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Oleo style legs on grass.

Started by Andy Sayle, April 20, 2010, 14:00:58 pm

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Andy Sayle

Afternoon all,

I've been seriously impressed with some trailing link legs that I have installed on my Boomerang, and it has got me pondering about the legs that will go in my Harpoon.  I've been offered a set of Eurokit legs, that are just plain oleo type struts.  I was wondering if anyone could comment on their effectiveness on absorbing the bumps and lumps on a grass strip?  My instinct is telling me that a set of trailing links are going to perform better, but my wallet is telling me to not pass up the offer of cheap oleos!

Cheers
Andy
I can fix anything in the whole world, for I am armed with two rolls of Duct tape, and a rather large Hammer....

STORM

Andy, if your field is as rough as mine I wouldn't consider anything other than trailers.
I tried my Hawk and ripped the mainspar out along with all three retracts, due to a rabbit hole relaunching, just my opinion, Gary.

PDR

Trailing link legs behave the same as straight oleos with softer springs for a given amount of movement. Some maroon wrote a pointlessly long article and did a hideously complex spreadsheet to model and optimise undercarriage springs (the prat clearly doesn't get out much). It's in the downloads section of an obscure internet forum somewhere.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

darrell

first flight with my harpoon on Saturday with eurokit retracts and oleo legs ,by the sunday flight the legs where out and jet a1 in made such a difference.landings are ten times better on grass with jet 1a legs .
you can try my oleo legs out if you want .

p51p47

I've been running softly sprung straight oleo struts (with 4" main wheels) in the original Valkyrie sports jet prototype for 4 seasons now (33lb fully fueled)......the majority of my flying is off grass and I've never had a jot of bother.

I'm a BIG believer in wheel diameter playing as big part...small wheels create a HUGE amount of rolling resistance putting a lot of unnecessary force on struts and gear mounts........big wheel roll better basically...............

Phil
Real planes are green...anything without guns is a target. Fighteraces Warbird & Accessories

Pat Barnes

Larger wheels will indeed cope with rougher surfaces, as the bumps will 'appear' smaller to the wheel.

Assuming you cannot increase the diameter of the wheel due to wheel well size, I would suggest that trailing link legs will cope with rough fields better.  If you consider the wheel meeting a large bump as the model rolls across the surface, there is going to be a force pushing the wheel backwards.  On a straight oleo, this dragging force is going to be seen as a bending moment 'felt' by the leg where it is mounted into the retract unit - either the wheel rides up over the bump or, if the bump is too large, the undercarriage is ripped out of the wing.  On a trailing link, the wheel is going to 'feel' the same dragging force as it hits the bump, but because the trailing arm can move backwards against the spring, the bending moment felt by the top of the leg will be reduced.  Also, the backward rotation of the trailing arm around it's pivot point causes the wheel to move upwards, helping it to ride over the bump. 

Obviously it all depends on the size of the bumps in your field. If you land across a ploughed field, even the largest wheels on trailing links are likely to get ripped out!

JohnB

Quote from: Pat Barnes on April 21, 2010, 00:22:46 am
Larger wheels will indeed cope with rougher surfaces, as the bumps will 'appear' smaller to the wheel.

Assuming you cannot increase the diameter of the wheel due to wheel well size, I would suggest that trailing link legs will cope with rough fields better.  If you consider the wheel meeting a large bump as the model rolls across the surface, there is going to be a force pushing the wheel backwards.  On a straight oleo, this dragging force is going to be seen as a bending moment 'felt' by the leg where it is mounted into the retract unit - either the wheel rides up over the bump or, if the bump is too large, the undercarriage is ripped out of the wing.  On a trailing link, the wheel is going to 'feel' the same dragging force as it hits the bump, but because the trailing arm can move backwards against the spring, the bending moment felt by the top of the leg will be reduced.  Also, the backward rotation of the trailing arm around it's pivot point causes the wheel to move upwards, helping it to ride over the bump. 

Obviously it all depends on the size of the bumps in your field. If you land across a ploughed field, even the largest wheels on trailing links are likely to get ripped out!

Thats exactly my thoughts and experience.

