How do you calculate rate of descent of a parachutist

[Centaurus]

Does the rate of descent of a parachutist vary with local air pressure?

Parachutist bales out and QNH is 989 mbs at planned landing field. Another bales out where the QNH is 1032.

Difference in average rate of descent is ?

[darkroomsource]

Parachute open or not?

[Capot]

Why not ask Felix Baumgartner? He's had recent experience.

[Centaurus]

Parachute open or not?

Parachute open. Why didn't I think of mentioning that in the first place you Dummy, Centaurus? ... And daylight. Wind light and variable. QNH as shown. CAVOK. NOTAM: Ground soft with recent rain. Now about the original meant to be serious question.:E

[Not Long Now]

Wish I had a better memory, but here goes...(similar thing discussed on a physics degree course many many years ago)
No difference.
If the parachutist was going to be primarily affected by drag, then QNH, which could be considered as a representation of density of the medium they are travelling through by extraction of the ideal gas law, PV=nRT, may be shown to have an effect on that drag and as such on the resultant velocity of the body. However the parachute is not a model that can be considered as a uniform body for which frictional drag is one of the major forces acting, as the open canopy produces resistance many orders of magnitude greater than 'drag' in an aviation sense.
As has been perhaps inadvertently suggested, it may actually be relevant to Mr Felix where initial QNH is probably below 1, but for majority lower atmosphere activities, the varying pressure will be greatly overridden by body position, canopy design, the light and variable wind etc.
The terminal velocity, which could perhaps be a useful comparable value between the two parachutists, equals (IIRC) square root of 2 x mass x force(g) divided by product of pressure x cross sectional area x drag coefficient. Presuming (big presumption!) area and drag coefficient same for both parachutes (not jusy varying in production, but also in fitting, angle of attack, body position etc etc) then 4% difference in pressure is minimal relative to all other local factors.

[Di_Vosh]

Centaurus,

I can only assume that this is a theoretical question, as the day-to-day realities of parachuting have way too many variables.

Some of these are: Size (obviously), shape (round, steerable round, or square), whether the parachutist is steering or braking, as well as the weight of the parachutist.

Off the top of my head, the old T10B-D (Australian military round non-steerable) parachute had a rod of between 5.6 - 6.4 m/s.

[Capot]

Does the rate of descent of a parachutist vary with local air pressure?

The answer to that is easy. It must do; consider the extremes; would you fall faster in a vacuum than in a bowl of soup? Hence the reminder of Felix's little jump...

perhaps inadvertently suggested,No it wasn't...

Calculating the difference? Now, that's another matter altogether.....let's start with the assumptions about the variables.....over to you.

[Centaurus]

Thank you all for your contribution to the original post. It was meant as serious question because I was simply curious as to the effect of air pressure and whether there was more chances of breaking an ankle in low pressure rather than 1032 for example. Or how about do you flare slightly earlier when landing a 737 at very low QNH's in order to allow for thinner air?:ok:

[Brian Abraham]

Centaurus, you probably realise that the modern parachute is a wing and has all the attributes of that on your airyplane, including stalling. Jumpers select the wing loading for the desired performance. New comer, large area for low ROD, low airspeed and slow landing. Highly experienced people may be seen using such small area canopies they barely rate as adequate handkerchiefs. Jumping at high altitude fields can necessitate upping the canopy size.

[darkroomsource]

And the question "parachute open or closed" was a serious question also.
Because with it closed you'll reach "terminal velocity", whereas with it open you won't. At different speeds air density will have different effects. You will see a greater difference in speed at "terminal velocity" than you will with the parachute open. Either way you won't notice the difference, but for different reasons...
You won't notice the difference with the parachute open, because at complete extremes, the difference will be like 4%.
You won't notice the difference with the parachute closed because you'll be too concerned with trying to get the parachute opened.

edit: "At different speeds air density will have different effects." Isn't very clear. What I mean is that it's a variable percentage, that still isn't clear... let's try this, in a vacuum, no air density, you will continue to accelerate until you decelerate very quickly at the end, parachute open or closed. On the opposite end, like air as thick as water (as can be demonstrated in a swimming pool), you'll accelerate at about the same speed with the parachute open or closed (at this extreme), in between, the effect of the parachute will increase the denser the air, up to a point, and then begin to have less impact (as shown in water). This is all too confusing to explain clearly. Let me try again. The density of the air will have some impact on your acceleration, but with the parachute open, it will negligible within the normal air densities encountered within Earth's atmosphere below 20,000 feet.

[Lightning Mate]

For Centaurus.

Air pressure alone will have no effect at all, it is air density which will have an effect.

A parachute causes drag. For a given parachute area, drag is dependent upon dynamic pressure.

Dynamic pressure depends upon density and true airspeed. The higher the density altitude then the higher will be the true airspeed for a given dynamic pressure.

True airspeed is the actual rate of descent of a parachutist.

Thus at a higher density altitude the rate of descent will be higher.

This is why the terminal velocity of a free falling body (the state in which weight equals drag) is higher at higher altitude.

[Agaricus bisporus]

And the question "parachute open or closed" was a serious question also.
Because with it closed you'll reach "terminal velocity", whereas with it open you won't.

Oh yes you will. You'll achieve the terminal velocity of the deployed parachute. TV is only a meaningful figure if you relate it to the object in question, which in one case is a parachutist with a backpack and harness, in the other a deployed canopy with the parachutist supported by it. The terminal velocity of a feather, cannon ball or a hamster in a cage are all something different again.

[dubbleyew eight]

Thank you all for your contribution to the original post. It was meant as serious question because I was simply curious as to the effect of air pressure and whether there was more chances of breaking an ankle in low pressure rather than 1032 for example. Or how about do you flare slightly earlier when landing a 737 at very low QNH's in order to allow for thinner air?

centaurus they are totally different questions. ...well maybe they are not.

you dont flare slightly earlier in an aircraft because the aircraft will fly slightly faster through the thinner air resulting in the aircraft experiencing the same dynamic pressure.

my indicated approach speeds dont change with qnh differences but my actual speed over the ground probably does imperceptibly. wind variations mean you'd never detect the difference.

[RodH]

Slightly off topic but funny .
I recall quite a clever quote re parachuting/ skydiving:

" You don't need a parachute to skydive , you only need one to do it twice " .

:):):)

[Di_Vosh]

I can only comment on this as all of my jumps (53) were mil.

whether there was more chances of breaking an ankle in low pressure rather than 1032 for example

The bitter reality is, IME, is that air pressure is low on the list of causes of broken bones (let's not limit ourselves to ankles :ouch:). More likely causes include the nature of the DZ (are we talking long grass, swampy ground, or clay that hasn't seen rain this year...), wind on the DZ and landing technique.

Jumping with military rounds, landing technique is critical. Not so much with modern square chutes, but if you flare too early/late, the results can be unfortunate.