drag vs. lift : questions about glide ratio and air density

[whatever 0]

hi there

I was wondering about the optimum air density for achieving the highest glide ratio with wingsuit flight.

Please correct me if I'm wrong, I think it works like this:


Factors affecting glide ratio in wingsuit flight:
(we're assuming a flier who wants to achieve max glide ratio)

-drag of suit traveling through the air
-lift generated by suit
-weight of flier


Drag of suit:
I think it makes sense to separate this into drag in a horizontal direction and drag in a vertical direction, even though the direction of flight is a combination of both, as higher drag in a horizontal direction will result in less horizontal speed and also in less lift generated by the wings. Higher drag in a vertical direction however will result in a slower descent rate.
The 'floaty' nature of the Crossbow vs. a Skyflier can be explained by this.

Lift generated by suit:
This is a function of the 'airfoil' size and efficiency, both of which are affected by the skill and ability of the flier. It is also a function of the speed of the airflow over the airfoil and this in turn is a function of the suit drag vs. the flier's weight. Furthermore there is the angle of attack to consider, as optimum lift for most airfoils happen very close to the stall point.

Weight of flier:
The heavier you are the higher force you will have downwards due to gravity. A flier’s weight is tied in to their size/area though, so a wingloading would be a more indicative factor here, as fliers with large arm and leg spans and light body weight will fall slower and also have more wing area to generate lift from.


Now, as air density will not affect your weight (or wingloading) my reasoning says glide ratio variability with air density is due to the effect of air density on drag and lift only.

That is where I cannot theoretically predict what the net effect of increasing air density would be, as increased air density will increase lift, but also drag and the net effect depends on the lift-drag ratio of the wingsuit.

So, in short, my question is how air density affects your glide ratio. Specifically changes in air density due to temperature, pressure (altitude) and relative humidity.

You know, will you have a better glide ratio near sea level on a cold, high pressure day with low humidity, or at altitude on a hot, humid, low pressure day?

I know you’ll fly faster in lower air density, but will your glide ratio be higher too?

soon to be gone

[billvon 2,991]

Glide ratio is essentially equivalent to L/D. Air density/pressure does not affect it, nor does weight. (I know, these seem counterintuitive, but they are part of the basics of flight that all glider pilots learn.) However, shape and design of the suit does greatly affect L/D, so a big guy with a protruding belly might well have a worse L/D than a skinnier guy. In addition, things like weight and air density _do_ affect independent variables like sink rate and forward speed.

[whatever 0]

Quote

Glide ratio is essentially equivalent to L/D. Air density/pressure does not affect it, nor does weight. (I know, these seem counterintuitive, but they are part of the basics of flight that all glider pilots learn.) However, shape and design of the suit does greatly affect L/D, so a big guy with a protruding belly might well have a worse L/D than a skinnier guy. In addition, things like weight and air density _do_ affect independent variables like sink rate and forward speed.

ok, I'm confused...

how do you define glide ratio?

I'm using the term to describe the ratio of horizontal speed to vertical speed.

I think:

horizontal speed = forward speed

vertical speed = sink rate

maybe you can point me at some literature that'll help?


thanks

soon to be gone

[Professor 0]

Quote

I'm using the term to describe the ratio of horizontal speed to vertical speed.

Right, and the speed cancels out, so it's just horizontal/vertical distance, which is unaffected by weight (a heavier flier will go faster, but have the same glide ratio, all else equal) and also unaffected by air density (you'll have more drag, but also more lift, -ie- you'll go slower in both directions). of course, body shape and body position play a role.

Wide open blue skies

Ted
Like a giddy school girl.