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Old 05-27-2020, 12:30 PM
Taltruda Taltruda is offline
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 333

Originally Posted by RV8AHopeful View Post
Charlie, interesting project!

I've been curious what all goes into adapting an automobile engine to work in an airplane. On the surface, seemed as easy as plopping it in and putting a reduction drive between the engine driveshaft and the propeller, but obviously things are never that easy haha.

I'm curious: what do you think of Mercedes' M139 2 liter engine outputting 416hp?

Seems like it would be the perfect automotive engine to stick in a plane? It's a Mercedes engine so you know it's going to be very reliable. And it's a 4 cylinder only 2.0 liters that is producing over 400hp. My guess is that weight will be the biggest problem? I believe it weighs 385 lbs. But given all of that power, the weight should be easily overcome so I suppose the real enemy will be CG?

Also, I'm no airplane engineer but how does one increase MTOW? Will extra power increase MTOW or is MTOW dictated by something else in the structure of the plane? Or will engine power increase MTOW slightly, up until you reach structural limits of the airframe?

So if engine power will increase MTOW then you don't have to worry about the extra weight of the engine, except for the CG, is that right? And then could you just add counter weights to rebalance the CG?
There is so much more to adapting an auto engine to aircraft than just plopping it on with a gear reduction. There are heat issues, lubrication, redundancy problems. they can be addressed, but when it comes down to it, the engine wasn’t designed for that application. I can imagine the engineer that designed whatever auto engine you are using , he would stomp his feet and pull his hair out saying, “if I knew that you wanted to do THAT with the engine, I would have DESIGNED it differently!”I have yet to see a successful auto engine conversion.. yes, people have flown auto engines on planes, but there is always problems that seem to take lots of time money and lack of promised performance.. eventually the plane either gets totaled, scraped, sold, or converted to an aircraft engine. Please someone prove me wrong and show me a successful plane with an auto conversion. If they were so wonderful, there would be a lot of them out there. There is so much more to an engine than water cooling, and prob reduction drives. Car engines don’t have the surface area on the rod bearings, or the beefiness in the crank and rods that aircraft engines do. Aircraft engines and their crank, rods and bearings are tremendously oversized for their hp load..low surface speeds on the bearings, wide crank journals, all add to their reliability. This isn’t meant to poopoo on the original poster and his project.. I support others experimenting with.. well, experimental aviation. I think it’s great that he’s doing something out of the box and having fun and learning from it. I was just addressing the guy who thought it may seem simple..

As for your MTOW question, there is a lot that goes into the max gross weight. One simple thing that I can think of is the main spar is only designed to hold so much weight, plus allowable “G” loading. Other factors to consider are fatigue cycles, motor mount, fuselage stringers etc...So no, you can’t safely increase takeoff weight based on more engine HP.
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Last edited by Taltruda : 05-27-2020 at 12:42 PM.
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