Debugging glider - which to tackle first: CG, incidence, decalage, elevator flex? - RC Groups
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Oct 27, 2004, 03:52 AM
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Debugging glider - which to tackle first: CG, incidence, decalage, elevator flex?

I have scratched built a dlg based on the bug and a bit of mod inspired from the seeker. it flies but the performance is poor and sink rate is horrible. I have posted similar question on another thread but I do not wish to rob it with my questions so I decided to start a new thread instead.

the plane in question has the same wingspan and weight (3.5oz) as the bug, the tail is a cruciform one. the incidence should be a close reproduction as the stock bug. being so similar to the bug i presume the desired cg should be the same as the bug. at present the cg is about 1.5cm ahead of that due to the battery (await lighter one)

problem is, even with optimal elevator trim, the sink rate is high with the plane assuming a nose down attitude all the time. when trying to do a dive test the plane does not recover despite being nose heavy supposedly

The possible reasons identified include incidence / decalage and elevator pushrod flex. i added cg to the list.

it is difficult to fiddle with the parameters all at once - so what should i deal with first?

CG: is nose heaviness the cause the plane has a nose down attitude all the time? if that is the case why does the plane not recover in a dive? is it because of...

Elevator pushrod flex - see the picture. if that is the case, what is the best way to fix it without drilling more hole in the boom?

or is it due to incidence / decalage problem? The incidence of the wing cannot be changed due to the construction of the plane. is changing decalage i.e. incidence of the horiz stab an alternative?

Thanks for all your input.

Pictures (please excuse poor quality):
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Oct 27, 2004, 04:22 AM
Registered User
Here's what I'd do.

1. sort out the CG - move it back - a fwd CG will cause the elevator response to be poor and could cause the nose down attitude.

2. Once the CG is sorted - trim out the glider for level flight.

3. Look at the elevator - is it up or down when trimmed - if its up then sort out the declage.

That should do it.

Oct 27, 2004, 09:01 AM
Registered User

I agree with Tim except for one thing. It is hard to see from the picture but there appears to be a whole lot of unsupported elevator linkage. Is this a cable? If so it definately can't be unsupported for that length. Simply put your finger on the trailing edge of the Elevator and see how much flex you have and how easy it is to move.
This issue should be addressed first. Good tight linkages are most important to good flight charachteristics.
Oct 27, 2004, 02:30 PM
Registered User

The pushrod is .032 music wire. it exits the boom 5.5" from the control horn so there is 5.5" of unsupported pushrod. The angle it makes with the boom is very shallow (may be 15 degree) to keep the binding minimal. On ground the control seems solid enough, although I must say when I try to force the elevator down hard the pushrod gives / flexes instead of the servo horn being moved so I guess force is not being transmitted to the servo directly. I can't really change the way the pushrod is run now without major hacking of the pod so my options are either changing the pushrod to CF (easiest), add support to the pushrod (difficult due to the geometry) or switch to pull-pull (still need some minor hacking to the pod). Don't know which one will work...

Last edited by dimple; Oct 27, 2004 at 02:31 PM. Reason: typo
Oct 27, 2004, 02:40 PM
Make Flying Fun
Without completely redoing the linkage(best), try unhooking from stab and slipping short sections of control tubing over rod and securing in places to remove flex in loose areas if possible. Denny is right on, in flight changes due to loose linkage runs make it impossible to correct trim problems cause moving surfaces give different reactions on every flight. Next time run the linkage outside attached to the boom for easy maintenance and no intrusions into the carbon boom compromising it's strength. Use readily availiable etched teflon tube and glue right onto the boom!
Last edited by KevinSharbonda; Oct 27, 2004 at 09:18 PM. Reason: added data
Oct 28, 2004, 10:42 AM
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windsox's Avatar

so which is it nose up or down

I am confused as to how flight characteristics relate to cg. does a forward cg cause a nose up or nose down attitude in level flight? My current understanding and my understanding of the utility of a dive test to identify optimal cg is that weight in the nose causes a tendancy for the plane to want to come up and can lead to a nose high flight attitude?
Oct 28, 2004, 12:09 PM
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ClayH's Avatar


Nose heavy planes (forward cg)require up trim to fly level in straight, level flight. When you put them in a dive, the speed increase will cause the plane to gradually pull out of the dive rather than holding the line. Things are just the opposite for a tail heavy plane, which will tend to tuck under or increase the downward direction when put in a dive.In both conditions the plane can be made to fly level with trim adjustments. The main characteristic that you're playing with is the pitch stability of the plane. In dlgs, a slightly nose light condition can be prefered so the plane will pitch easily, indicating lift more readily.

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