Steel tube for landing gear?

[duncan_rtfm]

Hi,

I'm at the stage where I need to start fabricating my main gear.  The design calls for three triangulated steel tubes without suspension, relying rather on the cushioning effect of the 450mm balloon tyres.  The plane is going to be very light (MAUW of about 300kg) so this should suffice.  I have 25mm OD steel tubing in the shop which I was going to use, but I'm beginning to think that the 1.2mm wall thickness isn't going to cut it.

 

Any suggestions?  Experience?

 

Regards,

Duncan

[facthunter]

The weight  you would add would go against having any thicker.  If you use chromoly you have to heat treat it or at least stress relieve near the welds. The test for U/C adequacy is to drop the plane (or something equivalent) from a certain height and it's allowed to deform as long as you can still taxi it. I can't recall  what the height is but it is not that much.  I think 15 feet would cover it but better to check. With only the tyre to yield it's going to be pretty savage reaction' Would you like to jump a motorbike with NO suspension? It's also better for the suspension to yield before the rest of the plane (and YOU) gets all the force that can be involved.  You've only got one spine. Nev

[kasper]

Duncan,

 

Sorry if this sounds like telling you what is obvious:

 

1. any round member in tension as its primary load path can be small diameter thin wall - its effectively a fat wire

2. if your round member is in compression as its primary load path you are in a balance - the greater diameter the thinner the wall that will work but you have to work out the collapse load of the tube - lots of fun engineering texts on that

 

If you are basing your undercarriage on the HM single seat plans then from severe experience the tube sizes work fine and the tubes can be the mild steel per the drawings.

 

In an undercarriage tripod:

1. the vertical member (the leg) is in compression and will end up the heaviest member

2. the cross member (wheel to centre of fusealge in the plane of the leg) is primary tension but with a strong secondary of compression - deals with side loads on the wheel

3. the drag link (wheel to fore/aft of the leg plane) is either compression if it goes rear of the leg (low load) or tension if it goes forward of the leg

 

And to be frank - the main issue will be the quality of the joints and how they work - welding needs to be good and I would recommend understanding and doing the correct post weld heat treatment is more critical than steel selection - its more likely the joint will fail ahead of the round steel members.  

 

And remember - its a 300kg single seater you are building and its not going to landing at 90kts - basic simple undercarriage will suffice

[duncan_rtfm]

Excellent info kasper.  You're right, mostly known, but I thin k I may have underestimated the compression on the secondary members.  Thanks for the reminder.  I've been looking at 25mm square aluminium also, rather than steel.  Not sure about this however.

 

Duncan

[duncan_rtfm]

22 hours ago, facthunter said:

The weight  you would add would go against having any thicker.  If you use chromoly you have to heat treat it or at least stress relieve near the welds. The test for U/C adequacy is to drop the plane (or something equivalent) from a certain height and it's allowed to deform as long as you can still taxi it. I can't recall  what the height is but it is not that much.  I think 15 feet would cover it but better to check. With only the tyre to yield it's going to be pretty savage reaction' Would you like to jump a motorbike with NO suspension? It's also better for the suspension to yield before the rest of the plane (and YOU) gets all the force that can be involved.  You've only got one spine. Nev

Thanks for the comments.  I know the Aeromax has rigid 3-point suspension and relies only on the fat 18-inch low pressure tyres for damping.  I should give David Trump a call and get his thoughts on this.

 

Duncan

[Geoff_H]

On 23/09/2021 at 2:56 PM, facthunter said:

The weight  you would add would go against having any thicker.  If you use chromoly you have to heat treat it or at least stress relieve near the welds. The test for U/C adequacy is to drop the plane (or something equivalent) from a certain height and it's allowed to deform as long as you can still taxi it. I can't recall  what the height is but it is not that much.  I think 15 feet would cover it but better to check. With only the tyre to yield it's going to be pretty savage reaction' Would you like to jump a motorbike with NO suspension? It's also better for the suspension to yield before the rest of the plane (and YOU) gets all the force that can be involved.  You've only got one spine. Nev

I think FAR23 requirement for certified aircraft is a downward velocity of 20 ft/second must be able to be absorbed and still be usable.  Have to look up height to give that height but if I recall my high school physics it is 20ft, don't assume that this is correct

