Hi All,

I recently picked up a 1956 C-172 amateur built project thats unfinished. I'm just after some opinions on a project path I was toying with.

I've been thinking of doing a tail dragger conversion on it but used Cessna parts to do it are very pricey in Canada.

I have a set of PA18 extended heavy duty gear with hydrosorbs and bungees off another project sitting in my shed. I was toying with a design using the PA18 gear. I think I could build it very strong by fabricating gearbox type assembly's at each attach point for the mounting tabs and re-enforce the bulkheads between. Not sure if the winters been too long and I've gone mad. Is this a stupid idea? Maybe it would look crap and very frankensteiny? I think with the HD chunky gear and sitting taller it might look good and be a very tough airplane?

I may photo shop a pic and post it. I'm obviously not worried about cruise speed with this design.

Converting landing gear used on a steel tube framed plane to that with riveted aluminum skin.... Anything could work with the right reinforcements. Might be a tougher task than practical.

Rogue wrote: Maybe it would look crap and very frankensteiny?

Almost guaranteed IMHO

If you want to use a "Cub style gear" instead of a leaf spring gear then you need to be looking at the Pilatus Porter and/or the Fieseler Storch. For a Cessna 172, the Porter style would probably be the least ugly or strange looking.

But there will be a boat-load of engineering required, because you are putting the landing loads in totally different places, through different load paths, etc.

A few things jump out at me immediately.... the Pilatus puts the lower pivot point in the center of the fuselage, and on the Cessna there is not as much structure in that location. All of the Cessna structural reinforcement is in the corner, and those reinforcements (hydroformed steel doubler) end about 12 inches from the middle. The Pilatus mounts the upper pivot point at the instrument panel bulkhead, and the Cessna also may not have enough structure or load path there. If I remember, the Pilatus uas a huge cross member or cross beam across the fuselage at the bottom of the panel. Can anyone here who is more familiar with the Pilatus verify this?

If you want to redesign a Cessna gear you could look into a one piece spring gear, that looks like a giant R/C model gear. The Zenair homebuilts use this very effectively. You would still have to put big structural fittings in the stock Cessna location, but you can eliminate the inboard part of the Cessna "gear box".

That said, overall, the very best, easiest, and fastest way will be to use the RV/Sonex style gear because you can do 90% of this with a custom engine mount, and maybe 10% adding skin doublers, stringers, and other load path reinforcements in the fuselage structure.

This would of course still require a lot of structural analysis in the fuselage structure to make sure your load paths are strong / stiff / fatigue safe between the (new) engine/gear mount and the (existing) door post bulkheads... the Cessna engineers have already analyzed the loads from that point onward.

But the good news is that if you do build an RV style gear you can do most of it with one complicated weldment, instead of having to deal with hydroforming steel parts, and doing major major surgery on the fuselage.

Only because of the COST of replacement Cessna landing gear parts, this would likely be the fastest and least expensive way to re-design the Cessna landing gear system as a homebuilt/EXP. If you value your time at anything more than $2.00 / hour, paying full price for new Cessna parts will still be cheaper.

I have to say that this seems like a whole lot of hassle. 180 gear works fine and you can build the gearboxes with 170 parts.

Thanks for the quick feedback guys!

That's exactly what it would look like I guess but with a straight tail. Maybe the taller gear with some larger tires would be the go. I wonder how it handles.

Good points EZFlap. My thinking was after measuring it up and you can see it in that pic the front attach point for the gear is right at the bulkhead where the wing strut mounts to and where the gearbox would be installed in taildragger configuration. The rear attach mount is right at the doorpost where the rear gearbox is mounted in tricycle configuration. So providing I fabbed up a strong box section type gear box with mounting tabs that would be attached to the gearboxes and the correct extra reinforcements all mounting locations would be super strong with the landing force exerted where Cessna originally intended and engineered it to be but across all four locations spreading the load. Does that make sense?

To be honest I would really prefer to do the standard TD conversion with the 180 gear but after pricing it out utilizing some second hand parts but new gearboxes it came in around $15K CAD for parts. The cheap guy in me started thinking about the free PA18 gear sitting in the corner of the shed. Just an idea I wanted to get some opinion on is all. I think realistically I'll pony up for the proper conversion parts or leave well enough alone.

