Starting a new thread instead of hijacking Phil's excellent "SQ2'n around" thread:

Zzz wrote:Blackrock, I think the weight of the BH tail will be a consideration for Dan's system. Those shocks are adjustable with air pressure but all the forces are a little amplified, though the 3200 and its clones don't really complain as much as the hardware holding them together and to the airframe. I will be curious to see how they hold up to side loads and stress-producing impacts. The spring rate is progressive on the shocks I believe but so is a leaf stack. I wonder how the curves compare.

Thanks Z,

All great points, especially on heavier AC. Have you seen these in person? I really like the idea and think TW suspensions are a great concept. Always takes a while to perfect new designs though. I'm thinking these have been vetted on more than a few cubs, including Bobby's, of course. As Maverick pointed out to me in some texts, our temperatures and altitudes around here vary a lot and the pressures in those shocks will as well. Couple that with a heavier plane and the uncertainty goes up as well.

It would be really good to hear from those with first hand experience. I looked on SuperCub.org but my search only turned up Bobby's product post. Anyone have more info?

I weighed the Bearhawk tail tonight on the ground while sitting on 29's. The TW weighed 199 lbs pretty much unloaded and with 15 gallons on board. Adding me in the left seat brought it to 230lbs. Some Q&D W&B calcs showed the weight would range from 230 to 450lbs depending on weight and loading. A typical scenario with 25 gallons, 2 people, and 40 lbs of cargo puts the TW weight at 300 lbs.

Temperatures here will vary 50 degrees from sunrise to afternoon and elevations vary from 5 to 10k feet so the pressure changes in those gas shocks will be significant as Maverick has pointed out to me.

I'm always one for pushing invention and innovation, as some may have noticed. I embrace the idea of EFII, etc. I really like the idea of tunability with suspension, even though it does add complexity and another mechanical system that can fail. But if you've ever felt the difference between damped shock/strut and a metal spring leg like a Cessna, you know this is huge for STOL landings and the ability to keep it on the ground without bouncing. A leaf stack or stinger sure is simple and tough to screw up.

The big deal for the tailwheel is that tailwheels are really subject to hard forces that get transmitted into the tubes and joints of the aft fuselage. A leaf spring stack that's heavy enough to support a larger aircraft is not going to do the best job of smoothing out the sharp hits. In the really rough stuff I think most guys are wheeling on and letting the big bushwheels do the work, and the tailwheel is likely suffering the most during taxiing over rough stuff. The biggest victim is the hardware, and even though AN stuff is pretty elastic, that's where it often fails. Battson's spring retainer clamp failed when the AN4 or AN5 bolts (which did you use?) developed the slop and began shock loading. Battson will likely correct me there.

Anyway, less sharp forces from some nice progressive hydraulically dampened shocks should keep the tail end tighter. I'll be curious to give it a try.

/ramble

Bobby said he's going to put together a report for us in January.

I believe the DC3 and the Fairchild 24 have air shock suspension tail wheels and they were designed for rough strips but I don't think they were design for where we are playing. Years ago I built a number of offroad stadium cars using the Fox Airshox system and they worked really well. They did change as they heated up but there was virtually no difference from day to day on the pressure setting. During a race they would get really hot and change but we are not doing that kind of operation in most cases. Watching different tail wheel springs and stinger systems work in the rough stuff it looks like a flexible stinger allows more movement in the lateral direction and I think this takes some strain off the air frame. A stinger with an air shock will have a single attaching point and transfer all the energy to a single point on the frame. There will be some very serious side loads on the frame at that point.

I'll add some of my thoughts (in random order for sure) to this discussion.

Unless you are doing hard core "big rock long props" off-airport flying (or are serious about STOL competition) your aircraft's stock suspension is most likely more then adequate for your needs. My comments will be directed to those few crazy pilots who get a perverse joy from using their airplane as though it was some sort of rock crawler.

All "inflight non-adjustable" aircraft suspension, be it leaf springs, stingers, single spring, bungees, AOSS, or tunable nitrogen filled shocks, are all compromises between suspending an aircraft's empty and gross weight.

Tunable nitrogen filled shocks are a clear winner because (.... [plagiarizing my friend MTV] .... wait for it .... ) they are tunable! Those obsessed with saving grams of weight can set the suspension softer then those who load the aircraft down with creature comforts. Or pressures can be adjusted for competition then changed back for rock crawling.

