Weak point in the Kitfox tailwheel spring attachment design

[wypaul 448]

i have seen several failures of the tabs that hold the tail spring in alignment at the vertical fin post when stressed.  The Avid MK IV DESIGN is much more robust and I suggest that if your fuselage is not covered that you consider making that modification and for that matter even if it is covered.  They never seem to fail at a convenient time.  If anyone is interested I can provide pictures of the Avid and also how we modified Larry's old plane (Old Yellow) after the tab failed at Johnson Creek last year.  If you land perfectly every time then maybe you don't need to think about this little upgrade.  This happened again today to another KF at a flyin that I attended.  I wonder if KF has made improvements in their later models, 5 and beyond?

i just looked at a drawing on the Kitfox site and they are still using the same design on the S7, to bad.

Edited 27 Jul 2019 by wypaul

[Willja67 144]

i have seen several failures of the tabs that hold the tail spring in alignment at the vertical fin post when stressed.  The Avid MK IV DESIGN is much more robust and I suggest that if your fuselage is not covered that you consider making that modification and for that matter even if it is covered.  They never seem to fail at a convenient time.  If anyone is interested I can provide pictures of the Avid and also how we modified Larry's old plane (Old Yellow) after the tab failed at Johnson Creek last year.  If you land perfectly every time then maybe you don't need to think about this little upgrade.  This happened again today to another KF at a flyin that I attended.  I wonder if KF has made improvements in their later models, 5 and beyond?

i just looked at a drawing on the Kitfox site and they are still using the same design on the S7, to bad.

Funny, I was just thinking about my tailwheel installation when i opened the site and saw your post. My goal is to have my bird to Oshkosh next year and I'm trying to address all known issues before that time so yes I'm very interested in some pics of your tailwheel mounts. 

I frequently entertain somewhat off the wall ideas for the improvement of my plane. Often they're just dead ends but sometimes they lead to a different idea that does get used. 

My interesting idea today is to move the tailwheel forward to the next place on the longerons where some diagonal tubes are welded on. The advantages to this are a steeper deck angle for better takeoff aoa and in the event the tailspring fails in some manner the rudder doesn't get crunched.  Disadvantages are that tail wheel steering goes away, you may end up crunching the aft fuselage which is probably harder to fix than a rudder and if you really hang it on the prop coming in you might just get your rudder anyway (probably less of an issue for avids than foxes because of the rounded bottom on the rudder) and you would  have to do some modifying of your aft fuselage.

Since most aircraft are running differential brakes losing tail steering isn't a big deal, just add a mechanism that locks/unlocks the tailwheel based on stick position like the P-51 Mustang. 

Anyway that's my crazy idea for today.

[wypaul 448]

Ok Will I will get some pictures and forward them to you.  Maybe tomorrow if this older brain kicks in when I get to the hangar.

[Willja67 144]

Ok Will I will get some pictures and forward them to you.  Maybe tomorrow if this older brain kicks in when I get to the hangar.

Thanks.

Decided to go out and do some measuring just to explore my idea. So i hung the rudder back on the plane and the minimal distance between the tailwheel and the rudder has me worried. 

1 3/4" seems awfully close for a rookie tailwheel pilot. Definitely going to have to think about this some more. 

[1avidflyer 1983]

I think the spring is sprung.  Not intending to be funny, but my eye is telling me that a Kitfox spring has more of a curve to it between the fuselage and tailwheel.  Also to back that up, your verticle axis of the tailwheel should be more verticle.  That also says the spring is to straight.  With a load on the tailwheel, the axis should be about 90 degrees to the ground.    That would also give you more room between the tailwheel and the rudder.  In this picture, you can see the tailspring on my Kitfox 4 after it came here from Texas.  JImChuk

[Allen Sutphin 705]

My model 2 tail wheel. Pretty robust, I think. The black piece is a stiffner plate that acts like a weight distrubition hitch to spread the force over a larger area.

Edited 28 Jul 2019 by Allen Sutphin

[wypaul 448]

Hey Allen, it appears your 2 has an additional piece of metal (black) above the spring.  Is that something that you added or did someone else add that.  That appears to be a good viable solution to the weak point without the need for welding.  It gives added reinforcement to the tabs and also helps hold the spring in alignment with the downward facing ears.  Thanks for posting the picture.  I believe I will make a part like that for my friends plane and see how it works out.

