[naturewoo]

when a person is hang gliding, does lift increase with speed?

[PilotGuy]

That depends.

There are several factors that determine the total amount of lift that your wing is producing: speed, air density, size of the wing, and the "liftiness" of the wing design, called the lift coefficient. The size of the wing doesn't change, and let's assume a small change in altitude so the air density is constant. Then the only things that you can change are the airspeed and the lift coefficient.

Lift coefficient is dependent on the shape of the wing and the angle of attack. In a powered aircraft, you might be cruising along at constant speed and altitude with an angle of attack of 5 degrees (for the sake of discussion). If you increase the power to the engine, you will increase your airspeed and if you don't change the angle of attack, the total lift will increase, and you will climb.

However, in a glider, the only way to increase airspeed is to lower the angle of attack, which will reduce your lift and cause you to descend. It is the descent, due to the lowered angle of attack, that increases your airspeed. Once your airpseed increases, there will be a point at which the newly increased airspeed and the newly decreased lift coefficient (due to the decreased angle of attack) will balance the lift equation and the lift provided by your wing will once again balance your weight, and your vertical acceleration will stop.

So, in summary, with all else being held equal, especially the lift coefficient, lift does increase with speed. But the dynamics of gliders complicate things, since you change speed by changing your lift. Does this make sense?

[Jason]

that was a fairly good response

in order to produce more lift, you basically have two options, more speed, or more AoA.

For a given AoA lift is proportional to V^2, so say a 200 lb pilot wing combo trims at 15 mph, then a 300 lb pilot wing combo will trim at

sqrt(300/200)*15=~18.37 mph I say "~" because the wing will not be exactly the same due to some difference in twist/warp from the extra weight.

so as you can see if you double your airspeed, you have the potential to quadruple your lifting force, so if you stall at 15 and dive to 30, and instantly put the AoA back to trim AoA and you will pull about 4 gees, if you dive to 45, and do that you go 9 gees if you dive to 60 and it can produce 16 times more lift at trim AoA(16 gees)

now these numbers are slightly higher because the wing will flex and change shape do to the load placed on them but as you can see, this is where gliders can be capable of destroying themselves from their own pitch stability at highspeed

[naturewoo]

yes or no please...

[blindrodie]

Damn too technical for me this am ;)

This is a good subject for power guys as you can increase your climb rate by maintaining AOA and adding more power. If you just pulled back on the stick you could go into a stall. Another example: if you were on final approach and to high, instead of adjusting AOA (pushing forward on the stick) you cut power and wait for your flight path to change.

The book Stick and Rudder is a great short read on how to fly 3 axis with power. I believe it can be helpful to those of us that soar a great deal, and you get the bonus knowledge if you ever want to cross over to the noisy side!

(Small hilack there)

[blindrodie]

[PilotGuy]

In the simplest example, where nothing changes other than airspeed, than yes, total lift force will increase with an increase in airspeed and vice versa.

To make it as clear as mud, consider the beginning of an aerotow. The glider is sitting on a cart attached to the tug plane via a rope. The glider is sitting in the cart in such a way that you can't change its angle of attack, the only thing that will change is the airspeed. Just before the tow starts, you're sitting on the ground with no airspeed, so you've got no lift. As the tug starts rolling down the runway your wing starts to get some airflow over it and it gets a little lighter on the cart. As the tug pulls you faster the glider will lift off the cart once it reaches a given speed, and you have to pull the bar in to keep it from zooming up above the tug plane.

So yes, with everything else being constant lift does increase with speed. This goes back to to what Jason was saying about G forces. When flying at high speed in any aircraft, an abrupt increase in angle of attack will produce so much lift that the structure of the aircraft cannot withstand it, and the structure will fail.

And I strongly second the recommendation for "Stick and Rudder". Great book that straightens out in your mind many aspects of flight.

[naturewoo]

it seems that some people would argue that a glider diving at 40 mph is creating more lift than a glider boating at 20 mph...does anyone agree?

[David W. Johnson]

Deleted my post.

dwj

[David W. Johnson]

Deleted my post.

dwj

[naturewoo]

if we slow down, raise our AOA, we produce more lift. If we speed up, and decrease our AOA, we produce more lift...which is it?

[blindrodie]

[blindrodie]

[CHassan]

No in general we raise our AoA to slow down, and we lower our AoA to speed up. Not to change the amount of lift!

For a constant AoA, more speed = more lift. Since we alter our AoA to alter our speed it gets hard to calculate (for a math scrub like me!)

You might say the airfoil produces more lift with more speed, but the total lift created by the wing will vary depending on 101 outside influences.

[TomGalvin]

[PilotGuy]

[Lobido]

I was a photon, once.

[FormerFF]

Dont forget that when you are diving, because of your nose low pitch angle, some of the lift generated by the wing serves to produce airspeed.

[PilotGuy]

Those equations can be simplified for low speed wings like ours and you can just consider the fact that the wing is, by one way or another, pushing down on the air and as a result, the air is pushing up on the wing. In its simplest form, a wing is just like putting your hand out the car window at 60 miles per hour and feeling it get pushed upwards. Forget about the top surface is curved, bottom surface isn't stuff, just consider the air getting pushed down by the wing. In fact, did you know that a flat sheet of plywood makes almost as much lift as an airfoil? We just don't use flat boards because an airfoil makes less drag.

Anyway, I'm sure you understand the action/reaction principle, if not you've got some reading to do. Because of that principle, a wing needs to put a down force on air that is equal to the weight that the wing is trying to support. For instance, to support 100 lbs of load a wing needs to exert a 100 lb force on the air that is flowing around it. This can be done either by moving a little bit of air with a lot of force, or a large amount of air with little force.

Not to draw too many analogies, but think of standing on a skateboard and holding up a pillowcase like a sail on a sailboat. You can be pushed down the street by a strong wind that catches your pillow case. Since the pillow case is small, only a small amount of air is hitting it, but that fast moving air is transferring all its momentum to your pillowcase as it screeches to a halt.

On the same skateboard you could hold up a king size bedsheet in the same manner. Now, it will take much less wind to push you down the street because the huge sheet catches lots of air. This large amount of air does not need to be moving as fast because there is a much larger mass of air transferring its momentum to your bedsheet in order to get you moving along.

[MorphFX]

Actually, we're all in a computer simulation and we don't really exist.

Follow the white rabbit.....

Okay, back to lift, one really simple explanation I like is this one (it's really easy to understand):

http://morphfx.co.uk/hang-gliding/lift.htm

And from my humble understanding with all things being equal; more speed = more lift..... but, as we all know, it's a tad be more complicated than that!

Next time you go out in the car, stick your hand out and shape it into an wing, then if you can, experiment with different speeds (I used to when I was a kid!). .... dad was driving!