Oleo's on larger models

[Andy J]

In the process of re-working a 84 inch Sparrowhawk wing to add flaps and repair the odd ding to the structure given Seagull Kits balsa is like soft butter.

Whilst the undercarriage has been modified by a previous owner to be a double wire strut thinking it would be far better to cut these off and use a pair of HK sprung oleo's for each wheel.

Given landing forces are equal and opposite, will a sprung oleo be better or lead to a significant bounce back into the air on touch down?

Edited By Andy Joyce on 19/05/2020 17:41:06

Edited By Andy Joyce on 19/05/2020 17:41:53

[Jon H]

I dont have any bounce issues with my oleo equipped models as they are all pretty porky and those two little springs have quite a bit of work to do if they want to rebound all that mass

[Andy J]

Good point Jon. Is there any real benefit from adding olo's then?

[Cuban8]

Don't forget that wheels, and in particular the quality of the tyres, will play a big part in how an undercarriage will perform and will go hand in hand with how the rest of the undercart is sprung. Fit a wheel with a tyre that's as hard as iron and with heavier models the chances are that the remaining springing (or lack of) will cause all sorts of problems. Fit decent quality low-bounce wheels that are well damped and they can transform a model that is otherwise a tad 'lively' on landing. The car lads spend ages getting their suspension setups just right with springing, dampers, tyres etc - so just fit anything and forget it on a model aeroplane usually doesn't give ideal results.

On small and light models, the low mass involved usually doesn't have us run into problems and we don't normally worry too much if a model hops and skips about, usually not damaging itself even when flipped over - not so for something bigger and heavier and not so easy to repair.

[Jon H]

Posted by Andy Joyce on 20/05/2020 08:13:16:

Good point Jon. Is there any real benefit from adding olo's then?

Oh absolutely.

The oleos take that hard shock out of touchdown/bumps as the energy is absorbed in the direction it is received. With a wire leg, it absorbs energy by moving back and this not only makes the leg longer due to geometry but also moves the wheels back. This can make the model skip and bounce before ending up on its nose.

I also had a problem with my ESM P39 that was solved by an oleo mod. The model had an oleo on its nose leg but it was a very short travel with a very stiff spring as it was basically a repurposed main leg. Due to the balance (or lack thereof) of the model on landing you could catch the noseleg on the ground before the main gear and this would bounce the nose way up in the air leading to a stall. My solution was to modify the leg to double the travel, and then fit a softer spring ahead of the stiff one. The weight of the model now just about half compresses the oleo at rest and now when i touch down i get no bounce. Admittedly, my landings are way better with it now, which helps, but it was a vast improvement and it allowed me the stick time i needed to work out how to land the beggar! I dread to think what a solid leg would have been like.

The only downside to oleos is that if you really plant it, or if you come in at a very steep angle they have no give at all and your retracts/mounting plates will depart on their own adventure.

With everything considered, i would always go for oleos and make the effort to land smoothly.

Cuban is quite right about wheels too and on the P39 i also subbed the nose wheel (hard rubber tyre and ali hub) for a softer foam one. This helped, and that is what lead me to mod the nose leg.

[Andy J]

Thanks, will add these to the model then as I need to modify the undercarriage anyway as it sits way too high in relation to the scale drawings I have for the full size aircraft.

Also looking for some 5 inch low bounce tyres. Do you recommend Du-bro?

[Jon H]

dubro wheels are very heavy but that may not be a problem on a model this big. The tyres are also really fat so might not fit the trousers on the hawk. Radio active do some 5 inch wheels that are a bit narrower so they might be a better fit

[Chris Walby]

Someone mentioned to me that with wire only UC (even with the coiled loop) if you land hard and then bounce the compressed wire leg rebounds with the mass of the wheel, but has nothing to damp the action and its this energy that does the damage. Clearly if you just keep bending the UC back until it rips out of the mounting is the more short term effect, but wire legs (with or without a loop) can add to the bounce energy.

Oleo with rebound damping is the way to go...or a nice flat runway

[Stuphedd]

I know this might be " NON scale " but I use Gas struts from cupboard doors , On my big Porter , Stampe and Waco and they do the job well, All DLE 30 powered .

cheap as chips from ebay

cheers

[Nigel R]

" but wire legs (with or without a loop) can add to the bounce energy."

Technically, wire legs can't add energy... sometimes it does seem that way though!

The coil is really only there to provide some protection for the airframe where the leg attaches, by virtue of doing the spring action - it doesn't really help the landing, quite the reverse really.

Some interesting alternative ideas on plain wire trike undercarts on Andy Lennon's designs

**LINK**

Not strictly speaking on topic, that was a 0.40 size.

