HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jul 23, 2012, 11:04 AM
Registered User
DoctorG's Avatar
United Kingdom, Basingstoke
Joined Nov 2011
28 Posts
Help!
Tail wag Kwik-Fli

I have a Phil Kraft Kwik Fli MkIII. Flew it for many years in it's standard configuration with tri-U/C. Recently on a whim I recovered it and replaced the tri-U/C with a retracting tail dragger configuration. I also reconfigured the rudder and added ~25% aerodynamic balance ahead of the hinge line to see if it would improve knife edge.

The model flies brillantly and is really fast with the wheels up but there is a slight but pernanent lateral 'tail wag' i.e. side to side when in straight and level flight. Rudder, servo and linakges are as tight as I would expect i.e. no real play so I am assuming it is an aerodynamic instability.

Anybody got any ideas what might cause this - and more to the point what might cure it?

Thoughts that have occured to me: could it be an imbalance in the open wheel wells? is it a CoG issue? Have I cocked up with the rudder modification?

Hope somebody can help. Cheers.

Many thanks.
DoctorG is offline Find More Posts by DoctorG
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 23, 2012, 11:17 AM
CPA# 39
EscapeFlyer's Avatar
United States, MN, Brooklyn Center
Joined May 2009
743 Posts
This was a common issue with the Kwik Fli series. I believe it is an unintended design flaw.

I have heard that not every Kwik Fli will do this and I don't know if anyone knows exactly why a few never tail wag and the majority will.

I understand some examples from the Kaos/Super Kaos line from Bridi will do the same thing. The smaller 40-60 scale/semiscale F4U Corsairs will do the same thing.

It doesn't fix your problem. But it is a quirk that is as vintage as the design itself and should add to the fun of the nostalgia.

I hope this will help you some.

Brian
EscapeFlyer is online now Find More Posts by EscapeFlyer
Reply With Quote
Old Jul 23, 2012, 12:20 PM
That's so classic you.
Generic Member's Avatar
United States, NC, Stokesdale
Joined Jul 2006
360 Posts
Add some pretty decent sized wing fillets. That should help. The "wag" or "Qwick Fly Dance" is cause from turbulence. Adding speed by retracting the gear accentuated what was already there.
Generic Member is online now Find More Posts by Generic Member
Reply With Quote
Old Jul 23, 2012, 01:41 PM
Registered User
DoctorG's Avatar
United Kingdom, Basingstoke
Joined Nov 2011
28 Posts
Now there is a couple of interesting answers. Thanks guys. I will probably live with the problem - it's not serious - more entertaining. As Brian says it's all part of the nostalgia.

Thanks for the replies.
DoctorG is offline Find More Posts by DoctorG
Reply With Quote
Old Jul 23, 2012, 02:56 PM
Registered User
Hawaii
Joined Feb 2003
5,956 Posts
That's how you know it is a Kwik Fli. I love the tail wag and will always treasure it. It is nostalgia.
SeismicCWave is offline Find More Posts by SeismicCWave
Reply With Quote
Old Jul 23, 2012, 11:47 PM
Registered User
Joined Nov 2011
3 Posts
Kwik Fli and Kaos will do it, cure is to add a bit of 1/16 square balsa either side of the rudder trailing edge, 2" or so long. You can trim it back after flying until it just starts again, then add the last bit you just trimmed off...Several well known full size a/c did it too, their testing showed that there was an airflow breakaway over the fin that would initiate the yaw towards the breakaway on one side, which would re-attach, and then the other side would break away..., thickening the rudder trailing edge was a quick and effective cure.
Evan, WB #12.
pimmnz is offline Find More Posts by pimmnz
Reply With Quote
Old Jul 24, 2012, 04:42 AM
Oxford Panic
AndyOne's Avatar
United Kingdom, Oxford
Joined Feb 2003
3,659 Posts
Or you could make a rudder in the style of the Sebart WindS, this may show less than adding strakes.

A.
AndyOne is offline Find More Posts by AndyOne
Reply With Quote
Old Jul 24, 2012, 01:03 PM
Registered User
stuntflyr's Avatar
Los Angeles
Joined Jun 2010
1,353 Posts
The Unlimited air racer Tsunami had strips added to the trailing edge of the rudder, I went to the hobby shop to get 3/16th bass strips. Worked OK, allowed the flight test to go on to higher speeds.
Chris...
stuntflyr is offline Find More Posts by stuntflyr
Reply With Quote
Old Jul 26, 2012, 08:43 PM
Mach 1
pathfinder's Avatar
walterboro s.c.u.s.a
Joined Jun 2004
758 Posts
Amazing info.This explains a lot of things that were never explained to me when I was younger, flying all those Kaos's my dad had.
pathfinder is offline Find More Posts by pathfinder
RCG Plus Member
Latest blog entry: Nice
Reply With Quote
Old Sep 03, 2012, 09:55 PM
Registered User
vonkross's Avatar
Brazil, São Paulo, Santana de Parnaíba
Joined Nov 2008
141 Posts
Hi DoctorG

I have this same tail wag in my moulded sailplanes.
Did you try the suggested fix?
Did it work?

Cheers
vonkross is offline Find More Posts by vonkross
Reply With Quote
Old Sep 11, 2012, 02:02 PM
Registered User
DoctorG's Avatar
United Kingdom, Basingstoke
Joined Nov 2011
28 Posts
Not tried any fix's - I'll live with the problem I think.
DoctorG is offline Find More Posts by DoctorG
Reply With Quote
Old Nov 08, 2012, 04:26 PM
Edubarca
Edubarca's Avatar
Colombia, South America
Joined Oct 2009
1,211 Posts
I have flown Kwik Fli since 1968. As a matter of fact I still have the one I built in 1970 and still flying!!! So, I really do know this most fantastic plane. The "Kwik-Fli dance" as someone stated above, is a characteristic of this model dating way back to the Kwik Fli Mk. I and Mk. II. I have been told that Mr. Kraft modified the slim fuselage of the Mk. II to a wider one and he also increased notoriously the rudder and fin area with little help solving the problem. My Kwik Fli also dances but I agree, this is part of this beautiful airplane and apart from a funny looking attitude, it does not harm at all its wonderful performance. No wonder this model was World Champion in Corsica way back in 1967 when the Beatles launched Sgt. Peppers Lonely Hearts Club Band!!!!
Edubarca is offline Find More Posts by Edubarca
Reply With Quote
Old Nov 08, 2012, 04:27 PM
Edubarca
Edubarca's Avatar
Colombia, South America
Joined Oct 2009
1,211 Posts
Look at my Kwik Fli when my daughter Paloma turned 2!!! I also had dark hair!!!!
Edubarca is offline Find More Posts by Edubarca
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Tex 450 tail wag Patitifa Electric Heli Talk 6 Apr 16, 2012 01:01 PM
Help! 600 3GX Tail wag midohioboarder Large Electric Helis 0 Mar 28, 2012 08:25 PM
Help! Tail wag on spool up and spool down tonycopter Beginner Training Area (Heli-Electric) 9 Mar 17, 2012 02:54 AM
Discussion 450 pro tail wag dgibbons 3D Electric Heli Flying 3 Mar 11, 2012 12:00 PM