New Super Cub suspension system

[geoffmeyer]

Since my last post, I've put about 70hrs on Frankenmaule. The gear has been through some rough stuff recently and handled it very well, although I have mixed feelings about its overall design. Although I am biased because I've put so much time into it, I'll try to be as objective as possible...

Negatives:
* Adds drag and hurts cruise performance.
* Scary on the ground when taxiing with a strong/gusty crosswind.
* Heavier than stock gear
* Gear scrubs (lateral movement) on landing & takeoff, probably adding to tire wear.

Positives:
* Very plush and forgiving on landings.
* During gusty wind conditions, drive the plane to the ground with no flare (no resulting bounce). Spot landings are easier under these conditions.
* On takeoff, gear starts at relatively low aoa, then quickly extends to a high aoa as wings create lift - allows for better visibility.

Overall, I think this landing gear has been superior under certain conditions, and inferior under others. As they say, everything is a compromise - and my biggest compromise is that Frankenmaule is more sensitive to strong crosswinds when on the ground.

Here is a link to some photos & videos that I recently added from a trip to Moab, UT. Elevation is roughly 5000msl.

http://www.supercub.org/photopost/showgallery.php?cat=500&ppuser=1360

You can sort of see the gear working on the three videos.

Geoff

 

[jimboflying]

Geoff.

What I have noticed with my Racerunner airstrut system is that the suspension is in the airstruts rather than basically in the tires. I had what I thought was too much droop down of the tires so I added some internal shims to block some of the downtravel. That helped alot so the tires were more vertical for landing. With the pressure I have in the struts I find I only use about 2-3 inches of travel during landing or takeoff on the sod. Thanks for sharing the cool videos in a wonderful place.

 

[geoffmeyer]

Jimbo,

I like the idea of internal shims. On a "normally firm" landing, I'd say I use at least 8 of the 18". I've also noticed a big difference with oil volumes. Running about 205ml gives the shock almost a 2:1 compression ratio - making it extremely plush, but also very "tippable". Running 275ml gives it a 5:1 compression ratio, which is a bit too progressive. Have you experimented with the different compression & rebound dampening?

Geoff

 

[jimboflying]

I have not changed the rebound rates or oil quantities other than by adding 2 one inch shims to each cylinder. I ordered them directly from Racerunner. Without removing oil the shim addition does change the stiffness. I am also running 500 psi nitrogen in each at this time.

 

[Fortysix12]

After 200 hours on a spring type gear I am ready to build an experimental solution. Read every post hear and we were off to a good start. By any chance does anybody have a the compression load when sitting on the gear. Specifically the load between the cabane V and the gear. I may have to build some kind of hydraulic load tester for drop testing. Is there any interest to get a cooperative started hear for the design and production for experemental Super Cubs. There's strength in numbers here. Those of you with 31's or equal on a standard cub hydrasorb gear, do you bounce more with those tires, specifically should dampening testing be done on or off those tires. I don't own a pair but wondered if they contributed to the problem of bounce or actually were part of the solution sort of a combined component approach.

 

[Rob Murray]

One of the nice things about S.Cub gear is that it is external and this makes wheel to float to wheel changeovers about as easy as it gets. Certainly better than a Husky or Pacer set up.

 

[Fortysix12]

Here's some information we need.

Stock cub Hydrasorb as the bench mark or starting point.

I need to know how many pounds of deflection force it takes to maximize full travel on a stock Heavy duty bungeed shock strut assemble. Anybody have this info?
I've read that the coil type spring gear requires 1000 lbs for .7 inch deflection,
Which brings up another question. What has CC done to the original hydrasorb to handle 2300 Gross weight?

 

[nanook]

Fortysix12, Wipaire had to do the drop test for their 2000lb stc...Someone there might have that info. As for the large tire question, I never bent my gear attach bolts on 30" airstreaks....I always bent my gear bolts on straight skiis....The big tires make a huge difference in how hard the hydrasorbs work....I think you would have to be out of your mind to put 2300lbs in a cub....I don't care what mods you incorporate....I'm mostly doing off airport cub stuff....2300lbs would be a disaster....

 

[Fortysix12]

nanook,

You are probably correct on the 2300 LB. but , Cub Crafter's Top cub is rated at 2300 pounds. Seems like they would of had to change the static load and shock aborbing load for the additional stress, or did they? Reasonable assumption don't ya think. I will look into Wipaire's info.

 

[Amy]

nanook,

You are probably correct on the 2300 LB. but , Cub Crafter's Top cub is rated at 2300 pounds. Seems like they would of had to change the static load and shock aborbing load for the additional stress, or did they? Reasonable assumption don't ya think. I will look into Wipaire's info.

I would presume CC had to do the same drop testing that we did for our certification. I would have to confirm, but I believe the drop testing was done with 31" Alaskan Bushwheels. We have numbers somewhere, and the AOSS stood up just fine. The failure number on it was something ridiculous--I think over 3,000 lbs (aircraft loading) with a 4G drop (ouch!). We did have to beef up the Cub gear to make the 2,300 lb number.

