Topic: Undercarriage for Grumman Avenger

OK guys,

As Steve has mentioned on the other build thread for this project http://www.rcmf.co.uk/4um/index.php/topic,35919.0.html, I am attempting to produce an undercarriage for this project. Sierra produce a complete system for this particular model, they're a bit expensive, but where's the challenge in handing over a large wedge of dosh.........Far too easy........      

I am preparing the design drawings at the moment............For those of you who are not familiar with the Avenger, and that includes me, it was a huge, mostly carrier borne single engined torpedo bomber/bomber/strike aircraft. Excuse me if I have some facts wrong, I am on a steep learning curve too. The model is 1/6th scale, my YT Spitfire is 1/6th scale and has a wingspan of 72.5", this model has a wingspan of 108". A big heavy old bird, therefore, the undercarriage has to be full of testosterone to take all the pounding of those carrier landings.

The main oleo legs are braced for fore and aft movement with a massive drag brace and also braced for sideways loading with hinged upper and lower side struts. Our first thoughts were to make the side struts non functional, but if they can be designed to do their intended job they add much stiffness to the whole structure, effectively reducing the length of the legs by a half. The problem here is releasing, the lock first, before retraction, I have a solution, but not a very elegant one at the moment, but it does work.......     ...Hey Ho

What I could do with some help on, is.......................

The required rate for the springs, the whole model is designed to weigh 38 lbs................Is there a rule of thumb for the spring force required, 1st case just to support the weight of the model and 2nd to absorb the shock loading of Steve's landings. If one of you learned gentlemen could point me in the right direction...........    

My plan at the moment is to show as much of the design, machining and build of this undercarriage system as I practically can, so with your 
forbearance I will do my best to 'get on with it'...........    

Tally Hoooooooooooooooooooooooooooooooooo

Andy

Spring rate rule of thumb = double the models weight per oleo. 38lb model required a 76/80lb spring per leg.

My B-26 has 150lb springs for a 70lb model, Skyraider has 250lb springs.

Was chatting to me inspector last weekend.............he's given up with springs now, instead he's using small 'gas struts'.......like the ones that hold your car boot open...................apparently these are available in small diameter, short stroke, and have a better 'damping' effect than a standard spring. He's fitted them inside the oleo as a direct replacement for the spring.

Food for thought.

Phil

The model is 1/6th scale, my YT Spitfire is 1/6th scale and has a wingspan of 72.5", this model has a wingspan of 108". A big heavy old bird, therefore, the undercarriage has to be full of testosterone to take all the pounding of those carrier landings.

Big indeed 

Took this pic at Yeovilton last summer.


And the gear, with the far side tire pasted in

Phil,

Thanks for that, just the sort of answer we were looking for, I like the idea of the gas struts, I will investigate that one.

Sverrir,

Good picture     , it's that side strut that's posing the prob at the moment, the rest of it is sorted. The picture is a good example of why we want to make the side struts functional, as I noted in the first post the length of the leg is reduced by a half.

Thanks guys...........      

Tally Hoooooooooooooooo

Abdy

We use a fair amount of Gas struts at work for doors and the like Andy, Stabilus have a pretty good online catalog showing the different types available, gas springs, dampers etc, the ones in the link below may not be the exact type your after but browse around the rest of the site, theres loads of different ones on there.

http://www.stabilus.com/frameset.asp?sprache=englisch&headline=Products&bereich=produkte&subsite=menue.asp

I can not see it on the pictures but how does the side strut bend?
If you can give me a good picture or drawing of the side strut, maybe I can help you with your problem.
As I can see on the pictures, if there is enough room in the wing I think you can make it working by using 1 big servo. This should be connected to the side strut. Then you do not need pneumatic power or an electric motor to move the oleo.

Carlo

Carlo, it's similar to the Hurri, the actuator operates directly on the sway brace which pulls the leg up


Mark.

Carlo, it's similar to the Hurri, the actuator operates directly on the sway brace which pulls the leg up


Mark.

