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I happened to be walking past a small outdoor fair today, and saw someone flying a paramotor. The craft took off and started a steep ascending turn losing speed rapidly, seemed almost to come to a stop (relative the ground, but the flags nearby didn't show much wind, either) before the pilot brought it back to more level flight and got some way again.

I'm not a pilot, but I'm roughly aware of the mechanism of stalls and the consequent danger of high angle of attack, low speed maneuvers and I was briefly worried that there was going to be a problem.

In hindsight, the craft clearly has a very low stall speed (not much more than walking speed, I guess) but it did leave me wondering.

Are these craft are more or less at risk of stalling than conventional fixed wing aircraft?

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  • $\begingroup$ And why should we actually care about stalling? Even without any lift, aren't we supposed to soar down gently, para(chute) way? $\endgroup$ – szulat Sep 26 '15 at 12:21
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I suppose with paramotor you mean a powered paraglider, as pictured below.

Paramotor

Paramotor (picture source)

Since they obey the same laws of physics, they also stall, just like a fixed-wing aircraft does. There are, however, some peculiarities which should be mentioned:

  • The shape of the wing is produced by ram air filling the parafoil, so speed will help to keep the wing inflated and in shape. At low speed and in gusty winds the risk of the parafoil collapsing (severe deflation) is higher. This was the reason for several fatal accidents in the early times of paragliders, when the aspect ratio of high-performance parafoils was increased beyond safe limits. Today's designs will normally quickly recover from a deflation event, but not without a substantial loss of altitude.
  • The pressure forces on the parafoil will change its shape depending on its angle of attack. At low speed the angle of attack is higher, resulting in a suction peak close to the leading edge which in turn will increase the local camber of the foil. This leads to severe lift loss in a stall, and recovery needs some altitude. A fixed-wing aircraft, on the other hand, can employ an airfoil with only little lift loss in a stall, so recovery needs less altitude. This is by no means universal - there are also fixed-wing aircraft with less benign stall characteristics.
  • If the stall on a fixed wing starts near the wing tip, the aircraft might enter an uncontrollable roll. This can happen in steep turns and with badly designed aircraft. This is not possible in a parafoil; it might instead slide sideways with a partially collapsed parafoil, but stay upright due to its low center of gravity. If the pilot panics and pulls his brake lines on one side during a stall, the parafoil can enter a spin. Similarly, if the pilot of a fixed-wing aircraft insists on applying rudder when stalled, the aircraft can also enter a spin.

Generally, due to their low wing loading and the drag of the raiser lines, paramotors fly at much lower speeds than aircraft, so accidents involve less energy, but the pilot has no protection around her/him, so fatal accidents do occur more regularly than with fixed-wing aircraft. For paragliders, Wikipedia gives a number of 2 fatalities per year for 10.000 pilots.

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  • $\begingroup$ But how bad it is when it stalls? Since parachutes rely purely on post-stall lift and paraglides are not much smaller I would expect the remaining lift after stall to still be enough to keep the rate of descent in survivable range. $\endgroup$ – Jan Hudec Sep 26 '15 at 20:12
  • $\begingroup$ Paragliders might also fly more often, so I'm not sure the statistic is comparable $\endgroup$ – Antzi Sep 26 '15 at 21:03
  • $\begingroup$ @JanHudec: Good fixed-wing airfoils have the same lift coefficient for several degrees of AoA into the stall. Parafoils go from approx. 1.5 right before stall to 1.1 with separated flow, and their effective area shrinks because the parafoil starts to collapse. Stall speed (forward) is slightly less than the vertical speed when used as a parachute, so a couple of meters are needed to accelerate downwards. When dropping in parachute mode, a paramotor pilot will get hurt and might kill himself if he is unlucky. $\endgroup$ – Peter Kämpf Sep 27 '15 at 21:01
  • $\begingroup$ fatal accidents do occur more regularly than with fixed-wing aircraft... I disagree. Do you have a source? $\endgroup$ – Cloud Jun 3 '19 at 12:00
  • $\begingroup$ @Cloud: Yes. Not only Wikipedia (see last line of the answer), but also statistics for paragliders and fixed wing aircraft. Unfortunately, not normalized to flight hours, but 34 accidents with severe injuries or death with fixed wing aircraft versus 139 with paragliders support my position. $\endgroup$ – Peter Kämpf Jun 3 '19 at 16:51

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