J
No longer an active participant.

jason

Trailing link all the way, I fly of grass all the time and the Trailing link on my Elan behave tottally differently to oleo legs,  Oleo work great on the vertical component of the landing but have no give forward or aft, 
Next time you see a jet  taxing with trailing link look how much energy and movement  is not being transferred to the fuselage.   their was a reason the BAE 146 landed so well and that was to due the Trailing link u/c. my choice would be wire leg or trailing but not to bother with oleo,   

jason

Quote from: PDR on April 20, 2010, 18:14:05 pm
Trailing link legs behave the same as straight oleos with softer springs for a given amount of movement. Some maroon wrote a pointlessly long article and did a hideously complex spreadsheet to model and optimise undercarriage springs (the prat clearly doesn't get out much). It's in the downloads section of an obscure internet forum somewhere.

PDR
sounds very typical of a post by you. :af

PDR

Well I'm sure if the article's author is offended he'll let me know...

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

reaps

Quote from: jason on April 21, 2010, 07:57:56 am
Trailing link all the way, I fly of grass all the time and the Trailing link on my Elan behave tottally differently to oleo legs,  Oleo work great on the vertical component of the landing but have no give forward or aft, 
Next time you see a jet  taxing with trailing link look how much energy and movement  is not being transferred to the fuselage.   their was a reason the BAE 146 landed so well and that was to due the Trailing link u/c. my choice would be wire leg or trailing but not to bother with oleo,   


I would counter that wire legs on grass are a pain as you spend forever bending them back to shape not to mention forces being transferred directly to the airframe.

In my opinion (and i'm no expert) i would rate the 3 as follows:

best (£££)                worst (cheap)
trailing link - oleos - wire leg

Although the oleos can be very expensive if they are a scale part.

Oh and small wheels dig in as  pat barnes has already said :)

PDR

Quote from: Pat Barnes on April 21, 2010, 00:22:46 am
Assuming you cannot increase the diameter of the wheel due to wheel well size, I would suggest that trailing link legs will cope with rough fields better.  If you consider the wheel meeting a large bump as the model rolls across the surface, there is going to be a force pushing the wheel backwards.  On a straight oleo, this dragging force is going to be seen as a bending moment 'felt' by the leg where it is mounted into the retract unit - either the wheel rides up over the bump or, if the bump is too large, the undercarriage is ripped out of the wing.  On a trailing link, the wheel is going to 'feel' the same dragging force as it hits the bump, but because the trailing arm can move backwards against the spring, the bending moment felt by the top of the leg will be reduced.  Also, the backward rotation of the trailing arm around it's pivot point causes the wheel to move upwards, helping it to ride over the bump. 


Yes and no, but more usually no - it depends on the geometry. In most trailing-link installations the link angle is less than 30 degrees from the horizontal in the standing condition. At this sort of angle there's very little backward movement available to behave in the way you suggest, and the force required to push back against the spring is less than the rolling resistance of the tyre. So the response to a backward force would be to roll over it rather than push the wheel back against the spring. The spring DOES compress, but this is a response to the ground trying to lift the model (as for a linear oleo).

The design choices and principles of trailing links are all focussed around getting a longer apparent oleo travel (and hence softer net spring rate) from a shorter oleo leg. The shorter leg is substantially lighter, and the extra weight of the link assembly (arms, trunions etc) is usually less because it isn't a large-diameter forging that is housing and containing the oleo-pneumatic system. It isn't *always* lighter because it depends on the details of the specific installation and the particular forces imparted by a particular aeroplane design and the other knock-on consequences (larger wheel wells that reduce space for wing tankage, greater torshional forces at the U/C pintel which needs a stronger structure at the wing end etc etc). That's why some aircraft have them whilst others don't. Of course trailing link noselegs are far more common, but that's because they help adsorb the braking weight transfer and also provide castoring more easily than linear oleos.

If you can get the same range of vertical wheel movement with a linear oleo it will give the same soft, pilot-flattering bump absorbtion as the trailing link.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

Calljj

Quote from: PDR on April 21, 2010, 17:47:18 pm
If you can get the same range of vertical wheel movement with a linear oleo it will give the same soft, pilot-flattering bump absorbtion as the trailing link.

PDR


Very interesting, I didn't know that.

Pat Barnes

Quote from: PDR on April 21, 2010, 17:47:18 pm
it depends on the geometry. In most trailing-link installations the link angle is less than 30 degrees from the horizontal in the standing condition. At this sort of angle there's very little backward movement available to behave in the way you suggest, and the force required to push back against the spring is less than the rolling resistance of the tyre. So the response to a backward force would be to roll over it rather than push the wheel back against the spring. The spring DOES compress, but this is a response to the ground trying to lift the model (as for a linear oleo).

The design choices and principles of trailing links are all focussed around getting a longer apparent oleo travel (and hence softer net spring rate) from a shorter oleo leg.


