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Why radial engines? Why not?


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At a recent airshow, I asked everyone I could find who had flown one in why early planes had radial engines and why not now. No one seemed to know.

 

I've taken three wild guesses (I am not an engineer); can anyone tell me if they make any sense, and what the real reasons are?

 

  1. Radials are smaller, therefore provide more power with less weight
  2. Radials concentrate their weight toward the nose of the plane, moving the CG forward, and maybe this is a good thing
  3. By putting the pistons out in the slipstream, radials are easily air cooled and don't need more complicated cooling systems

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In those early days (perhaps 1920s, or so), the radial had a better power to weight ratio, better cooling, and better reliability than the water-cooled in-lines and rotaries. You could add another bank of cylinders without increasing the frontal area, so much more power for essentially the same amount of drag. And though the Allison and Merlin engines were used in some of the WWII fighters, most bombers and virtually all airliners after the DC-3 came along, used the radials, right up until the advent of jets and turboprops.

 

You might wish to see more in this Wikipedia article, which goes into a lot of detail.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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In those early days (perhaps 1920s, or so), the radial had a better power to weight ratio, better cooling, and better reliability than the water-cooled in-lines and rotaries. You could add another bank of cylinders without increasing the frontal area, so much more power for essentially the same amount of drag. And though the Allison and Merlin engines were used in some of the WWII fighters, most bombers and virtually all airliners after the DC-3 came along, used the radials, right up until the advent of jets and turboprops.

 

You might wish to see more in this Wikipedia article, which goes into a lot of detail.

 

Effectively, turboprops have taken the place that used to be occupied by radials: Small form-factor and air-cooling: All cylinders in the cooling air flow, in the case of single-bank engines, equally (well, `temperature` in the case of turboprops); inherent simplicity for greater power-to-weight and shortness, in the fore/aft sense.

Turboprops bring a level of primary balance even the best radial engine can't match.

 

Where radials might still have a future is in the lighter end of the market, and powered by diesel/kerosene.

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