[DanH]

I like tweaking my airplanes. Thought this particular tweak might be of interest to others.

I'm using an Aviation Products tailwheel on my RV-8:



The connection between tailwheel and rudder horn is more-or-less conventional; pull cables and drawbar springs. I like to use nicopressed cables with thimbles and shackles instead of chains (because they're bulletproof and because I have the cable tools), but that's beside the point. This is a report on springs.

The first set came from Aircraft Spruce:

http://www.aircraftspruce.com/catalog/lgpages/comp_springs.php

The airplane was overly sensitive to pedal input at taxi speeds, but that was no big deal. However, crosswind runway work was tricky. It was necessary to remove most of the rudder input momentarily just before allowing the tailwheel to touch, then reapply a smaller deflection. Not doing so resulted in a swerve when the tailwheel made contact. The springs were obviously too stiff, and didn't allow the tailwheel to caster straight with the runway while maintaining some aerodynamic rudder input.

First step was to quantify the springs in use. A pull with the electronic fish scale found the above springs to be about 65 lbs for 1/2" of deflection, far stiffer than I would have guessed.

A little searching found these (and there are other sources):

http://www.reidsupply.com/Detail.aspx?itm=CDBS-7052

Same 4" long but 50 lbs per inch, about half the stiffness of the originals. Made up a new cable set and went off to fly. Huge improvement. Now the rudder dominates directional control when moving down the runway with any real velocity. Taxi became very stable, but close quarter maneuvering was not affected. After 5 hours or so I'd estimate a spring rate around 70 lbs per inch would perfect for this tailwheel, but I'm not going to change.

Anyway, these spring values may help someone. The standard Vans tailwheel may incorporate a bit more trail (distance from the point where the swivel axis intersects the ground, to the tire contact point). If so, you may want a slightly stiffer spring rate.

I have about 800 hours TW in all kinds of airplanes. The original setup wasn't dangerous or uncontrollable, but it wasn't optimum either. Sometimes it's a real surprise to see how something as mundane as a spring swap can change some aspect of an airplane's performance. We enjoy the right to tweak our experimentals, so go ahead, make it perfect.

 

[hydroguy2]

Nice pirep Dan.

 

[jimgreen]

Hi Dan. I'm not airborne yet but have bought a Rocket steering link. Have you had a chance to try one? Doesn't look like they can be tweeked.

 

[DanH]

Jim, no, I've not tried any brand of single-sided tailwheel link, but the same spring rates should apply given similar arm lengths.

 

[rvmills]

Hi Dan. I'm not airborne yet but have bought a Rocket steering link. Have you had a chance to try one? Doesn't look like they can be tweeked.

Jim,

I have a Rocket Steering Link, and find it to be very smooth and contollable in a variety of wind conditions. Inside that black shrink-wrap on the Rocket Link are two springs separated by a roll pin, so there is some give.

I don't know how the JDAir link or the Silver Bullet link function, so I can't comment...probably some similar give as well.

On the Rocket Link, should it feel too stiff, I guess you could change out the springs (but better ask Wayne...not sure if various spring stiffnesses are available).

FWIW, I've never felt like I had to back off the rudder input at touchdown and re-apply after lowering the tailwheel, as Dan describes, but his report is good info, just the same.

Caveat, the Rocket Link is all I've flown with on my RV, so no comparison to give...its just been good to fly with. (Would like to fly some RVs with springs...but that's just me trying to mooch more RV flying...all the guys up here that let me fly their planes have wheels up front...I keep the stick back on them too! )

Good report Dan!

Cheers,
Bob

 

[Sam Buchanan]

Hi Dan. I'm not airborne yet but have bought a Rocket steering link. Have you had a chance to try one? Doesn't look like they can be tweeked.

I've flown the Rocket link on my RV-6 for many years and have been completely satisfied with its handling. Prior to the link I flew the standard chains--I can't tell any significant difference in the Rocket link and chains that are properly adjusted (very little slack).

I've never noticed any need to make any rudder pressure adjustments as the tailwheel departs or arrives on the runway. Maybe I'm doing it subconsciously...

 

[mike newall]

Tailwheel -

I'm guessing these are the the guys from Ojai in California.

If so - used their tailwheel on 4 aeroplanes now, very good indeed.

Springs -

We are still on standard Van's tailwheel and toilet chain steering, however, when it warms up enough to work on the aeroplane, I will be fitting Tail Lynx springs which look all together better.

Sort of ties in with Dan - swaged cable and a firmer spring.

 

[DanH]

Note the short trail dimension with the AP tailwheel.....



.....compared to a Vans tailwheel or a Bell tailwheel:



The AP's short arm provides less force to extend a spring and allow the fork to caster straight with the rudder displaced. The AP would also steer with less pedal pressure during taxi.

The longer arm of the Vans or Bell would allow it to caster while using a stiffer spring.

 

[scsmith]

steering axis slope?

Hi Dan,

In your picture of the AP tailwheel, it looks like the tailspring rod is nearly horizontal, causing a much greater slope to the steering axis. Is that orientation correct as it would sit on the ground on your airplane?

It looks like even with the steering axis vertical, the AP would have less trail than the Bell or Vans, but not quite as short as your picture implies. If the steering axis really is sloped that much, its because of the angle built into the pivot block (can't think of a better name for it). Do they sell them with different angles?

 

[DanH]

It looks like even with the steering axis vertical, the AP would have less trail than the Bell or Vans, but not quite as short as your picture implies.

Good catch Steve....exactly right, and a poor illustration on my part. I'd estimate one third less trail for the AP.