Mark is correct....................

Carlo thanks for the idea.................stored away if needed, but read on please........  

I have sorted the design tonight............   The air cylinder I am using has a 50mm stroke, I need 47mm for the 90 deg rotation of the leg to fully retract. So, I have 3mm movement to spare, I will allow the body of the cylinder to move backwards 3 mm and this movement I have used to release the lock on the side strut. The solution is somewhat neater than I was thinking it would be last night, not quite in line with the full size but very simple............

This is a drawing of the fullsize arrangement.................and of course the tailwheel...........I haven't even thought about this yet....  



Steve hasn't seen the final design yet , he's not around this weekend, so I can hardly show you guys before him can I?..........

If he likes it and we can work out how to post a CAD drawing, I will show you my thoughts...........    

Tally Hoooooooooooooooooooooooooo

Andy

Andy,

I like the idea

How are you letting the 'torque tube' rotate after the side brace unlocks?

Do you have another sketch showing the 'up' position?


Mark.

Andy, if you need very thin walled st/st pipes of all sorts of dia. check by Toni Clark http://www.toni-clark.com/Default.htm  I'll use his stuff for my Wildcat gear.

  Johan

Steve hasn't seen the final design yet , he's not around this weekend, so I can hardly show you guys before him can I?..........

If he likes it and we can work out how to post a CAD drawing, I will show you my thoughts...........    

So Andy, now Steve's back, has he seen it, does he like it, and when can we see it? 


Mark.

Hi guys...................

he likes it         

Johan,

Thanks for the link could be very useful...................

And now the drawing.......................tear it to pieces please, it costs nothing to make changes at this stage.

The air cylinder is a std industrial double acting unit SMC part No. CJ2D16-50 producing 25 lbf  at 80 psi and 31 lbf at 100 psi. Not shown on the drawing is the fore/aft drag brace, this is attached to the same mid leg collar as the side brace and pivots aft of the retract unit on a pivot pin extension. It will be machined from ally with lightening holes as the full size.




Thanks in advance for your comments

Tally Hoooooooooooooooooooooooooooo

Andy

Andy, that looks very good

What diameter cylinder are you using, as 25lbs and 31lbs doesn't sound a lot with that moment arm, Sierra are using a 1" unit, which gives approx 45lbs at 100psi on leg retraction, 35lbs at 80psi, (by my calculation anyway )

To improve rigidity, I'd suggest having the lower flanges out at the end where the side brace attaches.  They don't need to be continuous, just at the ends

I really like your idea with the spring   helps maintain a geometric lock


Mark.

Really nice job Andy, looking forward to seeing these in the flesh

Steve

 Andy,
      That"s very impressive, wish I had your machining skills!

 Cliff 

How do you command the side brice to bend when wheels up and how do you lock it when wheels down?

Carlo

Carlo,

The leg is locked in 'wheels up' and 'wheels down' position by the curved slots in the retract unit side plates, 47mm of the 50mm cylinder stroke. The side braces and the fore/aft drag links are there to effectively shorten the length of the leg.

Wheels up sequence is this............

Air on to cylinder to extend piston rod..........

The piston rod will try to extend and move the leg, the leg cannot move because the side brace is locked over centre. The cylinder body can, by means of a 3mm long mounting slot, so will, move backwards towards the RH end of the slot. When it moves backward it pushes on a inner lever which rotates the torque rod and lifts the outer lever which in turn, lifts the upper side rod link, thus, the over centre lock of the side brace is removed. The leg is now free to retract into its up position. At the end of the cycle the piston in the cylinder is fully extended 50 mm and the leg is locked up, by the lock on the retract unit.

To lower the gear...........

Air on to cylinder to retract piston rod.............

The retract unit up lock is removed by the piston rod tee bar, the leg lowers. The side brace links are pulled down by the leg. When the piston reaches the end of the stroke. The mounting pin is at the LH end of the 3mm slot, the leg is locked in position by the retract unit the side braces are locked in their over centre position.