I agree with your points on geometry.  However, the OP is restricted by his models retract recess size, so the oleo needs to be a fixed length when unloaded, whether it is straight or trailing link.  As you point out, for the same overall oleo length, the trailing link gives more travel and is 'softer' than the straight oleo. In my experience, in a like for like situation, the trailing links behave better.

PDR

I wouldn't argue with that Pat - all I was disputing wre the common myths that (a) a trailing link suspension always gives a softer ride, and (b) that trailing link suspensions soak up bumps by moving backwards rather than upwards.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

Andy Sayle

Cheers for the input folks, much appreicated.

I'm going to have a shot at designing some trailing link struts myself.  I like the idea of using a soft oleo, because of the neater packaging.  However, I'm not sure if I can get a decent range of movement to make it comparable to a trailing link.

Andy
I can fix anything in the whole world, for I am armed with two rolls of Duct tape, and a rather large Hammer....

ron sweeney

Andy, you will end up with trailing link struts and less heart ache if you start with them,..............30 years of flying jets from grass, come and visit deeside on Saturday.   Ron.

Andy Sayle

I've already got a set of trailing link struts drawn up on the computer, just need to speak to a friendly machine shop who owes me a favour ;)

I'm having another dilemma now though, install the vectored thrust pipe?  Or leave it out, and save a few ounces?

Andy
I can fix anything in the whole world, for I am armed with two rolls of Duct tape, and a rather large Hammer....

SWM

May 02, 2010, 23:37:50 pm #18 Last Edit: May 02, 2010, 23:42:51 pm by SWM
Quote from: Andy Sayle on May 02, 2010, 20:29:18 pm
I've already got a set of trailing link struts drawn up on the computer, just need to speak to a friendly machine shop who owes me a favour ;)

I'm having another dilemma now though, install the vectored thrust pipe?  Or leave it out, and save a few ounces?

Andy


Vectored thrust its a must

I only said that cos it rhymes. ;D

I sent you a PM Andy - hope you are well. It was just a 'catch up'

I was wondering -  when you'd be out and about flying your jets in Wales? - might get a  chance to visit for a couple of hours in June if your about one weekend?

Steve
I love all things Radio Control, so much it hurts.

Andy Sayle

Well as it rhymes so well Steve, I'll have to install it!

Thinking about it a bit more, if the vectored nozzle is left off, there will need to either be a fixed short pipe put in, or the engine moved back a bit.  Me thinks the vectored nozzle stays now :)

I got your PM, been a bit busy to reply!  Will do today though.  As for jet flying days, then I don;t have anything planned for June yet (other than I'm camping at Weston park this year and a couple of concerts, plus the good food show ;)  weekends are always best, that means I don't need to skip a days work (meaning more money for the toy fund!).  Just let me know when you were thinking of heading down, and I'm sure we can arrange to meet up.  One thing you might like to do, is pop over on a thursday.  We usually have a club night at the field on a thursday afternoon/evening throughout summer, and being the club chairman, I tend to go to most of them :)

Cheers
Andy
I can fix anything in the whole world, for I am armed with two rolls of Duct tape, and a rather large Hammer....

tomkfly

You could both come to Deeside on Saturday ;D


   Tom
When you change the way you look at things, the things you look at often change.        www.deesidemac.co.uk
Gravity, thou art a heartless bitch

Andy Sayle

I'm hoping to be there on Saturday afternoon, all being well :)

Andy
I can fix anything in the whole world, for I am armed with two rolls of Duct tape, and a rather large Hammer....

SWM

Quote from: Andy Sayle on May 03, 2010, 09:01:35 am
Well as it rhymes so well Steve, I'll have to install it!

Thinking about it a bit more, if the vectored nozzle is left off, there will need to either be a fixed short pipe put in, or the engine moved back a bit.  Me thinks the vectored nozzle stays now :)

I got your PM, been a bit busy to reply!  Will do today though.  As for jet flying days, then I don;t have anything planned for June yet (other than I'm camping at Weston park this year and a couple of concerts, plus the good food show ;)  weekends are always best, that means I don't need to skip a days work (meaning more money for the toy fund!).  Just let me know when you were thinking of heading down, and I'm sure we can arrange to meet up.  One thing you might like to do, is pop over on a thursday.  We usually have a club night at the field on a thursday afternoon/evening throughout summer, and being the club chairman, I tend to go to most of them :)

Cheers
Andy


Cheers Andy

I went off line with family 'stuff'. I think i'll pop by on a Thursday afternoon in summer. Its a good way of saying to the wife - 'fancy a drive into Wales' - 'oh look model aeroplanes'...

