motor mount question

[pistoncan]

Hello, I am building a corvair motor mount for my Wagabond and wonder if anyone can tell me what thickness the 3/4 inch tubes generally are on a motor mount, from the engine back to the firewall. .035 or .049? I have to order steel Thanks Gary

[SuperPacer]

Hi Gary,

I have a friend here in at OGD (Ogden UT) that has two Dragonfly(s) that he has designed Corvair motor mounts for. He ran the design through a stress analysis program and he would be a good source for the tube size question you have asked. I will see him this weekend and get the info to you.

What HP / engine RPM do you plan to "shoot for"? With the Dragonfly, he uses a rather short prop (direct drive) and will see low 3000 cruse and 3800 + for full power, and plans on producing 100 hp (maybe a bit more at full power.

John Kobbeman
UTAH SWPC
801 499-0413

[Hillbilly]

Hello, I am building a corvair motor mount for my Wagabond and wonder if anyone can tell me what thickness the 3/4 inch tubes generally are on a motor mount, from the engine back to the firewall. .035 or .049? I have to order steel Thanks Gary

Gary, considering the mount is likely to be only a few feet of tubing total, and that the corvair engine is likely heavier (?)
I would go with the heavier of my options. look at it like this, lets assume your motor mount will use 10 feet of 3/4" tubing.

10' of .035 X 3/4" tubing - 2.67 pounds
10' of .065 X 3/4" tubing - 4.76 pounds

So for a gross increase of slightly more than 2 pounds you'll have a motor mount that is likely 4 times stronger...

And of course I was using 10' for mathematical simplicity - also note I nearly doubled wall thickness.

Your mileage may vary, best of luck- Hillbilly

[pistoncan]

Billy,
The corvair is a little lighter than an 0-200 Cont. ready to fly (220 lbs with a starter and altenator) Its also 4 inches narrower than a Lyc (which is why I was asking Steve about using a nose bowl from a 85 cub) On an airplane of the SWP type most guys are using a 68 inch prop and turning 3200 RPM. Somewhere around 100 net hp. (It was interesting that the corvair guys tested a 0200 on their dyno and found the net horsepower was closer to 90 than 100) They also said the 187 lb advertised weight of a O-200 was baloney.
The big thing for me is that it can be overhauled for 600 instead of 10,000. dollars. The parts to do the conversion are under 1,500 dollars.
The conversion uses a special distributor plate with two sets of points and dual coils, so it has redundant ignition with exception of the plugs. The engine runs on one set or the other, never both. There are about 500 planes flying today with corvair engines so I was told.

[Steve Pierce]

Bernie Pietenpol said it was the best engine he ever flew behind. I would check with William Wynne. He has built a Vag I think with a Corvair the last time i talked to him. I wonder if over building a mount can cause any sort of problem, maybe not enough flew? Not sure, just wondering outloud. :idea:

[pistoncan]

Bernie Pietenpol said it was the best engine he ever flew behind. I would check with William Wynne. He has built a Vag I think with a Corvair the last time i talked to him. I wonder if over building a mount can cause any sort of problem, maybe not enough flew? Not sure, just wondering outloud. :idea:

Steve, Sorry for the size of that corvair engine pic, I didn;t realize how large it was.
Yep I asked. I actually bought the tray (square) from WW but he is notorious for looooong delays and not responding to questions or emails. I know from his website that they figured the weight and balance for Dave's Wagabond at 17 1/2 inches forward of the firewal for the tray, but getting any more info out of him is near impossible Thats why I decided to try to find out from you guru's. The mount weighs less than 10 lbs. here is a pic

[Steve Pierce]

I have the Piper engine mount drawing. Working from memory it is .035" wall thickness. I will check tomorrow to verify if that will help.

[pistoncan]

Thanks Steve, thats kinda what i figured but Its good to hear it from you.

[Hillbilly]

Gary, thats cool that its lighter, I dont have any problem with the corvair engines I actually quite like them.
I used to hang around an old one armed mechanic that only worked on air cooled engines, he took a porsche engine and a corvair engine apart and compared everything, after about a week of studying he looks over at me and says "you know, Porsche ain't got a damn thing on Chevy"...

I tend to overbuild stuff if the penalty isnt too stiff...
Good Luck

[Frankenpacer]

So for a gross increase of slightly more than 2 pounds you'll have a motor mount that is likely 4 times stronger...


What is the advantage of "4 times stronger" if the lighter tubing will not fail in the first place?

[Steve Pierce]

The mentality around here is oil field tough however sometimes you don't want it that tough. Case in point my buddy wanting to know why he shouldn't use grade 8 bolts instead of AN. I relayed to him how many bent bolts I had pulled out of landing gear that held but bent. What would happen if a grade 8 was used? Probably shear and then what? :shock: Wish I had a little more engineering background. I just try and look at what the manufacturers have done and figure out why.

