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I'm looking for an authorative textual source (and indeed any guidance would be useful) on the relationship bewteen a given airframe and the maximum power that it can realistically harness/cope with from a piston powerplant.

For example; if looking at something like a warbird, is there a theoretical way to work out the engine power beyond which the basic airframe design would be unable to cope with the excess torque/force/weight or is it more of an art than a science?

Thank you!

J Bronk
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    The art comes from balancing dozens of competing objectives and limitations in a series of compromises to produce the final product. There are just too many variables. – John K Feb 11 '19 at 17:58
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    Hanging a different engine is not a simple bolt-on. There are often other modifications involved, so that raises the question of how much you are willing to modify the airframe, and to get the appropriate analysis and paperwork, to back that up. Just to give you an idea, here's a turbine c206: https://www.soloy.com/8203soloy-mkii-turbine-206.html – Adam Feb 11 '19 at 18:34
  • I assume you mean a single engine? Because a 2-engine design with contra-rotating propellers has way less torque problems. Of course, that's not easily to retrofit to an airframe designed for a single engine, single propeller. – MSalters Feb 12 '19 at 14:33

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It really depends on how long you want to fly for. Big engines generally require a lot of fuel so dropping a huge power plant on a small airframe may burn through all the fuel you can practically carry fairly fast. But if we assume that you don't want to fly all that long then we run up against the next issue.

You can always try to bolt a merlin engine to a C-150 airframe if you are willing to get creative with your bracket fabrication and perhaps beef up the nose wheel. The issue you will run up against here is at least in part a CG issue. A 1600LB engine on the front of an 800 LB airframe is bound to get interesting. But lets say you move the wings around, and put some ballast in the aft areas to even it out a bit and get a workable CG. You now have nearly 1600 HP up front to have fun with. So what happens?

If you can get this beast to fly you will eventually hit aerodynamic issues depending on the airframe, like Mach Tuck and ultimately at some point you are just going to rip the airframe apart.

So you need to balance structural integrity, aerodynamic stability, CG, the ability to carry enough fuel to actually do something practical and the limits of actually building an aircraft mechanically. There are also practical limits on scaling piston engines in general.

Dave
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  • Thank you - that's helpful! Presumably though vibration becomes a serious problem at some point? – J Bronk Feb 11 '19 at 19:09
  • @JBronk at some point maybe, but we have gotten pretty good at mitigating that over the years and you could likely mount a lot of engine on a very tiny airframe without issue. – Dave Feb 11 '19 at 19:10
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    Uh actually there are quite a number of things stopping you from strapping a Merlin engine on a stock C-150, not the least of which is the airframe is probably not going to be able to support the engine weight (the dry weight of the engine exceeds the BEW of a C150!) nor the thrust loads on it without breaking.... – Romeo_4808N Feb 11 '19 at 19:28
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    @CarloFelicione it was more of a humorous example than a practical one. It would however be an interesting experiment to see how much reinforcement would be needed to pull something like that off. – Dave Feb 11 '19 at 19:31
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    Another problem that this "super 150" would need to overcome is engine torque. A F8F Bearcat could not use full throttle on the takeoff roll as the plane would tip over, dragging the wing along the runway. (Source: Norwood Hansen, WWII pilot of, and an owner of a Bearcat post-war, now deceased.) -Skip Miller – Skip Miller Feb 12 '19 at 02:35
  • I'd be extremely worried about torque adding a Merlin to a C152. More than the CG tbh, as the torque may mean you won't even get off the ground to be troubled by that CG :) – jwenting Feb 12 '19 at 04:49
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Whatever point the limit is they were at it by the mid 1940s, and went beyond it with lighter, more powerful jets. Make no mistake about it though, these aircraft had to flown extremely carefully at low speeds lest the engine/propeller torque become too strong to control. Jets greatly lessened these issues and offered even higher speeds.

But you may find more happiness in discovering how little power you need to fly. Try building gliders and then incrementally add power. You will see there is no point in going anywhere near the "limit" in recreational flying. You may have an appetite for higher performance for aerobatics or greater speed, but it comes at a cost in weight, fuel consumption, complexity, and risk.

It takes 4x more thrust for a plane to go from 100 to 200mph, 9x more for 300mph, and 16x more for 400mph. What are your goals? This is a ladder that can only be safely climbed from the bottom to the top. But throwing a huge engine on an airframe, as fantastic a garage project as it may seem, would probably be making life a little shorter.

All that said, assuming CG is worked out with counter weights, new engine mount, moving wing, etc., your airframe stress limits are already there in your POH as neg and pos Gs, and Vne. A larger engine makes it easier to reach Vne, but you can do it with your original in a dive. But with your heavier re-make, stall and maneuvering speeds will have to be carefully checked, as well as rudder authority in low speed/high power (TakeOff/GoAround) situations. Your new plane will have much better climb performance.

So, this may work out after all by taking a 170/172 engine and putting it on the 150. I would start by doing some research (so lucky to have the internet). The Screamin' Sasquatch might give you some ideas.

Robert DiGiovanni
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