You seem to be missing the spacers to the bottom plate the allows proper tightening without bending the bottom plate. You ca see them in Jim's picture above and on the KF site under manual, landing gear.

http://www.kitfoxaircraft.com/index.php/landing-gear-assembly#page/13

Edited 28 Jul 2019 by wypaul

[Allen Sutphin 705]

With the black additional plate, the tab bolts don't need to be super tight, just snug. The black plate has it captured till it can't move either way. I seen the plate on another Fox and thought it was a good idea so I made one using the "TLAR" method.  But for the sake of causing concern I'll probably add the spacers. Can't hurt anything to add them. When I got the Fox, one of the ears on the tail was cracked and would have broken off so I added another 3/16" bracket and welded it to the existing tabs. Then made the plate as a weight distrubition thing a ma jigger. If you don't know the "TLAR" method, there is a chapter on it in the experimenter's guidebook on it. "That Looks About Right".

[Willja67 144]

I think the spring is sprung.  Not intending to be funny, but my eye is telling me that a Kitfox spring has more of a curve to it between the fuselage and tailwheel.  Also to back that up, your verticle axis of the tailwheel should be more verticle.  That also says the spring is to straight.  With a load on the tailwheel, the axis should be about 90 degrees to the ground.    That would also give you more room between the tailwheel and the rudder.  In this picture, you can see the tailspring on my Kitfox 4 after it came here from Texas.  JImChuk

Your definitely right about the spring. 1 more thing to add to the list of things to fix. But better now than after it crunches the rudder. 

[FredStork 453]

That hard Maule tailewheel is a killer for the tailspring, it provided no damping what so ever and it is heavy.

While keeping the mechanism intact (I belong to the minority actually finding it good...) I replaced just the solid wheel with a pneumatic wheel 2 years ago. I get much less vibration fron the tail and it is more comfortable...

I wrote an article about it on my blog http://avidsimonini.blogspot.com/2017/03/finally-pneumatic-tail-wheel.html

[1avidflyer 1983]

That looks like a Matco wheel on a Maule top end.  I just picked up an unused 6" pneumatic complete tailwheel that I wont be using that I will probably be selling.  The plane it came with is maybe a bit on the heavy side for that 6" wheel.   JImChuk

[wypaul 448]

MK IV SETUP also see Fred's picture in his post. 

[Willja67 144]

How much travel do you guys think these leaf springs have? I mean obviously just before impacting the rudder might be max travel but that might also be a one time thing and the next time even a soft landing might damage the rudder. 

How much deflection would a normal 3 point landing cause?

Edited 28 Jul 2019 by Willja67

[Allen Sutphin 705]

I've always had the habit of bumping the tail wheel about a second before the mains touch. I had a pneumatic tail wheel on my Stinson and had a flat multiple times so not really fond of them, but would use one if I can locate one locally. Good landing technique is not super hard on the tail spring unless one does carrier landings, which we all do on occasion. Rough strips with rocks, driftwood, etc. a soft tire would be nicer than the Maule. But hundreds of Maule tail wheels out there in use everyday. It comes down to type of flying and personal choice. Touching down in the low thirties, the ground roll is 200' or less so not a lot of time for the wheel to get abused. But hey, everybody has a choice in the experimental world so if some like air in the tire or solid rubber, who am I to say what is best. I am thinking of taking the breakaway pawls off my Maule and just keep it steerable only. In a partial ground loop, once you hit the 30 deg breakaway point, hang on momma, we goin' for a ride. The castoring wheel is really not needed for doing a 360 in an aircraft like the Avid/Fox anyhow. A little forward stick and a shot of power does the trick. Me, I don't play much on the black grass type runway.

[1avidflyer 1983]

There is a Maule 8" pneumatic tailwheel for sale on barnstomers for fairly cheap right now.  JImChuk

MAULE SFS-P8 • $100 • FOR SALE • Maule SFS-P8 W/ good tire. + shipping. • Contact Richard Benedick, Owner -  located Jack, AL United States • Telephone: 334 897 0279

[Willja67 144]

I asked this question above but didn't get an answer so I'll try rephrasing in multiple questions. 

