Anatomy of a high performance landing

[raymod2 1]

I've put a lot of thought recently into the principles involved during a high performance canopy approach and landing. I figured I'd write them down, share them here, and solicit comments.

A canopy pilot is flying at full glide with the toggles all the way up against the guide rings. The canopy is flying a straight line path towards the ground at a constant airspeed. The canopy/pilot system is in perfect equilibrium: there are no unbalanced forces and no rotational moments acting on the system. The total aerodynamic force (abbreviated T.A.F.), which is the sum of the lift and drag vectors, is equal and opposite to the gravity force and colinear with the center of mass.

The pilot reaches up, grasps both front risers, and pulls them down a few inches. This shifts the center of mass of the system forward. Now the T.A.F. is no longer colinear with the center of mass. A moment has developed which causes the canopy/pilot system to rotate counterclockwise (as viewed from an observer to his left) until the center of mass again lies on the same line as the T.A.F. At first, due to inertia, the direction of flight and the airspeed remain unchanged. Due to the rotation of the canopy its angle of attack has decreased. This causes a decrease in lift and drag. The canopy now experiences an unbalanced downward force. The system seeks out a new glide angle and airspeed that produces equilibrium in the new configuration. The result is a steeper glide angle, a higher airspeed, and a lower angle of attack.

As the ground approaches the canopy pilot begins the roundout (this is the phase of the landing where descending flight transitions to horizontal flight). First he eases off the front risers. The center of mass shifts backwards, the system rotates clockwise, and the angle of attack increases. The increased angle of attack causes an unbalanced lift force which alters the direction of flight (making it more horizontal). This tends to decrease the angle of attack but it also causes the lift and drag vectors (and the T.A.F.) to rotate clockwise. The canopy/pilot system must also rotate clockwise to keep the center of mass colinear with the T.A.F. This tends to increase the angle of attack again. Thus, once initiated, the roundout is a self-sustaining process. There are two other processes, however, which will oppose the roundout. (1) The component of gravity which opposes lift will increase. This will tend to eliminate the unbalanced lift needed for the roundout to continue. (2) The component of gravity which opposes drag will decrease. This will cause the airspeed to decrease which also tends to eliminate the unbalanced lift.

The pilot may need to apply some toggle input to complete the roundout. As he pulls down on the toggles the tail of the canopy deflects downwards. This inherently increases the angle of attack of the wing (similar to lowering the flaps on an airplane). In addition the new shape decreases the lift to drag ratio of the canopy. In other words the T.A.F. rotates clockwise. The canopy/pilot system also rotates clockwise to keep the center of mass colinear with the T.A.F. This rotation further increases the angle of attack. The higher angle of attack provides the extra lift needed to complete the roundout.

At the end of the roundout and the beginning of the surf (horizontal flight) the lift force must be equal to the gravity force. If there is too much lift at this point the canopy will continue past horizontal flight and transition to ascending flight. The pilot can ease up on the toggles (decreasing the angle of attack) to correct for this. During the surf all of gravity opposes lift and none of it opposes drag. With nothing to oppose drag the airspeed will steadily decrease. As the airspeed decreases the pilot steadily pulls down on the toggles (increasing the angle of attack) in order to maintain constant lift. Before reaching the stalling angle of attack the pilot touches down and completes the landing.

[skydive84 0]

my head hurts!

[grega 0]

great explanation.

Would you mind explaining in same style normal toggle turn and front riser turn please

keep up the good work

"George just lucky i guess!"

[Austin 0]

It looks like you have put some thought into your assessment of canopy flight. I just wanted to point out one thing about your comment on flaps.

Quote

This inherently increases the angle of attack of the wing (similar to lowering the flaps on an airplane).

Lowering flaps on an airplane does not automatically increase the angle of attack of the wing. This may create a pitching moment, however, this pitching moment would usually be countered by application of other flight controls (i.e. elevator). Lowering the flaps on a wing simply increases the camber of the wing, which in turn, increases the the lift produced by the wing and makes it possible to provide more lift at lower speeds.

Austin

[andy2 0]

Great job, this is a straight in approach youre talking about, right? Your writing style "clicked" in my brain. Post some stuff about toggle/riser turn landings!