[Capt Kremen]

Interested in this post for two reasons. My current project has a nose oleo only (VQ Tri-Pacer) wondered if it was worth it and would it stand up to 'everyday' use. Also, wheels. I put a post up a week or two back asking folks experiences with different types of wheel e.g. brands, hard/soft, foam/plastic/rubber etc. lots of reads but surprisingly little feedback or comment.

[Martin Harris - Moderator]

I suspect that the main benefit of our simple oleos* is more in the friction provided by the lack of sophisticated bearing surfaces! The springs don't actually absorb energy - they store virtually all of it, and as anyone who has driven a car with faulty shock absorbers will tell you, that energy is soon sent back where it came from...leading to very uncomfortable handling due to changes in suspension/steering geometry and loss of tyre adhesion.

In our units, I believe that the spring compresses and friction is generated between the sliding components, slowing both the compression and rebound and dissipating some of the energy as heat, reducing the tendency to bounce back. Ideally, some form of one way damping on the rebound would be provided but with the additional elastic hysteresis provided by the deformable rubber wheels that we use, these simple units seem fit for purpose.

Some full size light aircraft e.g. DHC1 Chipmunk, use rubber in compression as the springing and damping medium, as does my car trailer.

*Technically, oleo means relating to oil, as in an oil damped device.

P.S. Sorry if some of this overlaps with other postings - I started the reply over an hour ago but was interrupted!

Edited By Martin Harris on 21/05/2020 11:07:25

[Nigel R]

Didn't the original Mini use squishy bits of rubber instead of a spring?

"Also, wheels."

Mostly...

**LINK**

Medium sort of weight. Air filled. Rubber (?) tyre. Work nicely with glow models. Effective enough. Cheap. Good lifespan.

**LINK**

Light. Open cell foam. Ok for electrics. Seem to dissolve into a sticky mess with glow use. Look kind of ratty after any amount of use.

[Chris Walby]

Hoping for good things from my Focke UC (RC car dampers) in it mock up display. The trailing link should reduce the bend back force.

Tim,

I was not implying that there was any more energy, just the unrestrained whip effect once the wheel is released from the ground.

[Martin Harris - Moderator]

Posted by Nigel R on 21/05/2020 11:37:00:

Didn't the original Mini use squishy bits of rubber instead of a spring?

Yes - I was tempted to mention them but having managed to largely [if that's appropriate] avoid Minis I'm not sure if they had additional conventional dampers. Then, I believe, they moved to combining fluid and rubber in the infamous Hydrolastic system which again, probably fortunately, I've never had experience with.

[Martin Harris - Moderator]

Posted by Chris Walby on 21/05/2020 11:47:36:

Hoping for good things from my Focke UC (RC car dampers) in it mock up display. The trailing link should reduce the bend back force.

Very elegant Chris. Incorporating model car dampers has flickered through my mind on more than one occasion and as long as the spring rates and to a lesser extent damping are appropriate it should be a great improvement over simple friction damping.

[Andy J]

Well was going to ask if anybody had used RC Car oil filled dampers.

Wondering how they could be employed for a fixed undercarriage. Has anyone else utilised them?

For the sparrowhawk wing current set up is two heavy gauge struts possibly 5 swg with a single back stay

[Denis Watkins]

That U/C Andy, is either the original twin bar Sparrowhawk set up

Or a copy of the original setup and handled controlled landings well.

I do like oleos though and will try and get a pic of my damper experiments

Edited By Denis Watkins on 21/05/2020 18:12:52

[Andy J]

Well made some progress but not as originally wanted. 169mm Oleo's I had thought suitable are out of stock at HK so thought I would see if the existing 5mm wire undercarriage could be removed as this would have some influence on what mod to subsequently perform. This was far easier than expected so quickly had the parts removed.

Noted the Seagull design for the Sparrowhawk sits way to high to be semi scale, so thought I would at least shorten the existing legs by a couple of inches. Use of a 6 inch angle grinder and blow lamp to soften the piano wire quickly had the legs adjusted but can now see that if I cut the legs off to provide a peg for the oleo the increased diameter of the oleo leg will fowl the wheel spats.

So not sure how best to proceed. originally thought I would fit two oleo's per leg but its clear I would need a very thin 5 inch wheel to allow sufficient clearance with the wheel spats. Use of one oleo would be possible but it would mean losing the affect of one torsion bar which is not good given my landings leave a lot to be desired.

Suggestions welcome!

[Martin Harris - Moderator]

I fear you may have ruined your undercarriage legs if you've heated them to red hot for bending. Piano wire can't be heat treated and it may have been softened, losing most of its torsional strength. Don't be tempted to recover them by quenching and tempering as you would with ordinary spring steels - they will become as brittle as carrot sticks!