Piper's original engine mounts for the PA16, 20, 22 and 150 hp Pawnee were 1025 steel .035" wall thickness. Univair now uses 4130 on theirs.

[Frankenpacer]

Steve,

It's far from my intent to pick on anyone here, rather, it's to foster critical thought. "Stronger" is only better if the design is deficient to start with. "Just a little extra weight" is a slippery slope indeed...

BTW, I don't have the drawings in front of me, but by the looks of the bed type mount pictured for the Vag, it would easily handle a Big Block Chevy V8. It reminds me of the story I read where a guy built a prototype mount for a V8 that he was attaching to a Pawnee. He constructed the mount from PVC pipe in the same OD as the steel he intended to use, and glued it together with PVC cement. Once satisfied with the shape, he decided to test it to destruction to see where it would fail first. He had to load it to something like 1000 pounds before it gave way. This was PVC and glue, remember... After that, he figured the 4130 tube should hold.

[Steve Pierce]

I didn't see anyone picking just relating my experience with my oil field buddies. They use oil field pipe for everything around here. :lol:

[Gilbert Pierce]

Sometimes building stronger is not better. It just moves the failure point somewhere else where it may be more devastating.
A aquaintenance of mine busted his landing gear so he "made it stronger". The next time he landed way too hard the landing gear did not fold up. The whole fuselage did. Now he didn't have to fix the landing gear but he sure made a mess of the fuselage. That is not to say you should not beef up a poorly designed part. If you beef up a properly designed part so it won't fail when you overload it you are asking for trouble.

When I inspect homebuilt aircraft one of the things I ask the builder is "What did you change from the plans"?
Often times the builder thinks he knows more then the designer so he makes changes. Sometimes those changes are dangerous. Sometimes they just add uneccessary weight. Light is better.

[pistoncan]

See! thats what I love about this site, So much expertise. I will build it out of 4130 3/4 x .035 I would go with what the plans say but they say "BUY wag aero part number xxxxx" for a lyc. but of coarse, no specs. and mention nothing about a corvair. Thank you all Gary

[Hillbilly]

I only suggested the heavier wall tube because I wrongly assumed that like most things automotive, that the chevy was heavier than the aircraft engine.
Apologies, BrayingHillBillyGoat

[skyking897]

I liked that "buy wagaero part #". Guess you can build/weld a whole airplane but they don't think you can build/weld a motor mount.
Looked at a set of Zenith Ch200 plans I have. The motor mount calls for 3/4" X .055" 4130 tubing.

[Hillbilly]

I liked that "buy wagaero part #". Guess you can build/weld a whole airplane but they don't think you can build/weld a motor mount.
Looked at a set of Zenith Ch200 plans I have. The motor mount calls for 3/4" X .055" 4130 tubing.

Well, I figure its only a matter of time before Frankenpacer contacts Chris Heinz and straightens him out concerning the slippery slope of using such a heavy tubing on a motor mount. Obviously Chris has no idea what hes doing...

[Frankenpacer]

Well, I figure its only a matter of time before Frankenpacer contacts Chris Heinz and straightens him out concerning the slippery slope of using such a heavy tubing on a motor mount. Obviously Chris has no idea what hes doing...

As a designer, I'm fairly confident Chris knows what he's doing. On the other hand, if he simply took someone elses design and arbitrarily made it "4 times stronger", then yes, he needs to be straightened out.

Each application must be evaluated on its own merit. Just because the birdcage mount on the back of an aircraft engine is constructed of X material does NOT mean the same material is acceptable for an engine of similar weight on a completely different type of mount (like a bed type for a Corvair). Each element of any mount (each single tube) sees a different load, and the tube in that spot should be sized to meet that particular load. A blanket statement that Piper used .035 wall on a Continental mount, so .035 will work on a Corvair is just as arbitrary as upping the wall thickness to make it "4 times stronger". I'm not about to perform a structural analysis of anyone's design here based upon internet pictures, but I will without hesitation call out someone who simply increases the weight of a component with no basis for doing so. Such activity is the bane of homebuilding, and always has been. My contention earlier is one simple thing - increasing weight needlessly is bad in airplanes. I invite ANYONE to prove otherwise.

I have been polite and I fully accept being proven wrong (I like to learn), but my patience wears a bit thin when the smart asses come out. If your feelings get hurt because you can't substantiate your position, don't resort to personal attack- take an engineering class instead.

