Doubled Tailwheel main spring

[Forrest Walton]

Decided to change out the tailwheel springs and discovered the previous owner had doubled the main spring. Two main springs, one on top of the other.

Is this common? Should I replace with just one? It sure seemed to work fine for ten years:)

Forrest

[zero.one.victor]

How many leafs are there, on what year 170? Are you sure that it's two main leafs? What a couple friends did in the past to their 140's was to cut the second leaf off flush with the main leaf, then drill thru it & bolt the tailwheel thru both leafs. Two reasons:
1) stronger (no tailspring breaking)
2) increased angle of attack in 3-point attitude
I don't necesarily agree with what they did, shouldn't need 2 leafs for strength, & don't necesarily need more AOA. I also think that this mod puts a lot of shear action on the tailwheel attach bolt as the 2 springs are moving against each other ( or trying to) as they flex.
Maybe that's what's been done to your 170? I've seen it done to a B model once, it resulted in a noticable (to me) increased nose-up stance on the ground.

Eric

[Bruce Fenstermacher]

Forrest
My aircraft has the same arrangement you describe and as far as I can tell it's been that way for 15 years and several hundred landings.

I asked the same question about 6 years ago on the old yahoo site and the idea was thought by members of the forum at the time to be dangerous. The thought as I remember it was that the scissor action of the 2 main springs on the bolt passing though them would shear the bolt and you'd loose the tail wheel. Seemed to make sense to me except for a few things things.

One because of other tail wheel maintenance I was intimate with this bolt and there was no sign of damage. Second at least the Piper PA20 and converted PA22s and possibly other Piper models use this arrangement. The Piper airframe uses the same Scott 3200 and the airframe is about the same weight although the distribution of the weight I'm sure will be different.

Since it was thought to be a bad idea yet having no signs of impending doom I decided to monitor this arrangement and closely inspect the bolt and tail spring brackets. To this date I have found no problems.

The only negative I've experienced which I understand may happen with the stock arrangement is I frequently have to tighten the bolt. I include this check on my preflight.

I have flown several 170s besides mine with the stock set up, one on a regular bases and find no difference in the handing or stiffness of this spring arrangement over stock.

I'd think this would be a popular bush modification and perhaps some of our newer forum members can shed some light on this mod from personal experience.

The bottom line:
With this arrangement you would have to snap 2 springs or the bolt before losing the tail wheel. We all should know the single spring will snap with out warning in most cases so this seems to be a positive point.

On the BIG negative side is that like so many modifications, with out some basis of approval such as an STC or approved 337 using 2 main springs is illegal.

[Roesbery]

We've been through this before. The reason for doubling the main leaf is to avoid the bending that is common in the stock leaf with heavy loads and or landing on gravel runways. Back in the 70s' a number of 170s' in the Fairbanks area were setup that way. The problem is that there is not enough flexing when landing on rough surfaces with heavy loads. At that time several planes in the area developed cracks just forward of the tailwheel bracket assembly # 31 pg 49. Fellow I was working with had that problem. There are three main leaves that can be used that I am aware of one is 1/4" inch thick ( standard ) another is 5/16" ( heavy ) and 3/8" ( L-19 ). Having been through the bent and broken spring problem for a number of years in my early flying and buying springs pretty regular, I received a spring one time that had L-19 printed on it, 3/8" thick, I installed it and have not had any tailspring problems since and that was around 20 years ago.

[mvivion]

Roesberry,

Amen, I too have the L-19 spring, and it is a MUCH better arrangement. We did this as a field approval before such got shut off, but its possible others could get it done using a prior approval as a basis. Who knows nowadays?

Bottom line is though, this big spring leaf really sorts out all the problems with the wimpy spring leaves on the stock airplane. And, basis or no, I doubt there's an FAA guy out there could pick it up. The stock spring is a POS in my opinion, and the breakage rate would seem to support that notion.

Yeah, I know, George...

But Happy New Year anyway.


Mike V

[lowNslow]

The whole idea behind a tailwheel spring is to absorb landing and rollout shock. Seems it would be better to occasionally change out the main spring which isn't that expensive rather the adding to much thinkness to the spring and damaging the airframe.

[mvivion]

Sorry, I don't buy that one. My stock tail spring broke as we rolled the plane across a set of tracks going into a hangar. In other words, it was about to turn loose, and could just as well have done so out in the woods somewhere.

What's it going to cost you to get a tailwheel flown out to you, let alone the hassles of flight plans expiring, etc?

The early 180 and 170 rear ends are pretty similar. I don't know that they are identical, but they are pretty close in strength.

In any case, I doubt that putting a beefier tailspring in there is going to do any more damage than dropping the rear end onto the tail cone after the spring breaks. Sure you transmit a little more load into the airframe, but frankly, the stock springs are VERY weak.

Before the flak hits, it is worthy of note that I virtually NEVER three point the airplane, and I take good care of the tailwheel, so it's not abuse.

I just don't like things that are apt to break out in the woods and leave me stranded.

Mike V

[GAHorn]

Of course, the extreme example is to simply install a cast-iron rear end and forget all about springs.
Cessna did a good job on designing this thing. All we have to do is inspect and maintain it properly.
I recommend that the proper spring be used, and that it be inspected and replaced accordingly. I also recommend that the spring immediately above the mainspring have it's rear/lower edge radiused/polished to avoid it leaving a stress-riser on the mainspring. L-19's and similar aircraft also have a history of neglected springs breaking.

