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Old May 30, 2011, 01:11 PM
Just a plane Junkie
Portland, Oregon
Joined May 2002
461 Posts
Help!
DLG flies parallel to ground on hard launches?!

When I launch my LightSpeed III (SG2 clone) it flies parallel to the ground for about 30-50 feet before abruptly pitching up to a 45 - 80 degree climb out (depending on how long I hold down my preset).

This causes two problems:

1) Danger to other fliers - Anyone within 50 feet in front of my launch could get hit. Obviously everyone has this problem to some extent but most DLG's I have seen are above head-height within 10 feet of release.

2) If I forget to use my launch preset, or my finger slips off during windup, it may result in instant vaporization of my plane as it plunges into the ground at full speed.


*) The harder I throw, the further the plane flies before ascending. I throw to maybe 150' max.

*) I use up elevator for my launch preset on a spring loaded switch. I hold the switch throughout my entire launch so it's constantly applying up elevator (about 1/8").

*) When the plane does pitch up, it does so extremely abruptly, as if a switch was being used. Using more elevator preset does not alleviate the problem - it just make the change in plane pitch more abrupt and usually results in a 90 degree or looping launch.

*) The plane is trimmed for level flight and pulls up slightly in a dive test. It requires very little down elevator to fly levelly when inverted.

*) There's no slop or play on the full-flying stabilizer. I recently re-did the linkage (push-pull with stainless wire in teflon tubes) and made it absolutely tight and stiff.

*) I replaced the horizontal stab servo (HS-50) with a Hyperion DS-09 digital servo when I redid the linkages.

*) I'm using a 4-servo wing and have the flaps / ailerons set even with the wing root at launch (speed setting - no reflex).


So, is this normal? Is there something I'm doing wrong?

I'm considering adding some camber to my launch preset, but I want to cure the 'problem' if there is one, rather than alleviate the symptom.
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Old May 30, 2011, 01:54 PM
Do chickens soar?
VH-ASC's Avatar
Snoqualmie, Washington USA
Joined May 2005
108 Posts
CG too far aft.
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Old May 30, 2011, 01:57 PM
Registered User
San Diego
Joined Aug 2004
2,315 Posts
Is your pushrod fully supported along its entire length? Is it set up to push for up? It sounds like it could be buckling elastically somewhere under high load. When the load decreases it pops back to its proper shape giving you the intended elevator throw.

This is easy to check for. Turn on the launch preset and press down on the elevator to simulate the air loads. Use at least several ounces of force. If the elevator pops down you know the pushrod is buckling. So even though the linkage may feel tight and stiff, under load it may not be . . .

Another possibility is your boom is flexing under the launch loads negating the effect of your launch preset.

FWIW
Tom
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Old May 30, 2011, 02:20 PM
Registered User
United States, SC, Mt Pleasant
Joined Nov 2003
1,209 Posts
Best bet is just to come up to our June contest and have me, or one of the other gu us check it out for you!

James
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Old May 30, 2011, 05:00 PM
Registered User
United States, MA, Waltham
Joined Dec 2001
7,133 Posts
It sure sounds like SOMETHING is flexing or moving. I have seen behavior somewhat like this in the old days with a hlg that had an arrow shaft for a tailboom.

The suddenness of the pitch up suggests to me that Tom is on the right track.
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Old May 30, 2011, 06:24 PM
ground penetration specialist
Nathan Schmoekel's Avatar
USA, MI, Grand Rapids
Joined Apr 2007
1,819 Posts
Quote:
Originally Posted by lincoln View Post
It sure sounds like SOMETHING is flexing or moving. I have seen behavior somewhat like this in the old days with a hlg that had an arrow shaft for a tailboom.

The suddenness of the pitch up suggests to me that Tom is on the right track.
I also have to agree with their assessment. You are looking for a flex gremlin somewhere. Hold the elevator preset and push the elevator down hard with your hand to find the problem.....if it breaks it breaks....better on the ground than when it's aimed at someones head.

Boom flex may also contribute, or a loose/ flexy servo mount or tray.....your just going to have to torture it by hand a bit and make it tell you the issue
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Old May 30, 2011, 06:53 PM
Yes..ok..maybe..lol.....
Tucson Avra Valley, Arizona, United States
Joined Jul 2004
5,729 Posts
Quote:
Originally Posted by kiesling View Post
is your pushrod fully supported along its entire length? Is it set up to push for up? It sounds like it could be buckling elastically somewhere under high load. When the load decreases it pops back to its proper shape giving you the intended elevator throw.