Any other feedback is appreciated!

I like te idea Rogue. But I do think it would he hard to accomplish. I would contact grove and see if you could get a set of aluminum spring gear made up. This should be cheaper. And also a bit lighter then 180 gear...

I'm currently investigating the possibility of converting my C172 to tail dagger also, but i need to keep it certified.

What did the $15,000 quote cover parts wise? I was going to contact Cessna with a parts list and get a quote to find out if it's viable for me or not....

You can build up the gearbox using the Bolen or Bush method with all bulk flat stock then, use the original 172 main gear with a short extension to achieve the 11 degree 3 point angle of attack.

Yeah that's a good idea contacting Grove, I'll do that. The gear is actually pretty cheap though for the early 180 gear, I guess everyone wants the heavy gear but maybe they can make it up as a full piece and lighter.

I priced out cessna parts new and salvaged. Salvaged parts came out at around half that but they are old parts so who knows what sort of abuse they've had, and condition they're in until I have them in hand, probably need to NDT. I can send you my pricing and parts spreadsheet if you like.

I'm thinking maybe leaving it a tricycle now and keeping the Scout for fun and the 172 for family trips. I originally thought maybe I could build it light with TD conversion and have a four seater that performs close to the Scout that I could fit a growing family in but still similar fuel burn tot he Scout. Probably wishful thinking, maybe be better money spent putting it on floats instead.

Early "wheels aft" 180 gear is generally $3k plus. Another option is early 170 gear (1948-mid1953) which I've seen go for $500. Or there's 170 "Lady Legs" (mid 1953- 1956), those are generally cheaper than early 180 gear legs but still spendy.

Grove can make any gear legs you want of course, but it's not going to be inexpensive.

I have custom Grove gear for my 172 conversion project, but they were built to my specs and used my engineering data, so you can't buy that design through him. You would have to have your own gear legs first designed by an aerostructures engineer, and then built by Grove, at "full retail" price for both.

What you need to know before you waste any money is that NO Grove gear leg will be able to fit into ANY standard or aftermarket STC Cessna gear box. So you will have to have an aerostructures engineer redesign those gear boxes too, just like I had to. And then you have to build them, which is not that big of a deal. And then you will need to come up with a way to align and install them in an existing airplane WITHOUT the benefit of the factory Cessna assembly jigs. That's a bigger deal than it seems. Did I ever mention the six months that this took?

Then you will have to find a way to install the bulkhead reinforcements. Put a few bucks into the savings account for the engineering and design work there.

After you get to that point, I'll tell you about the cost of drop testing and the deflection vs. load curves that need to happen so you don't shock load the rest of the airframe.

Remember the part in the second Star Wars movie where Yoda looks at Luke and says... "you will be..." ?

This forum already has the answer engraved on the walls: Just buy a Maule

I came accross that method of using the original gear and fitting extensions. It actually gives the C150 it was fitted to a very nice stance.



The box section in this one looked to have had the front and rear bulkheads replaced with new ones, a thicker lower belly skin, and a massive plate that screws down accross the top of it that was at least 0.080" or more thick. This was done in accordance with an STC, but i'm not sure which one.

Keeping the 172 and the Scout seriously sounds like the cheaper option. Plus, a scout is a ton of fun...

You can build up the gearbox using the Bolen or Bush method with all bulk flat stock

Hey Bush Master, do you have/know any more info about this? I've tried finding info on the Bolen STC with little to no luck (unless someone here can help out). What is the Bush method, i've not come across that in my searches? I'm more than happy to pay for an STC or engineering information that can lead to this being a certified mod if someone has it available. Feel free to PM me if it's getting off topic.

Thanks
Brady!

Brady,

Bolen is no longer around, but I believe Bush bought the STC. It was a good design and many were sold, however I understand you can no longer get this from Bush. I designed a Tail Wheel airplane based on a 175 using features of the above and my own designs. I used the original Cessna 172/175 main gear with extension to achieve the desired 3 point angle of attack. Many people mistake the airplane for a Cessna 180, I have about 800hrs Back Country flying on it an love it. All the parts are easy to make, no compound forming, just simple milling of the 4 slots for the landing gear. Anyone thinking of doing this needs to get PAZMANY LANDING GEAR DESIGN FOR LIGHT AIRCRAFT. I will PM you.