[Hillbilly]

Frankenpacer,
First of all if you can make an "attack" out of what is obviously only a bit of sarcasm, you have my sympathy.
Second I never took anyones design and modified it in any way, I simply gave my opinion and gave a very vague EXAMPLE, of the weight increase (in reality it would be much lower, I used .065 wall to show how small the increase was) the statement you have focused on "4 times stronger" was indeed arbitrary it could be as little as .5 times stronger or 10 times, I don't know, I didn't do any analysis. I readily admitted assuming the corvair engine was heavier I also readily admitted my error.
My feelings were not hurt. As to being able to substantiate my position, If Gary wants me to I'll be happy to discuss it with him
But trying to persuade against the prevailing popular opinion, I really have better, more profitable things to do with my time.
Rest assured, I will not resort to personal attacks here on a web forum or calling anyone a smartass, Politeness is a matter of others perception- beyond basic etiquette. Lastly as to engineering, I rarely speak of my credentials, I sir, am a journeyman aerospace technician so I am reasonably confident that my own expertise is at least on par with your own. There are many people here that I have a great deal of respect for, I continue to try and foster friendships and relations with. I have read your posts on this forum and the other one and seen your handiwork and your attitude. You Mr Frankenpacer are not one of them. So please by all means continue to waste your time and breath on my behalf.
Sincerely, Hillbilly

[pistoncan]

Well I opened this can of worms so I should probably jump in here.
First off, Obviously, I don't know any of you personally, but I should say that I am in awe of all of you guys, Franken, Billy, Steve etc. You guys have done it. I am just an ole knuckle buster without degree one, or the money to get one. I just want a plane to fly. I tend to use the copycat method. It would be nice if WW just answered my question and said "we used .0xx) and be done with it, but not everyone is willing to take the time to share their thoughts like you guys. I will evaluate what you have said and make my decision and live or die by it. (Hows that for personal responibility?) lol This is the third project I have started in my long life (the other two were sold unfinished when life's other problems got in the way) I hope to finish this one as I am running out of time. LOL
One last comment, It seems to me that more than one person here has had their feathers ruffled and resorted to personal attacks. I sure didn't intend to bring that on, but thank you brothers. Gary Richmond

[Hillbilly]

Well I opened this can of worms so I should probably jump in here.
First off, Obviously, I don't know any of you personally, but I should say that I am in awe of all of you guys, Franken, Billy, Steve etc. You guys have done it. I am just an ole knuckle buster without degree one, or the money to get one. I just want a plane to fly. I tend to use the copycat method. It would be nice if WW just answered my question and said "we used .0xx) and be done with it, but not everyone is willing to take the time to share their thoughts like you guys. I will evaluate what you have said and make my decision and live or die by it. (Hows that for personal responibility?) lol This is the third project I have started in my long life (the other two were sold unfinished when life's other problems got in the way) I hope to finish this one as I am running out of time. LOL
One last comment, It seems to me that more than one person here has had their feathers ruffled and resorted to personal attacks. I sure didn't intend to bring that on, but thank you brothers. Gary Richmond

Gary I'm truly delighted to offer my opinion anytime, I rarely give "advice" because people like to cast blame for bad advice :( but not so much for a faulty opinion :)
I really have not gotten "ruffled" at franken or anyone else, shucks I've been using the internet for almost as long as its advent,
I've seen people say things that were not only horrible but just plain evil and they would never say them in person.
I learned long ago to take the wheat with the chaff and not set too much store by it. I was only being humorous when I offended Franken, but sadly I would have made the same comment had he been standing there. I've never been known as the guy that always says the most appropriate thing at the most appropriate time. But when pushed I will speak my mind,
I have and its there for anyone to see. If the occasion should arise that you'd care to hear my possibly flawed opinion, I will be happy to rend it. Have a great day, and best of luck with your project, Hillbilly

[Frankenpacer]

Well I opened this can of worms so I should probably jump in here...

No "can of worms" as far as I can see... I've been on these discussion boards for a long time myself and I know that there are people who like to contribute items of substance, and there are others who failing that, resort to sarcasm. I respond to each according to individual merit. No problems here.

I hope that we can get past this nonsense and uncover the information you need to complete your project.

[Hillbilly]

Well I opened this can of worms so I should probably jump in here...

No "can of worms" as far as I can see... I've been on these discussion boards for a long time myself and I know that there are people who like to contribute items of substance, and there are others who failing that, resort to sarcasm. I respond to each according to individual merit. No problems here.

I hope that we can get past this nonsense and uncover the information you need to complete your project.

Franken, Look I assumed things about the Chevy engine and I was wrong, Then you proceeded to educate me in engineering ASSUMING I didn't already know...is one of us more wrong than the other? I don't think so.
but now for you to come back with what is purely and plainly a retaliatory response pointed at me and me only CLAIMING I do not contribute any "items of substance"... Well thats just playground bull****. "Resorting to sarcasm?" I defy you to claim with a straight face you've never made a sarcastic remark. From now on I'll be filtering your posts so I am not forced to gaze upon your childish drivel, so for all practical purposes you will be rid of me. Have a nice life.

[Frankenpacer]

...From now on I'll be filtering your posts so I am not forced to gaze upon your childish drivel, so for all practical purposes you will be rid of me.

With that unpleasantness behind us, hopefully the thread will get back on track.