB
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Old Apr 19, 2004, 12:11 PM
I'm big in Japan
Ducman69's Avatar
Houston, TX
Joined Feb 2004
439 Posts
Quote:
2) Put a small wedge under the back of the wing to tilt it up at the back a bit. That'll change the angle of attack to be a bit more flat.
There was a guy (I suck with names) here who had recommended that and then retracted it after finding that sometimes in a dive the natural tendency of the plane to nose up changed to nosing down... will have to try and search and find the exact quote. Apparently it worked great flying stock, but with the added weight of the camera equipment it felt unsafe in certain conditions.

I would think that this is just a normal response from a properly setup plane... with a gust of wind the planes airspeed increases adding lift... this pushes the plane up. The adjusting for slightly nose heavy sounds like a good idea though.
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Old Apr 19, 2004, 12:49 PM
Did You Hear That?
wattnoise's Avatar
Tucson, Arizona
Joined Aug 2003
2,153 Posts
I use the transmitters elevator trim to compensate... I've found that if I have it trimmed for normal climbout under power, I can smoothly and simultaneously reduce the power to zero while bringing the trim to full up on the elevator and the airplane will be in a perfect glide angle to the camera mount (about 10 degrees nose down)... Then i just use the rudder to stear it around the sky... When I power back up, I smoothly and simultaneously increase power and reduce elevator trim to level or normal climbing attitude for the power setting...

Kevin
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Old Apr 19, 2004, 12:55 PM
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Jacques Flambeau's Avatar
Backwoods Alabama
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"sometimes in a dive the natural tendency of the plane to nose up changed to nosing down..."

This "dive test" is a standard test for trimming out and balancing a sailplane. It's a compromise: nose-heavy adds stability, but tail-heavy makes for responsive handling.

--Bill
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Old Apr 20, 2004, 12:01 AM
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Warm-Sunny-no-Wind, Az.
Joined Dec 2003
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I usuallly go vertical to climb to AP altitude anyway.

I consider this problem to be a feature. :-) 30 seconds of vertical is good for 500 ft. RichieRich's 300 watt SS setup is twice as fast as mine straight up.

Gotta love the big brushless power. Middle aged testosterone !!

Rgds,

Mike
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Old Apr 20, 2004, 07:03 AM
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jgleigh's Avatar
Manassas, Virginia, United States
Joined May 2002
390 Posts
"sometimes in a dive the natural tendency of the plane to nose up changed to nosing down..."

Highly undercambered wings seem to be more susceptible to this effect - nose up at moderate speeds, changing to nose down at higher speeds. I think it has something to do with center of pressure movement at different angles of attack.
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Old Apr 20, 2004, 07:49 AM
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Jacques Flambeau's Avatar
Backwoods Alabama
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Ah, yes, the extreme nose-up attitude could be a big plus with a 300W BL where is is enough _oompf_ to go vertical; with a stock 70W 400 can this gets into a stall.

I did a little testing after work yesterday and found that shimming the TE of the wing _did_ decrease the pitch up tendency on power but the plane also required much more up-trim on the elevator to give a good glide. We're looking at two issudes here, IMO: the flight character under power and while gliding. The behavior while gliding is important since that is where photos are taken.

Data point: trim the SS to give a good glide with the elevator and decalage (wing incidence relative to the stab), control the power-on attitude using motor down-thrust. Three degrees ain't enough, think 6-10 degrees.

Since my motor mount piece is fixed to the fuse tube I'm not going to remove it and revise it to the larger downthrust. For the time being, I'll see how mixing down-elevator with the throttle works.
I'll do more testing....

--Bill
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Old Apr 20, 2004, 08:17 AM
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jlballou's Avatar
SF Bay Area
Joined Jan 2003
909 Posts
Bill - FWIW, I agree that glide characteristics are the most important part of preparing your platform for AP. I would never change the incidence angle away from the setting that gives me a good, smooth glide. The two things that you have to work with are thrust angle and elevator mixing. I personally would try to change the thrust angle, but a close second is the elevator mixing.

The reason it's close is that when you mix in down elevator, that turns the stab/elevator combo into an undercambered airfol, which does provide some lift, which is what you want during climb-out.

John
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