I hope this all makes sense. please shout, if it dosen't.

Tally Hoooooooooooooooooooooooooooooooo

Andy

Looks great this way.
Are you going to 'hard anodize' the alu parts with the 3mm slots?

Carlo

I hope this all makes sense. please shout, if it dosen't.

 Bloody Engineers  

 Absolutley Cracking bit of CAD ,can't wait to see the start of the machining process  

Looks great this way.
Are you going to 'hard anodize' the alu parts with the 3mm slots?

Carlo

Carlo,

Probably not, I plan to have a hard nylon or PTFE top hat bush on the support pin. The bush will then slide in the ally slot.

Mark,

Sorry.........................that's who I is..............    

Thanks guys

Tally Hoooooooooooooooooooooooo

Andy

Very nice Andy, very nice indead  .................

I prefere  Nylon or POM for bushings,  PTFE is not so hard.

Carlo

I prefere  Nylon or POM for bushings,  PTFE is not so hard.

Carlo

Just testing..................      

Me too

Tally Hooooooooooooooooooooooo

Andy

Tiger, sounds like a nice and functional construction 
For bushings use POM as PA (Nylon) is very hygroscopic and changes dimensions all the time with the humidity.

  Johan

Guys,

Just to make sure..........I am only talking about the top hat bushes to allow the cylinder to slide.

All other bearings will be brass or oilite.

Johan

OK.......POM it is then.........thankyou

Tally Hoooooooooooooooo

Andy

Guys,

I thought you might like to see what I've been doing, while it's been a bit cold in the workshop..........  



Still a long way to go yet, but it gives a good impression of how things will look...........      

Tally Hoooooooooooooooooo

Andy

Andy.

Can't get an idea of 'scale'................but can I ask what are the 'footprint' dimensions ?. For a 108" 40lb model......they need to be BIG, and compared to the overall unit size, they don't seem massive at present?

Phil

Andy.

Can't get an idea of 'scale'................but can I ask what are the 'footprint' dimensions ?. For a 108" 40lb model......they need to be BIG, and compared to the overall unit size, they don't seem massive at present?

Phil

They are to the sizes directly off the plan and some.............the sideplates are 210 long 50.8 high with 80 x 25 wide flanges , the pivot block is 30 mm wide and the cylinder shown on this drawing is 20mm bore. The intermediate spacer plates are 6mm thk

They look massive to us..................what do you reckon now?

T H

Andy

Andy   nice work mate (been watching from the sidelines)
I expect nothing less from you and am impressed mate 

Shane

They look massive to us..................what do you reckon now?

T H

Andy

Ahhhh.......they LOOK massive to me now as well  ....................obviously hadn't got the 'eyecrometer' dialed in quite right 

Tiger,  very very nice work,  if you need any material,  we have all sorts of he30, and L168 Ally. silver steels, and many exotics, titanium etc,  if i can help, just holler

Titanium retracts 

Titanium retracts 

  I don't think you like to work with it    Unless you got an eroding wirecutter No fun to weld either....

Looks nice when heat treated though..

  Johan

Just about finished with the CAD solid modelling now................





Quite useful, for this kind of stuff........    

Onwards...............

Tally Hoooooooooooooo

Andy

Looking good Andy

Now I see why the spring won't assist the downlock


Mark.

Like the 4th retaining bolt on the separate lug on the 1st image..............

Like the 4th retaining bolt on the separate lug on the 1st image..............

Very nice work Tiger 

I agree with Phil though - the additional lug will provide a far superior installation as it doubles the footprint..... well worth the extra machining!! 

Like the 4th retaining bolt on the separate lug on the 1st image..............

Very nice work Tiger 

I agree with Phil though - the additional lug will provide a far superior installation as it doubles the footprint..... well worth the extra machining!! 

Just keep watching boys...............    

Just keep watching boys...............    

You finished already?

Mark.

Well I've done my bit, signed off the drawings that is 

Steve