How do hang-gliders fly, and how do paragliders fly?

Hi,

is it true that hanggliders and paragliders use different principles to fly? I.e.: hanggliders essentially 'swim' on the air but descend slowly (if there's no up wind) while paragliders use the mechanism of jet wings (air has a longer path on top of the wing, hence is not as dense, hence wing is drawn up).

This theory could be tested: it predicts that a paraglider can gain height when there's only horizontal wind, while a hangglider would loose height under these circumstances. Is this true?

Thanks, Axel

both are cambered wings and produce lift the same as an airplane wing. the difference between the two is that todays hang gliders use battens in the upper and lower sails to hold their airfoil shape while paragliders use ramair to inflate the sail to its airfoil shape.

it takes vertical movement of an airmass greater than the sinkrate of either one in order for them climb. although excess airspeed can be traded off for altitude just as any other type of wing will do.

I'm sorry to disapoint you, axel, but both hang gliders and paragliders fly using the bernoulli principle which (for our purposes) states that when air moving over a wing moves over the top surface more rapidly than the bottom surface, then there is a net pressure difference, which provides lift.

As for your second paragraph, you are getting slightly confused about relative and absolute velocities. Here's my attempt to put you right:

1. In the limiting stable case (ie all controls centred, flying at constant velocity relative to the air) all gliders move downward through the air.

2. The ratio of the horizontal to vertical velocities is known as the glide angle. (ie 10mph forward to 1mph down = glide angle of 10).

The glide angle (in the steady state case) is equal to the ratio of drag to lift.

MATHEMATICAL DIGRESSION.

For the mathematically minded this can be proved as follows:

consider a glider with a vertical lifting force L and a horizontal drag force D.

Since the glider is moving a constant velocity, it's kinetic energy is constant.

We can then balance energy gains and losses. The glider loses Gravitational potential enegery at a rate MS, where M is the mass of pilot and glider, and S is the sink rate.

(work done = force * distance) (power output = force * velocity)

However, since we are descending at constant velocity, there is no vertical acceleration, and thus M=L (lift = gravitational downward force).

So we lose Gravitational energy at rate LS.

Where does this energy go?

It is transferred to the air as a whole (by various complex mechanisms) this produces drag. (Intuitively, you 'stir up' the air by moving through it, giving it energy, and it provides a retarding force on you).

So... the rate at which energy is lost to the air = VD, where V = horizontal velocity, and D = drag force.

We know that these are the only energy inputs and outputs, and energy is conserved, so,

LS+VD=0 (conservation of energy).

now we can rearrange this as follows:

LS+VD=0

=> LS=-VD.

=> (L/D)S=-V

=> (L/D)=-(V/S)

This shows us that the ratio of lift to drag (L/D) is equal to the negative of the ratio of horizontal velocity to vertical velocity.

Note that the minus sign appears, because vertical velocity is being measured upwards, and we are gliding downwards. The glide angle G is thus -(V/S) and so L/D=G.

QED.

END MATHEMATICAL DIGRESSION.

The only reason gliders go up is because they fly through moving air, and so their net vertical speed is the sink rate + the vertical speed of the air they are moving in.

Powered aircraft fly upwards because they have engines that produce a net thrust. This thrust forces the aircraft forwards. We can increasing the Angle of attack of the wing, so a greater amount of lift (and drag) is generated, and the extra drag is cancelled out by the thrust of the engines, leaving a net lift.

The above paragraph is a gross oversimplification, but it'll do for our purposes... study physics & fluid dynamics to learn more

Does that answer your wuestion about how aircraft fly??

MH.

*********************************************** Martin Harvey Pembroke College, Cambridge University.

Uni web pages: http://www-stu.pem.cam.ac.uk/~mch24/ ***********************************************

Yes, this is true.

I don't think so!!

There is a conservation of energy problem here.

Think about the four forces in flight. The weight has to be countered by an equal amount of lift. The drag has to be countered by some form of thrust and that thrust for unpowered aircraft is in the form of gliding forward ... using some of the weight component. The only way heavier than air craft can go up (or stay up) is if the parcel of air we fly in is going up at a greater rate than we are going down through it.

This is called airspeed. A hang glider or a para glider or sail plane must always have airspeed to fly. We are always going forward and down. If the air around us is going up then we go up relative to the ground.

Cheers,

It would be wonderful if a HG or PG could take off and gain altitude in horizontal wind.

Intuitively this seem to defy fundamental natural laws (for example energy conservation) . However I have not yet seen any theoretical proof.

I can give you a somewhat similar *counterintuitive* example. I once saw a windpowered boat, sailing against the wind (a windmill was connected to the propeller in the water). Here kinetic energy was transferred from the wind to the boat (forward motion).

Likewise one could imagine kinetic energy in the wind somehow being transferred to potential energy of the glider (gain of altitude).

There is one strong argument against the possibility of altitude gain in horizontal wind: *evolution* - no bird can take off from flat ground without flapping its wings - allthough it obviously would be an advantage.