How much travel is there in the tailwheel on a normal landing?

How much on a "carrier" landing that doesn't damage anything(except the pilots ego)?

I'm playing around designing a different tailwheel and just wondering how much travel the standard spring gear has. How much travel is required (with the right spring etc) to avoid damage to the plane? This assumes nothing out of the ordinary. I know i could conceivably come up with a design that could take a huge beating but the rest of the plane would already have fallen apart and it would probably be way too heavy. 

 

Edited 31 Jul 2019 by Willja67

[1avidflyer 1983]

I've made some pretty lousy landings, and have never hurt the rudder except for one time when the wings were folded and the back of the plane fell off something, and the other time when a tail spring broke while taxing.  I would suggest you rebend your spring to the correct amount, and get a second leaf to add to it.  If you want, I could trace out the curve of the Kitfox tail spring and send it to you, then you will have a pattern of sorts.  Send your address in a PM if you like.  Sorry I didn't answer your distance of movement question again.  JImChuk

[Turbo 162]

I asked this question above but didn't get an answer so I'll try rephrasing in multiple questions. 

How much travel is there in the tailwheel on a normal landing?

How much on a "carrier" landing that doesn't damage anything(except the pilots ego)?

I'm playing around designing a different tailwheel and just wondering how much travel the standard spring gear has. How much travel is required (with the right spring etc) to avoid damage to the plane? This assumes nothing out of the ordinary. I know i could conceivably come up with a design that could take a huge beating but the rest of the plane would already have fallen apart and it would probably be way too heavy. 

 

Here's a suggestion:  Glue or sticky-tape a styrofoam block to crush when the TW spring compresses, then go out and make a few landings.  First, make sure the foam doesn't rebound, or if it does, by what fraction.  This way you can get the answer you seek, but nobody seems to have the answer to.  The foam block needs to be thick enough that it doesn't undergo column failure, though.

[Allen Sutphin 705]

I dropped mine from about 3' (uncovered) and the tailwheel flexed up approx. 1". But I have the double leaf spring so a single leaf would most likely flex 2" or more. My rudder to tail wheel clearance is 4".  But I also have the stiffner plate (black plate) which spreads the load over a little more area. Wouldn't recommend anybody try dropping a single spring from 3'.  I did this to test the added ears I welded on. On a light aircraft, it shouldn't be a issue, unless you really grease her in.

[Willja67 144]

I asked this question above but didn't get an answer so I'll try rephrasing in multiple questions. 

How much travel is there in the tailwheel on a normal landing?

How much on a "carrier" landing that doesn't damage anything(except the pilots ego)?

I'm playing around designing a different tailwheel and just wondering how much travel the standard spring gear has. How much travel is required (with the right spring etc) to avoid damage to the plane? This assumes nothing out of the ordinary. I know i could conceivably come up with a design that could take a huge beating but the rest of the plane would already have fallen apart and it would probably be way too heavy. 

 

Here's a suggestion:  Glue or sticky-tape a styrofoam block to crush when the TW spring compresses, then go out and make a few landings.  First, make sure the foam doesn't rebound, or if it does, by what fraction.  This way you can get the answer you seek, but nobody seems to have the answer to.  The foam block needs to be thick enough that it doesn't undergo column failure, though.

that's a good idea. Unfortunately i don't have the tailfeathers on at the moment and it'll be awhile before they go back on. 

I dropped mine from about 3' (uncovered) and the tailwheel flexed up approx. 1". But I have the double leaf spring so a single leaf would most likely flex 2" or more. My rudder to tail wheel clearance is 4".  But I also have the stiffner plate (black plate) which spreads the load over a little more area. Wouldn't recommend anybody try dropping a single spring from 3'.  I did this to test the added ears I welded on. On a light aircraft, it shouldn't be a issue, unless you really grease her in.

Thanks for the info. I didn't think there could be very much travel especially with the rudder so close. I imagine there'd be more when full of people and baggage but still not much or the rudder gets damaged. 

[Allen Sutphin 705]

A heavy plane would probably benefit from a double spring or maybe a heavier single spring. Pilot technique has a lot to do with it along with the strip surface conditions. I would guess that rough conditions would also benefit from a dual fork tail wheel housing, along with a pneumatic tire. But, what the heck do I know.