[N1478D]

Where my tailspring broke it did not allow the tail cone to hit the ground, in fact the broken spring that was still attached to the aircraft acted to keep the plane tracking straight as it dug a trench in the turf. The main spring broke at the end of the spring sitting on top of it. The damage was extensive though, rudder and elevators beat up from the tailwheel assembly banging around hanging by the steering springs. Change your tailspring frequently enough to ensure that you don't have this opportunity. It's a lot of work and expensive to rebuild a rudder. Just a taillight housing bracket is $80 for each side. The steering springs allowed the tailwheel to reach up to the upper skins, so 2 uppers, 2 lowers, steering springs, rebuilding the tailwheel, the taillight parts - it all adds up. I have seen double main springs, sure seems like that would add weight. A new spring seems plenty strong for around here in Texas. I think George has come up with a reccommended replacement time of 500 hours/can't remember the time.

[lowNslow]

Joe, I think you have the right idea. I would guess that most of the tailwheel springs that have broken have seen quite a few landings on them and have probably never been replaced since new. Seems a far better idea to just replace the main spring on some time interval. If you bang your tailwheel around a lot, then replace it more often.

[mvivion]

Its worthy of note that a LOT of Cessna 170's around are equipped with early Cessna 180 main landing gear.

Watch a 170 sometime taxiing on uneven ground, at gross weight. They waddle a LOT. By the same token, load your 170 up to gross weight, and load it to the center or near the aft limit of the CG range. Do this with a stock tailwheel spring, and you'll find that the tailwheel geometry isn't exactly as perfect as we might hope for.

I'm not saying the undercarriage is underdesigned on the 170, but if you get into any uneven stuff at all, stronger gear legs and a beefier tailwheel spring are worth considering.

If all you ever do with your 170 is fly 400 pounds below gross weight, from paved runways, with paved taxiways, forget it.

I can assure you that there have been a lot of 170 drivers call in from the bush for a "spare" tailwheel assembly and spring to get home with. And the consequent rudder damage to boot.

An L-19 spring is cheap, it works, and if you can convince your FSDO to field approve it, it can be legal. I for one, think its worth doing.

Again, if you don't work off airport, maybe replacing the stock spring at regular intervals is good enough.

On the other hand, there must be some reason all these 170's stand so tall on their gear up here....... Must just be the air.

Mike

[GAHorn]

For every action, there is an equal reaction.
I disagree with the opinion that beefier gear springs (either mains or tailwheel) is beneficial for a standard 170. I believe that Cessna engineering is excellent and the airplane was well designed from the beginning, and balanced for the model.
The 180 needed beefier gear because it's a heavier airplane with heavier engine, and longer prop. An otherwise unmodified 170 is best with it's original gear.
If you operate with heavier than original engine, or longer prop, out of rougher strips and need the prop-to-ground clearance....then a 180 gear is longer and will serve you well. But it's stiffer spring rate will transfer shockloads to the airframe accordingly. (A compact car with a truck suspension will not be a balanced machine.) The flexible, forgiving 170 gear was somewhat disconcerting to pilots accustomed to the stiffer gear of an Aeronca, Stinson, or Piper, but it was supposed to be "springy" to absorb the shocks of rough strips. Pilots who learned to operate it properly never had a problem with it.
Cessna did change the spring-rate after SN 25612, but the new shape served to dampen...not stiffen the gear.
Otherwise the aesthetics of a longer/taller or bow-legged gear are simply in the eye-of-the-beholder, in my opinion. For the original airplane, the original gear is a good one, and any gear on any airplane will need regular maintenance occasionally. Replacing a tailwheel mainspring which has become 50 years old with stress-risers engraved into it from the spring above is a good maintenance practice. Replacing it arbitrarily with a spring designed for a heavier airplane is not necessarily a good thing. (Being no engineer, I do not doubt the certificated work of those who are.)
I operate my 170 predominately from grass runways, and the tailwheel mainspring was replaced about 5 years ago, and is serving well. If, however, your tailwheel mainspring has been "improved" by a previous, unknowlegeable owner to bent it "to a better shape" or overloaded it with abusive landings, or otherwise used arm-chair engineering on it,...it'd be an excellent idea to return it to factory-new condition with a standard, 170 spring. Any other modification is an entirely different issue, and likely not as good as the original. My 2 cents.

[rudymantel]

Wheel landings prolong tail wheel spring life.
Rudy

[thammer]

The 180 needed beefier gear because it's a heavier airplane with heavier engine, and longer prop. An otherwise unmodified 170 is best with it's original gear.
If you operate with heavier than original engine, or longer prop, out of rougher strips and need the prop-to-ground clearance....then a 180 gear is longer and will serve you well. But it's stiffer spring rate will transfer shockloads to the airframe accordingly. (A compact car with a truck suspension will not be a balanced machine.)

Hi George,

Regarding 180 gear on a 170, I'm thinking that the taller gear puts more weight on the tail wheel and I'm wondering how big the increase would be. Do you know of any approximate numbers for the weigth change on the tail wheel? How much longer is 180 gear than 170 gear, 2 or 3 inches?

tye

[GAHorn]

I'm thinking that would be a trigonometry problem and that's another area I do not dabble. But I'd venture the increased weight would be negligible....certainly no more stress than ordinary flight operations or taxying uphill would impart.