31 yo Stiletto 660. Impossible to fly level......

[Paul Van Dort]

I took out my Stiletto 660 that I build in 1991. Tomorrow there is a classic contest in my club for pre 1980 designs.
Good ST46 power. Engine run is decent. C.G. is in the ball parc, leadout position is ok, controls appear friction free...
Wings and flaps are straigth. Flaps and elevator both neutral simultaneously.. Tipweigth is fine. So bench trimming was fine.

One problem. I cannot control the thing in level flight. Going up and down. No grooving whatsoever.  Flying at shoulder height is too risky.
I have been flying F2B since 1990, but I cannot understand what is happening. This model used to fly decently (25 yeas ago) .
This evening I have oiled the Bellcrank pivot. Perhaps there was friction under linetension. The old bearing ... Who knows.
Tomorrow back to the flying field for testing and the contest....
I will keep you posted. Hints are very welcome!!

[EricV]

Assuming a warp didn't creep in somewhere, then check the controls for drag "under load" so do it with the plane hanging from your fingertips holding the leadouts.
Next I'd check for a thrust alignment of the ST46, since you just installed that, make sure it is level or at least down and to the right, heh.
Hope it goes well in time for your event.
EricV

[bill bischoff]

Have you checked the CG recently? In the southwestern US, airplanes that have been dormant for a while have been known to pick up unwanted tail weight in the form of mud dauber or other insect nests inside the aft fuselage. 

[Bob Hunt]

I'd bet a bit that it is the wing/stab alignment. If the stab centerline is not parallel to the wing centerline, the model will be almost impossible to trim. That is especially true if there is negative incidence in the stab (nose of stab down). You can sometimes get away with some positive incidence in the stab (very little), but almost never with even the slightest negative incidence.

Bob Hunt   

[Jim Rhoades]

     Twice, at major contests of course my Apteryx started to pitch down sharply at the downwind point of the circle and pitch up on the upwind side in a breeze.  My leadout guide had loosened and moved full aft.  Moving it back to its normal mid slot placement cured the problem.  Can't hurt to check.
     The last time was at the NW Regionals where it suddenly pitched down at 2 inches off the ground on the takeoff run, taking out a 3 blade Brian Eather carbon fiber prop.  Ouch!

Jim Rhoades

[Joe Gilbert]

Ball links tight?

[John Carrodus]

Paul, last time this happened to me with a proven model, it was crap on and twists in the lines . I was in a hurry with rain approaching and 'assumed ' the lines were tickety boo. I ended up crashing my hurricane. It was a nightmare!

[Dick Pacini]

Maybe Les McDonald will chime in here.

[Ken Culbertson]

Have you checked the CG recently? In the southwestern US, airplanes that have been dormant for a while have been known to pick up unwanted tail weight in the form of mud dauber or other insect nests inside the aft fuselage. 

He is not joking!

Ken

[Ken Culbertson]

My current ride has a CG sweet spot that will start a drifting type of hunting if it is too far back by as much as a 1/4".  Our circles have a wind pattern that almost always makes it difficult to not drift up and down.  In light of Bob's post, if you have an adjustable elevator, I have found that a 1/4 turn of down helps if the plane is hunting this way and does not change the turns enough to throw me off. 

Ken

[Dwayne Donnelly]

Have you checked the CG recently? In the southwestern US, airplanes that have been dormant for a while have been known to pick up unwanted tail weight in the form of mud dauber or other insect nests inside the aft fuselage. 

I had a PT 40 rc plane I put on floats, had it for years always a good flier one day I went to take off and it went straight up and straight in, after it dried I checked the cg and sure enough huge tail heavy, no idea why.

[Paul Van Dort]

Hi all, Thank you for the many great ideas.
I tried to get the model to fly decently but no success: Not flying level, very unpredictable in maneuvres: bad scores.
I checked and verified your good ideas I received but still the issues were there. A bit of down thrust did not help either.

Now I think that I am flying the Stiletto with a too backward c.g. Probably the Stiletto was not designed to fly with backward c.g. And a lot of flap deflection resists the ability to do decent corners. So I now reduced the flap throw compared to the elevator and moved the c.g. about half an inch forward.
If I remember correctly, the plans of the Stiletto does not show the c.g. location.

So fingers crossed that the combination with a more forward c.g. and reduced flap travel will give decent flying machine.
Of course your ideas remain very much appreciated.

I keep you informed about the progress

[Paul Allen]

Hi Paul
         I had the same problem, not flying level, I created the issue with my model, by trying to
get it to turn better, ended up tailheavy and poor level flight.

[Paul Van Dort]

Hi all, I think I found the cause of the hunting. (still need to test)

Many top flyers will name 1 main cause of hunting: Friction in the controles.
I tested the controles: No friction, very little play. . I tested under load (Pull on the lead outs) : No friction.

So I cut some small windows in the fuselage between flaps and elevator, close to the formers and I think I found the problem.
The elevator pushrod is connected to the control horns with clevises. These clevises allow their pin to move a few mm axially in the control horn bearing.  I noticed that the carbon pushrod was able to touch a former when pushed 1 mm sideways. I am convinced that this contact under centrifugal force causes sufficient friction to cause hunting. I had noticed in the flight tests that during the final glide, the problem was smaller. The speed drops, centrifugal force drops. Friction drops. This explains a lot. So it was most likely not the thrust line of the engine.

The proof of the pudding is in the test flying. I will keep you informed.

(who knows the static friction coefficient of balsa agains carbon?  It must be huge!)

[Paul Van Dort]

A small note to confirm that the hunting issue was resolved by giving more room to the pushrod in the fuselage. Centrifugal force was pushing   the pushrod against the first fuselage former, behind the flaphorn, causing a binding that was not noticable statically, not even when stressing the leadouts. Now the ship behaves "nornmally". Some extra trimming required, but happy that the mystery is solved. Thanks for the hints!