Notice I said nitrogen filled, not compressed air filled. Just like you would never put compressed air in your bush wheels, you don’t want to put compressed air in your nitrogen shocks. Compressed air introduces oil, particulates, and lots of moisture (which is corrosive and affects pressure with temperature change).

In addition compressed air contains 20% oxygen which is also corrosive and an oxygen molecule is four times smaller then a nitrogen molecule, so is more likely to escape past seals (or through the rubber of a tire), resulting in a decrease in pressure. Oxygen molecules are so small that it is normal for air filled tires to lose 1-3 PSI each month from "permeation" which is the normal process by which the oxygen molecules in compressed air seep through a tire's carcass. Nitrogen filled tires, on the other hand, typically lose no pressure from permeation, even over many months of use.

So how about nitrogen shock pressure changes with altitude? Not as important as you'd first believe. The air weighing down on us from the edge of space to sea level exerts 14.7 PSI. At 10,000 feet it exerts 10.1 PSI. Currently the nitrogen shocks on my front suspension are set to 210 PSI at less then 1,000 feet. If I find myself playing on a 10,000 foot high bench the nitrogen shocks go up in pressure from 210 to 214.6 PSI. Am I capable of noticing a 5 PSI difference? Not unless I'm at the edge of the shocks performance envelope - so not really.

I am capable of noticing the tire pressure difference from 4.5 PSI to 9.1 PSI, but I’d be inclined to just leave it be for the reasons to follow.

Temperature of course also affects PSI. About 1 PSI for every 10 degrees Fahrenheit change. For discussion let’s assume a 40 degree temperature swing from sea level to the 10,000 foot high bench. That’s a 4 PSI difference. But it’s going down not up. So the altitude pressure increase has been compensated for by the temperature pressure decrease. And that’s for both my bush wheels and my nitrogen shocks. Cool.

So I think we can agree that temperature and/or altitude changes affecting nitrogen shock (and bush wheel) pressures isn’t really a big deal at all. If you are still concerned carry a small paintball canister filled with nitrogen and adjust the pressure each time you land. Just don't expect me to hang around while you do it.

Zane made an important comment about the hardware that holds the suspension on. It is a weak point to be sure. So much so that once a year I change out all the bolts and nuts on both the main and tail suspension. And if I’m dropping in (like in STOL competition) I’m periodically checking the tightness of the nuts. If I find one loose I know the bolt has stretched and it gets replaced ASAP.

Options

Tony at TK made this one


And Dans


Side loading is the question on all of them for me

Akt

Thanks Phil and Kevin,

All great information you provided. I pulled the trigger on the Dan's so will be looking forward to getting it installed and will post after seeing how the Bearhawk likes it. Fortunately, I haven't had too much issue with side loading of the spring, and that is somewhat surprising, so we'll see how the shock set-up does. Most of my landings are wheel landings, with the occasional 3-point. As Phil pointed out taxing in the rough stuff takes it's toll.

I'm on my 3rd TW spring in 1,000 hours and re-arched the the first one 2 times and the 2nd one once and then installed a stiffer 3rd spring which is working really well. That's a re-arch or new spring every year, but between rough terrain landings/taxing and heavy loads, plus the occasional idiot behind the stick f'ups that isn't too unexpected, I'm really hopeful the shock system will alleviate a lot of that strain on the airframe.

Zzz wrote:The big deal for the tailwheel is that tailwheels are really subject to hard forces that get transmitted into the tubes and joints of the aft fuselage. A leaf spring stack that's heavy enough to support a larger aircraft is not going to do the best job of smoothing out the sharp hits. In the really rough stuff I think most guys are wheeling on and letting the big bushwheels do the work, and the tailwheel is likely suffering the most during taxiing over rough stuff. The biggest victim is the hardware, and even though AN stuff is pretty elastic, that's where it often fails. Battson's spring retainer clamp failed when the AN4 or AN5 bolts (which did you use?) developed the slop and began shock loading.

Hole in one, AN5, cracked in the threads due to a combo of bending strain and cyclic stresses.

After fixing that, the whole hat section started to warp and fail around ~350 landings off airport. Surprised nobody else reported this yet.

I've since gone to a much heavier system including 1/4" thick steel retainer plate, 1" wide, in a style often found on Ag planes - this is also bending - about 1/32" at midspan so far. You got it in one Z - hardware is what gets killed - which is the ideal result. I like to have disposable hardware in that area, rather than repairing the super-structure. The whole maintenance programme is a damage mitigation approach for the fuselage, let everything else fail first.