But watching the seagulls´ aerobatics, it seems that they might be able to do the trick. Particularly in strong winds , gulls sometimes make wild 180 degrees turns against the wind, combining an initial dive with a final altitude gain. The gulls seem to be able to gain altitude without flapping their wings - (allthough they are carried a considerable horizontal distance with the wind). Could they be using the * wind gradient * in some way?

It would be interesting to hear some answers - the practical implications could revolutionize HG and PG ; )

Can a hang glider or a paraglider gain altitude with only a horizontal wind? The answer is 'Yes, but not for long.'

The solution is not intuitively obvious. Here are the forces in play on a hang glider (assiming a head wind):

^ vertical lift

drag <

I think our Zulu friend was trying to stir up the para - hang pot. Not add anything intelegent. Maybe figuring the world needs more confilict. It's quite a common mis conception with those who don't know about flying that horozontal wind can be used. I guess they figure it's similar to flying a kite. Seeing or hearing about someone ridge soaring probably adds to the confusion.

Close, but no cigar... it's part of the lift component that acts forward. Weight, by definition has no horizontal component. When a hang-glider (or paraglider) lifts you up, most pilots will attest to the fact that unless the wing in close to the stall point (ie bar out or brakes on full) it also pulls you forward.

*********************************************** Martin Harvey Pembroke College, Cambridge University.

Uni web pages: http://www-stu.pem.cam.ac.uk/~mch24/ ***********************************************

I believe that sailplanes have used this wind gradient technique in NZ, as you pull up into the wind gradient you climb and you trade speed for height, except with a strong gradient as you climb so does the wind gradient thus maintaining your airspeed (over a limited height)

I don't believe that HG or PG have anything like the efficiency to usefully use this technique at the mo

some dudes somewhere sunny attached a HG to a windsurfer via a longish rope and sailed/flew both over a mile, this is kind of cheating by using a 'tether' on the HG.

<http://www.wintermute.co.uk/users/ratbag/welcome.htm>

I was trying to not add anything intelligent. Of course I know it's the HG that gain in a hoizontal wind. I read on rec.aviation.soaring a first hand account of dynamic soaring in Sweden. So it has been done. (No one lies on the internet)

Bhaok wrote:

both are cambered wings and produce lift the same as an airplane wing. the difference between the two is that todays hang gliders use battens in the upper and lower sails to hold their airfoil shape while paragliders use ramair to inflate the sail to its airfoil shape..

This is one of the very common errors in 'normal' PG training, it only takes my re-training students a few seconds looking along a 'Tip Touch' (wing tip on the ground) PG wing from the front to see that in fact the cell openings on the glider are in fact on the underside of the wing rather than on the front of the wing...

Hence in flight there is no 'Ram air', rather it is the reduction in STATIC Pressure relative to the Static Pressure INSIDE the wing.... all due to the (Dynamic Pressure) air flowing PAST the surface (top & bottom) that lets the wing hold its shape in flight.

Murray Hay

David S wrote:

I'm sorry to disapoint you, axel, but both hang gliders and paragliders fly using the bernoulli principle which (for our purposes) states that when air moving over a wing moves over the top surface more rapidly than the bottom surface, then there is a net pressure difference, which provides lift.

This is a common misconception, in fact the air flows faster past both the upper and lower surface than it would in the wing was a flat plate, the great thing about teaching the physics of PG flight is that unlike with rigid (say a C152) wings on a PG wing we can see the effect of laminar flow on the bottom & top surfaces in both flight (gliding) and in the various stall configurations.

It is VITAL that when refering to Bernoulli's theorem it is correctly quoted (In a dozen years of teaching PG I've yet to see a PG text book correctly state Bernoulli!)

For Bernoulli to be applicable to a problem the 'TOTAL Pressure' MUST be a constant (for any given altitude~MH) hence, setting aside the diffrence in altitude between the top and bottom surface there is no alteration in 'pressure' (atmospheric pressure is the 'Total Pressure' value for PG flight)[/quote]


David S wrote:

.......
The only reason gliders go up is because they fly through moving air, and so their net vertical speed is the sink rate + the vertical speed of the air they are moving in.

Powered aircraft fly upwards because they have engines that produce a net thrust. This thrust forces the aircraft forwards. We can increasing the Angle of attack of the wing, so a greater amount of lift (and drag) is generated, and the extra drag is cancelled out by the thrust of the engines, leaving a net lift.

The above paragraph is a gross oversimplification, but it'll do for our purposes... study physics & fluid dynamics to learn more :-)

Does that answer your wuestion about how aircraft fly??

MH.

*********************************************** Martin Harvey Pembroke College, Cambridge University.

Uni web pages: http://www-stu.pem.cam.ac.uk/~mch24/ ***********************************************

Another common error in standard PG theory relates to 'Angle of Attack', the first thing to understand is that a PG wing NEVER ever has a single AoA due to both its basic shape and of course the way it alters direction... due to the basic physics which permits soft wings (PG's) to maintain their shape in flight paragliders flying fast will often have (center section/straight flight) a NEGATIVE (range) of Angle of Attack.

Murray Hay