Line flutter

Went out yesterday with a scratch-built weird design. It is a Mongoose with a corroplast wing and an old Fox 29. 300 inch wing, so I was afraid it would be faster than this old guy could handle. Finally got it off the ground, not enough tip weight and the single wheel gear not properly situated. Anyhow, the thing was SLOW - there was a pulse in the handle, and when I finally started looking for what was going on I could see both lines occilating in S curves. Never did quit doing it, I tried grabbing the lines as far out as I could reach, did what manouvering I could, this thing was STABLE, and nothing helped. This is a new one on me, I am certainly no expert but the lines have always had a nice backward curve. I'm sure what I was using was overkill, .018 52 feet, but I thought it was better to err on the heavy side. Any ideas what I did wrong? The flight was successful, aircraft ran out of fuel in the air.

I'm not sure what's causing the line flutter. I'm more interested
in how the wing is made. Is it a single surface or folded, or?
Sounds interesting. How well does it fly?

Stan Bidowski
Perfect Circle Flying Club

I'd guess that an airplane vibration is shaking the lines, which have a resonance at that frequency. That's just a guess.

Sounds like a standing wave in the lines caused by vibration from the most famous source of vibration ever, a FOX engine.

Saw the same thing on my FOX racers, that's why they now fly with modern plain bearing 25s.

>Went out yesterday with a scratch-built weird design. It is
>a Mongoose with a corroplast wing and an old Fox 29. 300
>inch wing, so I was afraid it would be faster than this old
>guy could handle. Finally got it off the ground, not enough
>tip weight and the single wheel gear not properly situated.
>Anyhow, the thing was SLOW - there was a pulse in the
>handle, and when I finally started looking for what was
>going on I could see both lines oscillating in S curves.
>Never did quit doing it, I tried grabbing the lines as far
>out as I could reach, did what maneuvering I could, this
>thing was STABLE, and nothing helped. This is a new one on
>me, I am certainly no expert but the lines have always had a
>nice backward curve. I'm sure what I was using was
>overkill, .018 52 feet, but I thought it was better to err
>on the heavy side. Any ideas what I did wrong? The flight
>was successful, aircraft ran out of fuel in the air.

Ron B.

As Howard said, vibration could be the cause. Another could be excessive offsets causing a sinusoidal yaw. Does it stop if you whip it hard, then resume if you stop whipping? If so, try moving the leadouts forward, and/or removing engine and/or rudder offset. In addition, check the CG location. Unless it has a very unusual force arrangement, it should be within a range of 15-25% of Mean Chord (at least for first flights.

Ron B.
F4Fguy

LAST EDITED ON Oct-14-06 AT 09:51 PM (CST)
 
Russell,

If it is the engine that is causing the vibrations, which cause the line flutter, then first off, I'd make sure the prop was as finely balanced as possible.

Severe vibration should be easy to spot before you get the model launched. The whole front end of it should show the results of vibration...

A smaller or larger prop might get it out of a vibration mode. Worth a try? You say "older Fox 29" - three-bolt backplate or four? The 4-bolt, 1970/something series slant plug Fox 29 and 36 were notorious for vibration until they had quite a bit of time on them, and for liking more RPM than the 3-bolt Fox Stunt 35.

A 'soft' front end will emphasize any tendency to vibrate. Hope yours is solid. Can't recall the Mongoose front end at the moment: hope it has plenty of 5-ply 1/8 ply, at least some of which runs back past the wing LE.

The stability is due to CG location. PERIOD. If you can reduce the vibes, the model may go enough faster that it responds better. If it is still a sleeping slug, move the CG aft in small increments until it turns better. The Mongoose was intended for slow combat-type flying, so it has plenty of turn POSSIBLE. That puts it up to you to get it to do it.

(edit to add) Speed may be affected by where the leadouts are located. If you find a CG where the model turns well, the leadout guides should be about 3.5° aft of that CG - more or less. That's about 3/4" per 12" from fuselage centerline to the leadout guides.

Just a SWAG, but it should be pretty close...

Flew the corroplast beast again yesterday. No line flutter this time, but I changed too many things to know what fixed it. 60 foot .015 lines, more tip weight and some lead in the tail. Also increased the control throw. I can still see the top of the wing and it dives much easier than it climbs, so back home and glued more lead to the wing and tail. I was asked about the wing construction, so here are a couple photos. For a short description though, just say "heavy". The good point is that it is close to crash proof. The whole thing is proof of the old saying that you can fly anything with enough power. The Akromaster gives an idea of the size. The blue and red plane is underpowered with a 35, but I have a 45 doing nothing and the thing is already built, so why not? I was also asked how it flys, let's just say the Akromaster is a whole lot better.