Knowing that the Wright Flyer was the first successful heavier-than-air powered aircraft, I'm fascinated by its operation requiring a prone position.

Searching further, I was surprised to find out that the recent EMG-5 motorglider also puts the pilot in a "prone" position.

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My question:

Can someone expand on the alleged minimizing g-force effect of the prone pilot position. Why isn't it more common, especially in fighters?

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    $\begingroup$ It looks pretty uncomfortable to me - imagine holding your head & helmet like that for 30+ of aerobatics. Ouch! $\endgroup$ Jun 20 '14 at 16:46
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    $\begingroup$ That's why I tagged the post human-factors :-) $\endgroup$
    – menjaraz
    Jun 20 '14 at 16:58

It is true that the body can withstand higher g-forces in the prone position. The reason is that in the prone position, g-forces don't pull blood down to the lower body and away from the brain, as happens in the seated position.

The concept was explored for combat aircraft with the Italian Savoia-Marchetti SM.93 and the British Gloster Meteor F8. These aircraft revealed some practical issues with the prone position:

  • It is much less comfortable for the pilot than a seated position, especially during long flights.
  • It is much more difficult to design an ejection/escape system. The Meteor F8 required the pilot to jettison the rudder pedals and crawl backwards to an escape hatch.
  • It affords much less visibility to the pilot.

In light of these issues, it was found more practical to stick with seated pilots. In modern times, g-suits and advanced training techniques like the "hook" maneuver have allowed fighter pilots to withstand higher g-forces than in the past. Both involve reducing blood pooling in the lower body by squeezing or flexing leg and abdomen muscles to restrict unwanted blood flow and pooling.

  • $\begingroup$ Escape is not a problem: The DFS 228 used a jettisonable cockpit section which would descend on its own, small chute, and when the cockpit was low enough, the pilot was extracted backwards by his own parachute. $\endgroup$ Feb 22 '15 at 21:49
  • $\begingroup$ @PeterKämpf ... which was probably a lot harder to design and more expensive than a traditional ejection seat. $\endgroup$
    – TypeIA
    Feb 24 '15 at 0:42
  • $\begingroup$ … not when the typical operating altitude is 60.000 ft. The DFS 228 was planned to be launched in piggyback position by another aircraft and to be powered by the same rocket engine as the Me-163 for flight far above any other aircraft. I would also dispute that this would have been harder to design than an ejection seat - at that time (1943) you couldn't get them that easily. $\endgroup$ Feb 24 '15 at 15:40

The prone position was researched a lot in Germany before anti-g suits were invented. The human body can tolerate g loads a lot better lying than sitting. Martin Eiband collected a lot of data on this, if you want to know more, google for "Eiband diagrams". If you want the full picture, read the Army Aircraft Crash Survival Guide. It comes in five volumes, and volume 1 covers the design criteria. The Eiband diagrams below are lifted from this source.

Eiband diagram for lying human

The frontal acceleration is quite similar to that of the lying pilot. Please note that for a duration of 0.1 seconds, the limit is 40 g! For 1 full second, this goes down to 10 g. Any more, and the pilot might show mild symptoms of stress. And if you go up to approx. 23 g for 1 second (logarithmic scale!), there is a risk of serious injuries.

Eiband diagram for sitting human

This diagram shows the limits for a sitting human. At 0.1 seconds, the limit is only 10 g. For a full second, this goes down to 5 g, even though the data does not cover this duration.

One notable research plane was the B-9, built by Akaflieg Berlin, a student group which designed and built airplanes during their time at Berlin's Technical University. This airplane was designed for 22 g and used a prone position for the single pilot. It was tested in 1943 and proved the validity of the concept. Previously, Akaflieg Stuttgart had built the fs-17 for the same purpose. Also, the Horten flying wing gliders were flown in a kneeling-prone position.

Horten IV pilot position

Horten IV pilot position. Note the chin rest!

There was even a ground attack aircraft designed for a prone position, the Henschel 132, a small, jet-powered single seater. A prototype was almost finished before the war ended, and it never was used operationally. Similarly, a rocket-powered, glider-like high altitude reconnaissance plane, the DFS-228, was only flown without the rocket engine.

The prone position has a major disadvantage: Visibility is poor where it counts most. Downward visibility is great, and even forward visibility can be considered acceptable, but especially in turns the pilot must be able to look up. A tight turn is mostly a horizontal looping, and a lying pilot needs to lift his head up against the g forces in a turn - this is extremely tiring! The whole concept is impossible for air combat, where g forces are highest.

The prone position has an advantage for small aircraft where the drag of a sitting pilot would be unacceptable. It has only found widespread use in hang gliding, but all other aircraft prefer a sitting or, mostly in gliders, a backwards lying pilot.

  • $\begingroup$ The chin rest make me think of violin :-) $\endgroup$
    – menjaraz
    Jun 21 '14 at 16:12

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