Fish-tailing

[GrumpyGnome]

Well, did some experimenting. I tried:

Adding a bit of area to the rudder by taping on some clear plastic - no discernable effect

Ditto to fin - no effect

Moved balance point around - no effect

Put a small triangular extension on rudder without increasing area - fish tail rudder - no effect

Just Gyro left now. I have a small e-flight one, as well as an old GWS one to try................. as they both seem to be between the receiver and rudder, I've no idea how I can switch them on/off in flight via my DX7s. Don't want to trash the plane!

GG

[YakMad]

You need a gyro were the gain is controlled form the transmitter if you want to be able to switcn it off by reduceing the gain to zero and it must only be used in "Rate mode" and not heading hold mode. I used a cheap Giant Shark 401 gyro.

Edited By YakMad on 18/11/2013 08:43:31

[Biggles' Elder Brother - Moderator]

GG - refering to my earlier post - fiddling with the rudder will never be a gaurenteed fix for all aircraft. Yes it will work on some I have no doubt - clearly it works with aerobatic style aircraft or the guys there wouldn't do it. But its not necessarily transferable because it depends on exactly what is causing the fishtailing.

As I said it happens generally because in the interaction between two yaw/roll couples neither of them is dominant and their correction action is out of phase. The generic approach to a solution has to be one of two things, either;

1. Strengthen/weaken one of the couplings so that one or the other does dominate - this is the rudder alteration approach. The limitation of this is, as I say, that it is not necessarily transferable. What weakens say fin effect (with rudder centralised) enough to remove the fishtailing on one aircraft may not weaken it enough on another aircraft to create the necessary dominance of one mechanism. Similarly what strengthens the fin effect enough for one model to create a dominance might not be enough for another!

2. So the other approach is to incorporate some smart technology that removes the phase lag between the two coupling effects - this is the gyro approach (and largely these days the full-size approach as well). This method leaves them balanced but stops them working against each other out of step.

BEB

[GrumpyGnome]

Aye, will get me virtual cheque book out and try a gyro.

Or save the money, live with the minor annoyance, and buy some balsa to build a pushy cat !

I'll let you all know.

GG

[Bob Cotsford]

As I said my litttle bit wing waggles a fair bit, increasing the rudder size helped but I do wonder how well a gyro would work on any model especially if it's a high frequency wiggle. How much would it reduce battery duration if the servo is constantly blatting from side to side at high speed?

[Bandit]

I wouldn't worry too much about the servo, I'd worry more about getting the settings of the gyro wrong as it may make it over correct and make it worse ending in..

Must say though such an interesting thread wow so much to learn!

[GrumpyGnome]

Hi Bandit

That's what scares me into needing a gyro I can switch on/off from Tx

GG

[YakMad]

Its easy to switch the gyro off so long as the gain can be controlled from the transmitter, the gyro plugs into the rudder channel on the receiver and the gain connection plugs into channel 5, if your radio has a gyro function in aircraft mode you can set the percentage gain in there (0% off 10-12% on) and assign a switch to switch gains. If you don't have the gyro function you could use the flap knob to turn the gain up or down but you would be wise to set the amount of up gain on the ground by moving the tail of the plane and adjusting the knob until the desired opposite rubber move is achieved and then just use the knob to lower or turn off the gain in flight if needed. You don't need a high speed servo for an aircraft as fish tailing is a much slower movement than a heli's tail expierences so the servo does not need to react as fast and as i said previously the gyro will sense and react faster than you can anyway.

[Piers Bowlan]

Stephen, The Fantrainer has a short tail moment and long nose moment, necessitated by the rear mounted engine/motor. it also has a small fin/rudder or at least much of the tail is in the turbulent slipstream from the fan, reducing it's effectiveness. A bigger fin and/or a longer tail moment would make it more directionally stable so less likely to fish tail.

Just my 2p worth.

Piers.

[Biggles' Elder Brother - Moderator]

Servo current draws are tiny - assuming their not stalled - so I don't thing that would be a particular problem. But Bandit is right - tuning the setting will be pure "hit and miss". Its not so much overcorrecting though as pushing the correction even further out of phase that would worry me. If it did that its quite likely the gyro could actually make the fishtailing worse! What you'd need to do is be prepared to evaluate it over a range of settings and try to home in on just the point where its in antiphase with the fin's tendencies.

Being able to switch it in and out should make this experiementation fairly safe provided you do it fairly high!

BEB

[Julian Thacker]

What a great thread. I was about to ask if anyone knew how to curb fishtailing and found this! I have an EFlite 70mm EDF A4 Skyhawk which fishtails enough to be annoying. This is quite an old model now and when compared to say the equivalent F86 Sabre the differing flight characteristics are enough to make me want to do something about the A4 which I have long lived with. As the model does not have an active rudder and I don't particularly want to go to the trouble of fitting one I think I will try Ultymate's solution first - as he says it may not be pretty but if it flies better I can live with the cosmetic irritation.

Thanks to all the useful input from thread contributors.

Julian

[chris basson]

THAT'S Exactly what this forum is about Julian!!!