Steerable nose wheel & Weight/Balance Numbers

[Sacha]

Hi Folks,

I have a tricycle gear Kitfox IV with a Rotax 912UL which has several mods, one being a steerable nosewheel of my own design. The main gear is a Grove kit. I wasn't particularly careful with weight during the rebuild that I recently completed, so it's quite a heavy beast: empty weight is 316 kg (695 lb) of which 69 kg (152 lb) are on the nose wheel.

The nosewheel works OK but is not completely satisfactory as it tends to suffer a form of flutter on uneven ground (and our local grass strip definitely meets the definition of "uneven"). I'm thinking it might work better and be subject to less stress if it had less weight on it .

I've already moved the battery aft of the seats, so the only way I could lighten it up a bit would seem to be by moving the main gear forward a few inches. But it's not exactly an easy mod. Has anyone ever heard of or done such a thing? Any ideas?

Regards,
sacha

For reference, here are my empty W&B numbers:
Nose Wheel: 69kg (152lb) x -88cm (-34.6") = -6,072 kg.cm (-5,259 lb.in)
Main Gear: 247kg (543lb) x 34cm (21.3") = 13,338 kg.cm (11,553 lb.in)
Total Weight: 316 kg (695 lb)
Total Moment: 7,266 kg.cm (6,293 lb.in)
CG: 23cm (9.1")

[jiott]

Weight on the nose wheel is not a choice relating to shimmy, etc. It must be based on the proper specified W&B for flight. Get the W&B in the specified range and THEN work on the shimmy issue.

[Sacha]

Thanks, Jim that's useful info. Shimmy is the word I was looking for, not flutter.

My W&B is already in the specified range (remember it's not the empty W&B that counts, it's the actual, and as soon as you put a pilot and a bit of fuel in there, the CG shifts back sufficiently for it to fall in the 10.2-16" range).

[Sacha]

Dave S sent me a very useful reply which for some reason is not appearing in the archive so I am reproducing it below. Thank you Dave.

I did a bit of research and it seems like the shimmy problem is very complex and the theory is not yet completely understood to the point where one can say for certain that a given design will turn out to be OK in practice.

A survey of the literature shows that there are lots of parameters which can affect shimmy, but the three that Dave mentions (caster angle, damping and tyre pressure) seem to be the three main ones. I'm going to tinker around with those then. Thanks again for your help.

Sacha
Trigear Kitfox IV
Italy

Dave S wrote:



1) A nose wheel can be succeptible to shimmy depending on the caster angle....the correct cater angle can be a little unique to the particular design.

2) The non steerable KF nose wheel relies on a friction damper (stack of bellville washers adjusted to a specific tension) to control shimmy. A lot of certified aircraft with steerable nosewheels rely on a hydraulic damper to control shimmy.

3) Air pressure can affect shimmy because it can change the caster angle (low pressure increases negative caster and higher pressure increases positive caster on a front wheel)

If the caster angle is different from the original KF design and/or there is inadequate damping a shimmy situation could result.....I am not going to say that is the only possibility or that this is the case with your experience; but, it is a couple ideas a person could consider as a starting place....guess that is why we call 'em experimentals.

Wishing you luck in finding the cause

Sincerely,

Dave S
KF7 Trigear
St Paul, MN
***************

[rogerh12]

If you have grove gear on your model-4, it's not hard to move the main gear up a few inches, just swap the mounting brackets on the grove gear, and it will move the mains up 3.5 inches as I recall.

I did this on my model-4 to make it easier to rotate on the runway, as my VW engine and noise wheel up front make it noise heavy. Note: This mod will change the way you calculate the CG, as the mains are now closer to the datum.
Hope this helps
Roger

[Sacha]

Roger
I just happened to see your quote when I was down at the hangar today and yes you're right... I hadn't thought of that simple solution!
I might try it to see how the aircraft feels with a lighter nose wheel. Even if it doesn't help with the shimmy, presumably this would improve operations on uneven fields and make it easier to lighten up the nose and rotate.
Regarding the shimmy I've had the nose wheel (statically) balanced and have tightened up the various loose joints in the gear. I'll be able to test this next week and see if it has had any positive effect.
Sacha

[quote=rogerh12;40560]If you have grove gear on your model-4, it's not hard to move the main gear up a few inches, just swap the mounting brackets on the grove gear, and it will move the mains up 3.5 inches as I recall.

[rogerh12]

oh, on that nose wheel. When I installed my factory nose kit, I noticed it had specifications on how tight to make the nut holding the noise pivit in place. I believe I had to measure the side force required to move the wheel, when suspended in the air, or something like that. Tightening the nut gave more drag when the wheel pivoted, but we were not to over tighten it. Have you done this calibration?
Roger

[Sacha]

Hi Roger
My nose wheel is not the standard free castering one, it's a steerable one of my own design. I suppose that the equivalent of what one might call the "castering torque/friction" for a free castering nose wheel would be the amount of pedal force required to counter a certain turning moment on the wheel, in other words the leverage of the steering mechanism. This is something which I may attemp to tinker with if the other approaches fail.
Sacha