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I've learned a lot of great information about aviation on this website. One insight I learned from a poster here is that, all else equal, for every doubling of engine power that you give an airplane, speeds increase by about 25%.

However you're not always comparing "all-else-equal" airplanes, for example sometimes I want to compare between a single engine airplane a twin engined airplane. Yes the second engine adds weight and drag, but the factor that I have the most trouble accounting for are the difference in aerodynamic requirements between the airplanes.

My understanding is that regulations require all single engine airplanes to have a stall speed of no greater than 61 knots, while a twin-engine plane could have an arbitrarily higher stall speed. The higher stall speed is directly correlated with higher aerodynamic efficiency, meaning that it should be able to achieve higher speeds than a single engine airplane of the same mass, drag, and power, would be able to achieve.

So I am looking for a simple rubric with which to be able to compare single and multi engine airplanes. I think that there are really two parts here: (1) identify realistic stall speeds for the multi-engine airplane and (2) formula for comparing top speeds between planes with X mass and drag and Y power and Z stall speeds.

Any ideas?

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    $\begingroup$ There's a pretty big difference between the engine in front of the hull or in front of the wing. I'm not sure a apples to apples is possible. $\endgroup$
    – ratchet freak
    Apr 25, 2017 at 16:14
  • $\begingroup$ @ratchetfreak thanks, it would definitely be illuminating to have a framework for comparing an airplane with a stall speed of 63 knots and one with a stall speed of 80 knots. If you are building an experimental or kit plane you don't have to obey the standards so you could build a single engine plane with whatever stall speed you want. $\endgroup$
    – Charles847
    Apr 25, 2017 at 16:27
  • $\begingroup$ The stall speed limit is 61 knots, as demanded in FAR part 23.49. $\endgroup$
    – Peter Kämpf
    Apr 25, 2017 at 19:58

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Your question really needs further bounding to be able to accurately answer. I will discuss a few aspects.

First @ratchet freak is right on the money. What engine mount are we talking about? A podded engine on the single and twin (or multi, since your one of your questions mentions multi) or some different configuration.

In the late 70's and early 80's there were a bunch of academic and not so academic papers and articles written on the topic. Generally the focus was on the Cessna 336/337, O-2 and Defiant type aircraft. There were also papers on a loss of engine scenario. This got rekindled for the Voyager aircraft design and promotion. The Voyager had a L/D of 27. And it was intended that one engine shut down in cruise flight.

The 61 knot rule for single engine is said to be influenced by crash survivability. I get that, but I will editorialize that several studies have shown that the chance of something bad happening with the loss of an engine is greater in a multi.

Top speeds, stall speeds and mass (weight) are different parameters, subject to different trades. Furthermore, putting this into the context of existing, production or some arbitrary subset of the universe makes your question hard to answer.

The real answer is that one can design for what you need. You can have a very slow stall speed in a single or a multi, what would you like? You can have a nice L/D with any number of engines, what would you like? And you can haul lots of stuff, although there are not heavy singles around. But to generalize all three parameters, of these non-linear trades, into one rubric, would likely challenge even the most mathamatically talented aerodynamcists that I know. If you can tighten the bounds on the problem, you might get a better answer.

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  • $\begingroup$ Thanks. My motivation is to answer the question, "if I had 2 planes of the same cabin volume and weight, same wing shape, and with the same total power, but one had the L/D for a 61 knot stall speed and Y max speed, and one had X higher stall speed, how would I compute the max speed of the second plane?" Really I am trying to understand the tradeoff between going single or double engine regarding the regulatory limits on stall speeds that pertain to single engine planes, and to what degree the tradeoff of being able to have a lower drag wing compensates for the performance tradeoffs of twins. $\endgroup$
    – Charles847
    Apr 26, 2017 at 3:16
  • $\begingroup$ And yes I understand that choosing a lower drag wing has other tradeoffs. I have read several articles that reference those crash statistics showing twins having higher accident rates than singles. Imagine a scenario where those crash statistics are mostly attributable to the skill requirements of piloting a manually operated, twin-engined plane, with an engine out, and that with modern fly-by-wire control systems that make piloting a twin with an engine out arbitrarily simple, that there is a new equation for evaluating the value of twins. $\endgroup$
    – Charles847
    Apr 26, 2017 at 3:23
  • $\begingroup$ So not only at least as safe as singles, but with new aero diesels lowering the percentage of fuel cost to total operating cost. $\endgroup$
    – Charles847
    Apr 26, 2017 at 3:24
  • $\begingroup$ Are you asking about the impact of a higher wing load on cruise speed? $\endgroup$
    – mongo
    Apr 26, 2017 at 13:17
  • $\begingroup$ As for the stall speed of the multi, how about 79 knots (~30% higher than the single). Piper Aerostar had an 81 knot stall speed so 79 is feasible even for a light twin. As for the engine placement, whatever is easier to compare. Maybe assume that both the single and the twin have their engines concealed in the rear fuselage so that the aerodynamics are more similar. My intuition is that assuming the engines are in the rear fuselage and pushing propellers that are closer to the centerline (than they would be if wing mounted) makes engine out controllability less of an issue as well. $\endgroup$
    – Charles847
    Apr 26, 2017 at 14:07

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