[duncan_rtfm]

Since objects fall at a rate of 32 ft/sec, the duration of the fall would need to be quite small - nowhere near 20 ft.  More like one or two feet at most I'd think

[duncan_rtfm]

I came across this:  However, nowhere is the term "S" defined.  They do define "W" as weight (thank you).  Aaaargh!  However, from 9 to 18 inches seems about what I'd expect...

https://www.govinfo.gov/content/pkg/CFR-2010-title14-vol1/pdf/CFR-2010-title14-vol1-sec23-725.pdf

Edited September 24, 2021 by duncan_rtfm

[Geoff_H]

Agreed.  It was late and I was tired.  I mixed metric and ft.......I did leave a caveat that it might be wrong.  Thanks for the assistance.

 

[duncan_rtfm]

Aha!  Came across this:

According to FAR Part 23 h = 3.6* SQRT(W/S) where W is max airplane weight in pounds and S is wing area in ft^2. H is in feet.

 

The Fleabike weighs 300kg MAUW (660lbs).  Combined wing area = 13.5m^2 (145 ft^2).

3.6*(2.13) = 7.68 inches.

 

I'll drop mine from 9.2 inches.

 

Duncan

Edited September 25, 2021 by duncan_rtfm

[Geoff_H]

That aircraft designed and made in regional Victoria had an undercarriage test device to prove the design.  They made a machine that automatically raised the undercarriage connected to a weight, which I expect was the aircraft AUW, and then dropped the weight and landing gear.  Don't know what results they got but it seemed a great testing device.

[duncan_rtfm]

Hi Geoff,
Sounds an excellent idea.  Not quite sure how to do mine, since I will also need to be testing to make sure the wings/struts don't fail (I doubt they will, but good to be sure).

Duncan

[Old Koreelah]

On 23/09/2021 at 2:56 PM, facthunter said:

…Would you like to jump a motorbike with NO suspension? It's also better for the suspension to yield before the rest of the plane (and YOU) gets all the force that can be involved.  You've only got one spine. Nev

If you’re determined to not have any suspension, then at least add a decent layer of polystyrene (or similar) under your seat.

[Student Pilot]

Light aircraft have been using triangulated steel tube for undercarriages since the 1920's. Depends on the welding process and the welders skill as to any need for heat treating/stress relieving after welding. Chromemoly tube is very strong 1.2 mm would be ok, what thickness do the plans call for?

[turboplanner]

Go Karts don't have suspensions; a lot of tube/welding technology there.

[duncan_rtfm]

21 hours ago, Student Pilot said:

Light aircraft have been using triangulated steel tube for undercarriages since the 1920's. Depends on the welding process and the welders skill as to any need for heat treating/stress relieving after welding. Chromemoly tube is very strong 1.2 mm would be ok, what thickness do the plans call for?

Plans?  What plans?  I'm it...

[duncan_rtfm]

22 hours ago, Old Koreelah said:

If you’re determined to not have any suspension, then at least add a decent layer of polystyrene (or similar) under your seat.

How about 18 inch balloon tyres with low inflation?  Works for the Aeromax...

[Student Pilot]

A design from the 80's was the Kitten, Jessie Anglin designed it as an ultralight. There are a few flying in Oz, the design was copied given a turtle deck then called the Pup. Both of those originally had triangulated undercarriage and reasonably small wheels that worked well.

[facthunter]

There's  more that were adaptions of a leaf spring. or used rubber bungee's.. The damped oleo is pretty universal in larger stuff. Damping reduces the severity  and structural loads of a firm arrival.. In most "loaded" circumstances if the U/C doesn't absorb it something else will have to. Nev

[Yenn]

Do  not rely on polystyrene to insulate you from vertical seat loads. that has been proved to be bad policy, but you can get Conforfoam which will do the job. Nowhere near as cheap, but it will not burn and give off poisonous gas, plus it stays comfortable for at least 18 years as proved by my own seating.

[Old Koreelah]

41 minutes ago, Yenn said:

Do  not rely on polystyrene to insulate you from vertical seat loads. that has been proved to be bad policy, but you can get Conforfoam which will do the job. Nowhere near as cheap, but it will not burn and give off poisonous gas, plus it stays comfortable for at least 18 years as proved by my own seating.

Where do u get it from?