I recently flew on a tandem paraglider for the first time. Since then I keep asking myself a question I have no clear answers to.

While flying I clearly perceived the lift generated by the airfoil, but the strongest feeling I had was as if the paraglider hung to a rail, like a suspension railway, or to a cable, like a cable-car. In other words, I perceived a very strong vertical constraint. I know the airfoil generates lift, but I understand the amount of lift is not enough to give you a stability feeling like when driving a truck! To be similar to the constraint produced by a rail, this vertical constraint should be in the order of thousands of kg, IMHO.

Any suggestions on how to estimate this amount, and its origins?

  • Let me paraphrase this question a bit: by "constrained", I believe the OP means "very stiff". The acceleration is sudden - e.g. it goes from level flight to ascending flight in a very short amount of time, as opposed to, a general upward acceleration (and associated increase in G loading) lasting for 10 seconds. – kevin Aug 10 at 2:31

What you perceive as "motion" is acceleration.

No acceleration, and you feel no "motion" -- your kinesic sense and inner ear will tell your brain you're sitting still on a solid surface.

What you experienced as "vertical constraint" was nothing more or less than the result of flying in a very stable manner relative to pitch -- no pitching, up or down, means no change in vertical acceleration (which ought to be exactly 1 G at all times, if you're to feel "still"), and no sensation of motion.

To be similar to the constraint produced by a rail, this vertical constraint should be in the order of thousands of kg, IMHO. Any suggestions on how to estimate this amount, and its origins?

The thing is that the constraint is something you "perceived":

I perceived a very strong vertical constraint

not that was actually there.

Only gravity, lift, and drag (and thrust, if powered) are acting on the paraglider, nothing else.

And the lift is easily estimated, as it is roughly equal to the weight of the passengers and the aircraft summed up and being this a paraglider, definitely not in the range of the thousands of kg.

Having this been your first experience, the sensation can easily be explained by your lack of familiarity with the transportation method.

Fundamentally, the "constrained" feeling comes from the steepness of the lift coefficient vs angle of attack curve.

The angle of attack is the angle at which the air approaches the wing. In level flight, this has a certain value, let's say around 5°. Now let's say you encounter a rising pocket of air. Instantaneously, the angle of attack increases. This directly increases the lift coefficient. As a result, the aircraft starts accelerating upwards, until it is ascending as fast as the rising pocket of air, at which point as far as the aircraft is concerned it is in level flight again (while it is in fact rising along with the air pocket). The "sharpness" at which this happens is your "rail-like" feeling of "vertical constraint" as you call it.

For thin airfoils, the lift coefficient is approximately $2\pi \alpha$ with $\alpha$ the angle of attack. That means that if we were in level flight at $\alpha=5°$, we would be accelerating upwards at $1g$ if the angle of attack increased to just 10°. I can promise you that you can't tell such a minute change in the angle of the incoming wind. Therefore, as far as you are concerned, the wind is always coming head on and you feel like you are flying "on rails".

By the way, "thousands of kilos" of lift for a tandem paraglider of let's say 250kg would mean over $4g$ of acceleration which would be enough to knock you unconscious if sustained for any amount of time.

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