I think you got it right with the comment about taxiing too. Repetitively landing 3-point on rough off-airport surfaces is likely going to break something in the tail wheel assembly at some stage. I did. Since then, the tailwheel is only on the ground for taxiing, but that's been enough to bend that steel plate I talked about. Taxiing on really rough rocks on a regular basis has the potential to do a lot of damage, gradually.

Personally, my other concern is rocks being thrown up and impacting the tail wheel assembly. Whatever else you hang back there needs to be robust, something you couldn't destroy with a slag hammer if you really tried... If it fails that test, I think it's not going to last back there, assuming you're doing a lot of off-airport landings on rocky surfaces.

2c

I build off road race suspension for a living and rebuild and tune these types of dampeners daily, so this is what im basing my thoughts off of. The tune-ability of these style shocks is incredible, and I have always been disappointing with the dampening (or lack there of i.e bungee gear) on aircraft. This made me start looking looking into different applications for aircraft. The most popular adaptations I have seen use an air spring dampener for obvious weight reasons. The issue with that is the same as I have run into with rock crawlers, Rock strikes to the stanchion will damage and deflate the shock leaving you in a crappy situation. I have been looking into using Ti springs on a traditional coil over as a compromise between weight and reliability as well as composite guards for the air spring dampeners. There is great potential but its still early in the game and it will take some time to figure out what works best. Just my 2 cents on the subject.

As Blackrock pointed out the tail on a BH can be quite heavy. It would be great to have some sort of softer suspension back there to protect the tail. The tail spring has to be so stiff to support the plane when fully loaded that it is quite harsh when the plane is empty. That is one of the reasons I put a 400-4 TW tire on my plane. I'm not a huge fan of air shocks for the reasons Battson and Districtfab pointed out. After I get my plane flying my going to mess around with developing a bolt on coil over based tail spring suspension.

The cool thing about Dan's design is the retro fit for a standard 1.5" spring with the Cub style leaf receiver on the tail of the fuselage. No major changes required. Bolt it on.

I can say the BH tundra tail wheel makes a HUGE difference over the 10" wheel, on very rough surfaces. That mostly comes from the operating pressure (20 psi vs 60-70 psi). However....

My biggest concern with my current arrangement is side loading, when I do spot-turns on very rough surfaces. Enivitably you are moving the tail fast, because you need to carry momentum (unless you want to suck stones through the propeller). That feels like the time and place where the most critical stress is exerted on the current system. It's not designed for side loads, the spring has zero give left and right. So all the stress from side loading goes straight into the tailwheel mounts, which are the only thing that can bend / wear before the fuselage feels the stress.

This is one reason why I was interested in a stinger. However, they are subject to other maintenance concerns; not to mention they are a totally un-damped system.

Iron Design has a direct bolt on stinger kit for the Bearhawk and others. I see he also offers a Titanium stinger. I seem to remember reading that Titanium gear has less bad rebound characteristics. I believe it was an article on the XP mods Titanium gear. Who knows.
http://www.irondesign-airparts.com/Plane%20Parts.htm

175,

Thanks for that information on Iron Designs. I had heard something was in the works, but that was some time ago and I haven't kept up. I may have to try one of those, too at some point.

I think it only applies to the Patrol?

Kevin,

That's a really clean TW suspension on your Cub. I like it. I thought about trying to retro fit something like that, but with mine covered and flying, it would have taken too much effort. I agree, time will tell with the side loadings. At least the knowledge base is increasing and we have the benefit of the experimental classification to, well, experiment. It will certainly be interesting to see where this leads and how much more suspensions will have advanced say 5 years from now.

I believe Stoots Aviation in Alaska was also putting together a stinger set up for the c172/175 tail wheel conversion he has. He said it would also apply to the c170's. I do not know what the status is on his program but it might be worth phoning him. When I told him I was putting an LS3 in a c175 he got really pissed and said he wouldn't sell me one because I was ruining a perfectly good 175 frame. OH well, next page.

Christmas came early today. My T3 suspension system is here.

Today I shot a before video. And yes the waterfall is in it but from vantage points never seen before. Here it is.



https://vimeo.com/149720442

Great before footage Phil. Thanks for posting. That waterfall fly-over looked cool!

blackrock wrote:Great before footage Phil. Thanks for posting. That waterfall fly-over looked cool!

Thanks blackrock.

You should come up to Graham one weekend and we can go out and play on the river