If you want to see what a 4G (approximately) drop is, check out the test Mythbusters did--they actually did the test themselves, sitting in airline seats.

 

[Fortysix12]

I would presume CC had to do the same drop testing that we did for our certification. I would have to confirm, but I believe the drop testing was done with 31" Alaskan Bushwheels. We have numbers somewhere, and the AOSS stood up just fine. The failure number on it was something ridiculous--I think over 3,000 lbs (aircraft loading) with a 4G drop (ouch!). We did have to beef up the Cub gear to make the 2,300 lb number.


Please forgive me Amy, but who is "We" "for our certification"?

 

[Amy]

I would presume CC had to do the same drop testing that we did for our certification. I would have to confirm, but I believe the drop testing was done with 31" Alaskan Bushwheels. We have numbers somewhere, and the AOSS stood up just fine. The failure number on it was something ridiculous--I think over 3,000 lbs (aircraft loading) with a 4G drop (ouch!). We did have to beef up the Cub gear to make the 2,300 lb number.


Please forgive me Amy, but who is "We" "for our certification"?

No worries! We=Dakota Cub. I am interning with them right now for my marketing/aviation management majors Having a blast, and they are a great company!

 

[AntiCub]

Also good for research is the system used by the Bearhawk folks, designed by Bob Barrows. Oil dampened die springs. I was thinking how it could apply to the Cubs:

http://www.mybearhawk.com/fuselage/gearstrut1.html

The Oil damped spring system used on the Bearhawk and Patrol isn't anything new. It's the same system Maule and I belive some of the early Champs use.

Phil

 

[Altair]

Different Solution

Hi guys, I stumbled upon this thread while looking for something else.
I then registered to propose my solution to the rebound damping problem.

What I'm thinking about is very simple to make, and will allow you to have any amount of rebound damping you like. It will be invisible once installed, and works with the standard bungees. It is something I designed some time ago to put on an aircraft I'm working on.
Please refer to the drawing.

It uses the actual shock strut in combination with its inner guide tube.
The lower end of the shock strut is plugged so it can hold oil.
The inner guide tube is made long enough to go down close of the end of the shock strut.
A small check-valve is machined to fit on the end of the inner tube. It is retained by a small spring attached to a small floating pin inside the inner tube.
The inner tube is filled with oil for about half its length.
On the drawing, it is shown horizontally, but consider the right end as the bottom, near the wheel, and the left as the upper end that go to the bungees.

The way it works is that on the compression stroke, the inner tube slides out from within the shock strut. This creates a vacuum at the bottom which pulls oil down through the valve, to fill the cavity.
On the rebound cycle, the valve closes from the pressure of the oil trying to escape.
The only place it can go is through a couple of small bleed holes drilled in the valve itself.
The size of these holes determines how slow the gear will come back to rest.

The drawing shows small tubes, 1/2" for the shock strut, and 3/8" for the inner tube, because the aircraft I'm designing is an ultralight. On the Super Cubs, the shock strut is probably 5/8" or 3/4" but the principle is the same.
This concept is obviously applicable to a lot of light aircraft, don't you think? It's super easy to make and costs almost nothing in materials.

A small O-ring could be placed at the end of the inner tube, to prevent the oil from creeping out between the two tubes, but I do not see this as a problem, because the oil is under pressure for only a short time on the compression stroke, and then the pressure falls immediately. Anyway it will be very difficult for any oil to flow up in the narrow clearance between the two tubes and drip at the top, near the bungees. The only time this could happen is if you're taxiing for a long time on rough terrain, which would create a lot of successive pressure cycles in the strut, but again, the loss of oil will be minimal.
And at rest, there is no pressure inside, so no oil flows out.

So there it is, I hope you will try it and report back here with the results.
Tell me what you think.

Guy Sirois

 

[qsmx440]

Hi guys, I stumbled upon this thread while looking for something else.
I then registered to propose my solution to the rebound damping problem.

What I'm thinking about is very simple to make, and will allow you to have any amount of rebound damping you like. It will be invisible once installed, and works with the standard bungees. It is something I designed some time ago to put on an aircraft I'm working on.
Please refer to the drawing.

It uses the actual shock strut in combination with its inner guide tube.
The lower end of the shock strut is plugged so it can hold oil.
The inner guide tube is made long enough to go down close of the end of the shock strut.
A small check-valve is machined to fit on the end of the inner tube. It is retained by a small spring attached to a small floating pin inside the inner tube.
The inner tube is filled with oil for about half its length.

On the drawing, it is shown horizontally, but consider the right end as the bottom, near the wheel, and the left as the upper end that go to the bungees.

The way it works is that on the compression stroke, the inner tube slides out from within the shock strut. This creates a vacuum at the bottom which pulls oil down through the valve, to fill the cavity.
On the rebound cycle, the valve closes from the pressure of the oil trying to escape.
The only place it can go is through a couple of small bleed holes drilled in the valve itself.
The size of these holes determines how slow the gear will come back to rest.