Monte

Ok, this is gonna sound really crazy...sell ur 172 n buy a 170 [emoji854]


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Ok, this is gonna sound really crazy...sell ur 172 n buy a 170 [emoji854]

Not sure if you're talking to me or the op. Lol.

My reason for not buying a C170 first and formost is that there are only 20 - 22 of them here in Australia, and none of them are for sale (that i can find)... Importing costs a fortune, and currently my 172 is dissasembled to a state in which it would be convenient to do the conversion.

Does anyone know if the original 172 main gear will fit into the relocated cessna box section?

Hey Brady & Bush Master thanks for the info, someone else was telling me about a conversion using the original tricycle gear and gearboxes, I sure would be interested in getting more info on that process. Still jumping between leaving it alone and doing the conversion.

Brady I haven't got my project back here yet but I'll be pulling off some floor and belly skins late in the month, I'll do some measuring up and see what I can come up with and let you know.

Ezflap, thanks for the insight man, sure sounds like you've done a lot of work in that regard. I think I'd just stick with some 180 legs, haha.

Skalywag, 170's with an 0-360 in Canada are priced steep, getting up in the same range as a C180, I need a 180 hp so a standard 170 doesn't work for me, plus it's just a fun project. I tried denying it but I love having a project to wrench on, especially in the winter. I also really wanted to go experimental/amateur built and this project came with a lot of the hard work done and an OH'ed 0-360 so I can do the conversion if I like, take my time and still come in pretty damn cheap, labour not counted of course. Win, win!

Brady wrote:
Does anyone know if the original 172 main gear will fit into the relocated cessna box section?

Sorry, not on my 1956 Cessna 172, and I doubt that any others would do it either. You (unfortunately) cannot take the tricycle gear boxes and move them forward. The distance between the "landing gear bulkheads" is different.

The "taildragger" gear bulkheads (front and rear door post bulkheads) are 7 inches apart, and with the various doublers in there the Cessna "gear box" fittings are 6 7/8 inches long between the mounting flanges. The "tricycle" gear bulkheads are, I believe, a bit further apart than that.

If you are interested, the reason for all this mayhem is that the (original taildragger) door post bulkheads are so close together that it makes everything hard to work on, and there is no room to put any sort of an adjustable shim or mechanism in there. The bolt heads are difficult to get to, busting your knuckles and creating previously non-existent curse words.

So in 1955 when Cessna engineers were told to develop a tricycle gear for the 170, they had the opportunity to redesign and improve the gear box system from a clean sheet, and to get away from the service difficulties that had become known in the years since the original 170/180 aircraft were in service.

THIS is why the tricycle gear box fittings and attachment methods look so much different than the original taildragger gear boxes. This is why the landing gear sticks through an oval shaped hole in the inboard gear box on the trike version... because of problems and cracks and PITA working with the previous (classic taildragger) design where there was very little "edge distance" for the hole in the end of the gear, and very little room for the end of the gear to sit in the I/B fitting. Cessna wanted to use cheap extrusions and a minimum of milling/drilling/lathe'ing operations.

If you can actually fabricate new, approved, acceptable fittings from flat stock using an old Bolen conversion, then find the documentation on the Bolen conversion and do it that way.

Also, pay attention to the aluminum angle reinforcement that runs spanwise across the entire top/front LG bulkhead. It needs to have a reinforcement there, to transfer and "balance" loads from one side of the airplane to the other. The 172 does not have this piece because it doesn't need the load path there for the trike gear. And let's all give Cessna a big hand, they saved fifty cents by not installing this piece in the 172 !!! Now try and install this piece into an existing (completed) aircraft without disassembling the entire forward fuselage, the control "tunnel" removing the skins, etc..

But wait, Cessna used a custom extrusion to get a "closed" angle of 70 degrees instead of 90 degrees... the angle of the forward floorboards slanting downward demands this. Now, go to your local metal supply house and find a 70 degree closed angle in 2024-T3. Don't worry, I'll be there waiting with the tranquilizer gun and "straight jacket" after you've tried six or seven times