When I got my Pathfinder I bought the Tent extension so we could camp out at various 'do's etc - of course my 'heart' still buys stuff I think I can do! Never used the tent yet. No way I could sleep 'out' now a days.

The wife mentioned the tent recently ' you should sell it as we'll never use it'

Me ' oh...not sure you never know - I might get better etc'

again the decision making by the 'heart' and not my 'head' that probably agree's - there' very little chance of me using it.

Keep in touch and we''ll try and meet up into summer. 

Steve

I love all things Radio Control, so much it hurts.

tomkfly

Quote from: PDR on April 21, 2010, 17:47:18 pm
Yes and no, but more usually no - it depends on the geometry. In most trailing-link installations the link angle is less than 30 degrees from the horizontal in the standing condition. At this sort of angle there's very little backward movement available to behave in the way you suggest, and the force required to push back against the spring is less than the rolling resistance of the tyre. So the response to a backward force would be to roll over it rather than push the wheel back against the spring. The spring DOES compress, but this is a response to the ground trying to lift the model (as for a linear oleo).

The design choices and principles of trailing links are all focussed around getting a longer apparent oleo travel (and hence softer net spring rate) from a shorter oleo leg. The shorter leg is substantially lighter, and the extra weight of the link assembly (arms, trunions etc) is usually less because it isn't a large-diameter forging that is housing and containing the oleo-pneumatic system. It isn't *always* lighter because it depends on the details of the specific installation and the particular forces imparted by a particular aeroplane design and the other knock-on consequences (larger wheel wells that reduce space for wing tankage, greater torshional forces at the U/C pintel which needs a stronger structure at the wing end etc etc). That's why some aircraft have them whilst others don't. Of course trailing link noselegs are far more common, but that's because they help adsorb the braking weight transfer and also provide castoring more easily than linear oleos.

If you can get the same range of vertical wheel movement with a linear oleo it will give the same soft, pilot-flattering bump absorbtion as the trailing link.

PDR

The Type of trailing ling these guys are talking about are almost straight and will absorb a rearward knock.
[attachimg=1]



  Tom
When you change the way you look at things, the things you look at often change.        www.deesidemac.co.uk
Gravity, thou art a heartless bitch

PDR

Quote from: tomkfly on May 08, 2010, 22:58:17 pm
The Type of trailing ling these guys are talking about are almost straight and will absorb a rearward knock.


How much are they deflected when stationary and weight-on-wheels?

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

tomkfly

Quote from: PDR on May 08, 2010, 23:01:26 pm
How much are they deflected when stationary and weight-on-wheels?

PDR

Hardly anything.



   Tom
When you change the way you look at things, the things you look at often change.        www.deesidemac.co.uk
Gravity, thou art a heartless bitch

PDR

Quote from: tomkfly on May 08, 2010, 23:03:34 pm
Hardly anything.


Then they are essentially doing nothing at all - the springs are far too hard. If the weight of the model doesn't shift them neither will any bump, and the spring frequency will be so high that any energy absorbed will be returned almost instantaneously.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

tomkfly

You should be able to see the deflection, on the main legs, in this photo.( Noseleg is a more normal type trailing link)
[attachimg=1]

They seem to work quite well and are not that difficult to push back by hand.


   Tom
When you change the way you look at things, the things you look at often change.        www.deesidemac.co.uk
Gravity, thou art a heartless bitch

HarryC

Quote from: PDR on May 08, 2010, 23:06:09 pm
Then they are essentially doing nothing at all - the springs are far too hard.

That would not surprise me Pete, the Jet 1A trailing link noseleg with my Grumania Eurofighter was really a main leg and it was sprung so hard that it would have been correct as a single leg for the entire model, it would be far too powerful even to be one of a pair of mains.  I suspect a lot of legs are sprung too hard.

PDR

I'm not going to comment further.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

Andy Sayle

The trailing links on my Boomerang deflect a lot more than the picture a couple of posts above.  They sit at an angle of about 30 degrees from the vertical.  When fast taxiing/taking off and landing, you can visibly see them deflecting, sometimes enough to take the trailing link past the horizontal.  Despite that, I am trying to get hold of some suitable replacement springs, as they could do with being a little softer for our field...

Andy
I can fix anything in the whole world, for I am armed with two rolls of Duct tape, and a rather large Hammer....