The drawing shows small tubes, 1/2" for the shock strut, and 3/8" for the inner tube, because the aircraft I'm designing is an ultralight. On the Super Cubs, the shock strut is probably 5/8" or 3/4" but the principle is the same.
This concept is obviously applicable to a lot of light aircraft, don't you think? It's super easy to make and costs almost nothing in materials.

A small O-ring could be placed at the end of the inner tube, to prevent the oil from creeping out between the two tubes, but I do not see this as a problem, because the oil is under pressure for only a short time on the compression stroke, and then the pressure falls immediately. Anyway it will be very difficult for any oil to flow up in the narrow clearance between the two tubes and drip at the top, near the bungees. The only time this could happen is if you're taxiing for a long time on rough terrain, which would create a lot of successive pressure cycles in the strut, but again, the loss of oil will be minimal.
And at rest, there is no pressure inside, so no oil flows out.

So there it is, I hope you will try it and report back here with the results.
Tell me what you think.

Guy Sirois

I used your quote because this is such a long thread. I think the problem would be the thickness of the tubes. Your talking a lot of pressure and so splitting the tubes or bulging is a very real possibility. It would require a lot of testing and ruined tubes before fitting to an aircraft and probably the engineering will dictate something that resembles a hydrosorb by the time your done.

 

[Altair]

Hi qsmx,
no, there is not a lot of pressure inside the tubes, because the only pressure generated comes from the bungees trying to pull back the gear at the rest position.
On a Cub, the bungees must generate a pull of about 1000 lbs, so that would be applied to an area of .3 sq. in. inside a 3/4" dia tube, equals around 330 PSI. For 4130 steel, and even aluminum,it's no problem.

I don't know about Hydrosorb, I'll do a search on that.

Guy

 

[Altair]

OK, I checked what a Hydrosorb is, it's just a different suspension system, to replace the bungees.
It might have a bit of damping, both on the compression and the rebound stroke, due to internal friction, but it's not a damping system per se, like what I'm proposing.
The system I propose adds damping to the rebound cycle, but not to the compression cycle. And I believe this to be a better behavior.

Guy

 

[azdave]

http://tiny.cc/qig0g -- check out the landings they get progressively more aggressive as the video progresses. No bounce.

http://tiny.cc/l6zwn -- watch the landing gear on the Carter during this test. Also impressive. Test is at 2:36.

 

[Altair]

Thanks for the vids, azdave.
Very interesting. This kind of landing gear reminds me of the Fiesler Storch of WWII.
The gear is made with lots of compression damping, and is made to stay completely compressed under the weight of the aircraft.
This is quite different from the standard airplane use, where you want the gear to stay mostly extended under the weight of the aircraft, and ready to absorb the bumps while taxiing.
It would of course be adaptable on a Cub, but in flight the gear would hang down 2 to 3 feet lower than its actual position under the aircraft... Would not be pretty LOL

 

[Gordon Misch]

Needs one more zero on the pressure - -

 

[Altair]

Hi geezer,
here are my calculations;
the inside area of a 3/4" tube with .058" wall is .3 sq. in.
If we have a load on this of 1000 lbs, that's 1000 / .3 = 3300 ...Woops ! You're right! I calculated mentally and came one one zero short.
OK so 3300 PSI inside 4130 tubing is still within acceptable limits heh?

Thanks for the heads up.
Guy

Note to self: I have to stop trying to do mental calculus.

 

[Gordon Misch]

Heheh - That's why I give my students extra credit for catching me in an error on the board - stuff happens - they keep me on my toes! A joke in engineering school was something like this - Student: "aw, it was just a decimal error..." Professor: "Yep - there goes the bridge - -"

 

[Dave Calkins]

Anyone come up with something new to think about????? I had an original thought, (I thought) and see we talked about it once before.

For the ultimate rough terrain gear, I think we'll need to mount up differently from the original Cub style.

Anyone??

 

[ksecub]

We have not come up with the ultimate solution yet, I think the AOSS system is the best available today. I am going to order a set for my new Cub.

I don't think we should throw in the towel, it is a difficult task with the assumptions we have made.

I will starting thinking about it again after I install the AOSS.

Doug

 

[Super11XP]

I have wondered about a leaf spring that is between the cabane and the gear, so when you hit a bump the tension pulls the leaf spring straight. I have no idea whether it would work or not. Has anyone ever done it?

 

[Dave Calkins]

My worry about a leaf spring would be the lack of dampening.

Really, all we want here is better dampening and maybe more travel if we can have it. D

 

[swoeric]

Need some education. do the atlee safety cables require an stc to put on. I didnt know since these were essentially a backup safety device if they had to be certified when he created them

 

[DW]

Dave did you look at the system on my cub? about 300+ landing on it so far.

 

[Dave Calkins]

Dennis, sorry, I don't think I looked close. Any photos??

 

[DW]

Here's a couple pics of the proto type on my old cub.