What limits high altitude powered paragliders other than the engine?

Question

Due to their extremely low wing loading, powered paragliders stall at very low indicated airspeeds compared to other airplanes. A typical paraglider can continue flying all the way down to 10-15 knots.

That suggests the idea of using a fabric wing for flight at very high altitudes. At 100,000 feet, for example, an IAS of 10 knots corresponds to a TAS of 85 knots, well below the speed of sound. Suppose that we take a power source that works at low air density – say, a solar-powered electric motor or an engine with an onboard oxidizer tank – and attach it to a paraglider. Would anything prevent such an aircraft from flying stably at 100,000 ft? For example, would flutter be an issue?

Answer 1

Stalling speed increases with altitude. Maximum altitude is the point at which stalling speed rises to meet maximum airspeed. To get higher, the options are to; increase thrust, increase wing area, reduce weight, or improve aerodynamic efficiency. These principles apply to all aircraft using aerodynamic lift.

In the case of the paraglider, its aerodynamic efficiency is not very good. So maximum airspeed is low and the stall speed will soon catch up with it as it climbs. It is worth sacrificing a little weight to greatly improve the wing profile and enclose the pilot and engine - but the plane is then no longer a paraglider.

The other main problem with a simple paraglider is pilot support systems. Time to altitude is measured in hours, during which time the pilot lives inside a pressure suit; the kind of thing worn by Lockheed U-2 pilots on an intercontinental reconnaissance flight. Refreshments, especially drinks, will be necessary. If you put all the support equipment in a great big backpack, like an Apollo astronaut but with longer EVA endurance, where are you going to put the engine? Something a step up from a paraglider again becomes a necessity.

Answer 2

1. The engine is an issue as you have already identified so I won't address it here. But suffice it to say that most paramotors run on two stroke engines and don't perform well in low oxygen air. You also have to adjust the carburettor in flight which is often impossible. There's a reason Gilo Cardozo took so long in designing a new motor for his Everest flight and why he went for a rotary engine design.

   Existing paramotors are more likely to be limited by fuel capacity (see point 2 below) than the engine though.

2. It depends on your definition whether this counts as engine related at all (it's not specific to internal combustion propulsion), but even if you ignore the lower oxygen density, fuel capacity is an issue for altitude flights. I don't know whether you are concerned about taking off from close to sea level or not, but even the most efficient wing/pilot/engine combinations take a decent amount of fuel to climb and there are obviously diminishing returns at lower air densities.

   For foot launch, there's a physical limit to the mass of fuel you can carry. The same also applies to powered paramotor trikes but the sub-70kg requirement often is more relevant.

3. Related to fuel is supply of other essentials for a pilot, predominantly oxygen and heating. There is only so much mass of pilot support gear that can be carried and of course there's a balancing between climb rate and takeoff mass. There's also only so much insulating clothing a pilot can wear and still launch a paramotor even if you use the highest fill power down and electronic heating. A flight above FL200 will take several hours.

   You also need to be aware of sweating too much at warmer altitudes and then your sweat freezing when higher up and in wind chill.

4. Aerodynamics. As @Guy Inchbald explains, stalling speed increases with altitude. Again, there's a balancing act here between airspeed and weight.

5. In many jurisdictions, above a certain altitude you're in Class A airspace. While it's not impossible to get clearance, airspace concerns are a limit that affects record attempts. In the US, everything above 18,000 ft msl to FL600 is Class A and all operations must be conducted under IFR.