This is easy to check for. Turn on the launch preset and press down on the elevator to simulate the air loads. Use at least several ounces of force. If the elevator pops down you know the pushrod is buckling. So even though the linkage may feel tight and stiff, under load it may not be . . .

Another possibility is your boom is flexing under the launch loads negating the effect of your launch preset.

Fwiw
tom
+1...........
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Old May 30, 2011, 07:04 PM
Thermal Naked!
Hossfly72's Avatar
United States, AL, Mobile
Joined Dec 2007
1,670 Posts
Quote:
(depending on how long I hold down my preset).
What's it do if you don't use a launch preset? You said it zooms up after you let go, right? Just put everything in neutral trim and try it.
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Old May 30, 2011, 07:54 PM
Jim C
ShadowFalken's Avatar
Joined Sep 2009
1,473 Posts
Tom K is spot on. I had the exact issue with a LHIII and the push rod sheath had popped loose near the servo. ASK sends steel for the push rods and they are thin. MUST be supported.
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Old May 30, 2011, 07:57 PM
Walter Roos
ROOSWH's Avatar
Canton, Ga.
Joined Aug 2004
3,196 Posts
Video of your launch would help.

Like the others posting here, I would first look at my Teflon Tubing and Servo mounting.


Many folks think that they throw flat to the horizon, but most don't, IMO. A close look at one of America's best launchers, Oleg, will show that he releases at a pretty high nose up angle most ? times. If you really release flat, you will need a bunch of up preset to turn up to near vertical, especially if you are neutral to tail heavy. That can fold a wing with a really hard launch. My opinion only, but I have folded / delaminated several wings lately and in all cases I had just added a bunch of up preset to do a straight up, overhead to downwind roll. That manoeuvre has been dropped from my play book.

If you think my opinon has some merit, then start off easy with getting the nose up on release, at the same time do not allow the wing to come too close to contacting the ground on the spin. Higher angle on release means the DLG is a bit higher on the backswing, dives down a bit then turns up before release. I'll try to find my video of Oleg's launch and post it here for ya.

All IMO Only, YMMV,

walt
PS Ailerons even with the wing root is 2 degrees of Reflex for almost all Bagged DLG wings, also known as Launch and Speed mode.

Oleg's Launch. I'm not saying that you should copy this launch, as it seems a bit extreme, but no way anyone can say that's a flat with the horizon launch.
If you get your glider that high on the backswing you will be in dangerous territory.


Oleg DLG Launch Slow Motion (1 min 10 sec)
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Last edited by ROOSWH; May 30, 2011 at 08:35 PM.
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Old May 30, 2011, 09:30 PM
Come out swinging
sporter's Avatar
San Diego
Joined Mar 2005
2,661 Posts
Totally dangerous.

I think Tom's assessment is correct.

If your CG was aft, and all the controls were rigid, it probably wouldn't track out 30ft before pitching up, it would just keep tracking straight or even tuck.

Maybe try to add a little more up-elevator preset...and hold it for a shorter amount of time, or until it pitches to about 80 degrees or so, then let off. Or fix the loose controls problem.


Sean
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Old May 31, 2011, 06:51 AM
garry thrall
leavenworth ks
Joined Jul 2004
52 Posts
Are you a right hander or lefty. It sounds like the pushrod is on the wrong side to me.
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Old May 31, 2011, 06:57 AM
Registered User
JonyEpsilon's Avatar
London, UK
Joined Jan 2009
581 Posts
It would be good to disentangle the possible CG problem from the possible "flex" problem.

A simple thing to do would be get it up high and then point it down 60 degrees in launch preset mode. It should pull out pretty nicely - any sign of tucking would indicate a rearward CG.


Jony
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Old May 31, 2011, 10:17 AM
a.k.a. Bob Parks
Glendale, AZ
Joined Jun 2008
2,383 Posts
I find that my LH3 has a very stiff boom, at least for an intermediate (150ft) launcher. I would hold suspect the pushrod installation.

On launch, my plane goes out about 10 feet before rotation, while I hold the launch preset until it is climbing. I do not throw straight out, just a little upwards, probably less than 10 degrees up. It then climbs out at about a 75 to 80 degree angle in calm air, flatter in windy conditions.

Bob P
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Old May 31, 2011, 12:14 PM
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Espaņa, PM, Palma
Joined Jan 2009
114 Posts
Quote:
Originally Posted by garry View Post
Are you a right hander or lefty. It sounds like the pushrod is on the wrong side to me.
how is the right side determined?

Monhty
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