I have some anecdotal evidence that a ram air sport parachute requires less toggle input to stall at higher altitudes. A pilot needed one wrap of the brake lines around the hands to quickly, definitively, and repeatedly stall above 6,000 feet or so, and two wraps below that.
The stall maneuver was:
- fly in 3/4 brakes for a few seconds to slow down
- quickly lock out arms to maximum extension
Although I'm not discounting the possibility of this pilot being mistaken, I'm curious if this is expected behavior and what the physics behind it are (or might be) that are specific to parafoil type gliders. See my thoughts below on why a stall might be particularly complex here.
Update: Three different very experienced (in acro and/ or XC) paragliders just told me similar observations about higher altitude stalls: "lower brake pressure", "less brake travel/ range", "more dynamic", "similar to flying heavy or on a smaller glider".
What I found so far:
AoA, pitch, and IAS are expected to remain constant across altitudes for an unpowered parafoil in a stabilized glide.
A rule of thumb to get TAS is to add 2% to the CAS/IAS for every 1000 feet of altitude.
This question about stall IAS varying with altitudes has an answer that seems to mostly apply to powered aircraft. It mentions Reynolds number for smaller aircraft. Would that really be significant here?
Random explanation of paraglider controls, which are essentially the same as for a ram air parachute.
More info in case it helps:
A parachute like this is about 200 square feet. The total system weight is around 200 lb. The wing itself weighs 10 lb including the lines/ risers, which are around 15 feet long. It attaches to the pilot's harness at only two points, one on each shoulder. The fabric forms an arch in flight around 20 feet wide tip to tip, 5 feet tall in the middle, and 10 feet at the deepest (tapered).
The indicated or sea level airspeed is around 40mph with no brake input and 20mph in deep brakes. The glide is between 2.5 to 3.0 for all speeds (so components are significant).
The stalls were practiced at random altitudes between 12,000 and 3,000 feet.
This type of stall is a dynamic maneuver, and a ram air parachute is a soft wing with a CG far below the CL.
Here's my current understanding of what goes on before the stall fully develops:
- pilot pulls down the brake lines
- the wing changes shape, increasing camber and becoming less streamlined
- the wing accelerates up and back as lift and drag increases
- the pilot (ballast/ CG) has more momentum due to being much heavier than the wing and continues moving close to the original velocity for a while
- line tension increases (increasing wing loading and therefore also stall speed?)
- the wing pitches up due to the rolling moment caused by the CG moving forward relative to the CL
- the stall starts to develop, reducing lift
- at some point, I presume the stall causes enough loss of lift that the wing starts to deform (is this due to loss of line tension?), causing more loss of lift, etc... and allows the stall to progress even as the CG returns to below the CL, the wing pitches forward, and the lines slacken
Is it possible to analyze all this complexity to see which parts would be affected by altitude (e.g. air density or TAS) and whether that effect is significant?
Or am I overthinking this and there's some simple well known explanation?