---------------------------------------------
let my inspiration flow,
in token rhyme suggesting rhythm...

[bdazel 0]

Interesting analysis. This makes a lot of sense. While I don't do high performance landings (yet), I've always been awed by really good canopy pilots.

[grega 0]

i just couldn't help posting here, but your avatar is great. I'm still laughing my ass off. it's so funny and sweet at the same time

"George just lucky i guess!"

[TIJ 0]

Dan(raymod2), I'm still looking to find you a girlfriend.

[Tonto 1]

--Quote--------------------------------------------------This inherently increases the angle of attack of the wing (similar to lowering the flaps on an airplane).-----------------------------------------------------------

This is not done by the flaps on an aircraft. It's done by the tail. Since we don't have a tail, it is the light high drag wing that decelerates and the heavy, low drag pilot that swings forward, thus changing the angle of attack when the brakes are applied.

t

It's the year of the Pig.

[raymod2 1]

Quote

Quote

As he pulls down on the toggles the tail of the canopy deflects downwards. This inherently increases the angle of attack of the wing (similar to lowering the flaps on an airplane).

This is not done by the flaps on an aircraft. It's done by the tail. Since we don't have a tail, it is the light high drag wing that decelerates and the heavy, low drag pilot that swings forward, thus changing the angle of attack when the brakes are applied.

Since you are the second person to comment on my flaps analogy I will elaborate. The most commonly accepted definition of angle of attack is the angle between the chord line and the direction of flight. The chord line is an imaginary line drawn between the leading edge of the wing and the trailing edge. When you lower the trailing edge of a wing (by extending flaps or pulling on toggles) you are moving the chord line and increasing the angle of attack. This (usually) causes an increase in lift and drag. It creates more drag than lift, however, so the lift-to-drag ratio goes down.

The decreasing lift-to-drag ratio is generally a bad thing for airplanes (which is why they don't fly around all the time with flaps extended). But for canopy pilots this produces a secondary effect which is very desirable. It tilts the T.A.F. rearwards which creates a moment that rotates the canopy/pilot system. This tilts the entire wing and causes an even greater increase in the angle of attack.

Note that this has nothing to do with deceleration or "swinging forward". Since the canopy and pilot are connected they both experience the same deceleration. And if the pilot simply "swung forward" then the lines in the front would go slack and there would be no change in the angle of attack. What you perceive as "swinging forward" is actually a rotation of the entire canopy/pilot system.

[BrianSGermain 1]

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{I thought I would drop in here and make subtle corrections to the otherwise excellent description of the process. BSG}

I've put a lot of thought recently into the principles involved during a high performance canopy approach and landing. I figured I'd write them down, share them here, and solicit comments.

A canopy pilot is flying at full glide with the toggles all the way up against the guide rings. The canopy is flying a straight line path towards the ground at a constant airspeed. The canopy/pilot system is in perfect equilibrium: there are no unbalanced forces and no rotational moments acting on the system. The total aerodynamic force (abbreviated T.A.F.), which is the sum of the lift and drag vectors, is equal and opposite to the gravity force and colinear with the center of mass.

The pilot reaches up, grasps both front risers, and pulls them down a few inches. This shifts the center of mass of the system forward. Now the T.A.F. is no longer colinear with the center of mass. A moment has developed which causes the canopy/pilot system to rotate counterclockwise (as viewed from an observer to his left) until the center of mass again lies on the same line as the T.A.F. At first, due to inertia, the direction of flight and the airspeed remain unchanged. Due to the rotation of the canopy its angle of attack has decreased. This causes a decrease in lift and drag. The canopy now experiences an unbalanced downward force. The system seeks out a new glide angle and airspeed that produces equilibrium in the new configuration. The result is a steeper glide angle, a higher airspeed, and a lower angle of attack.

{Actually, the angle of attack only changes at first. It then returns to about the same angle of attack as in full glide. It is a new balance of forces, as would be the case if the trim were altered. BSG}

As the ground approaches the canopy pilot begins the roundout (this is the phase of the landing where descending flight transitions to horizontal flight). First he eases off the front risers. The center of mass shifts backwards, the system rotates clockwise, and the angle of attack increases. The increased angle of attack causes an unbalanced lift force which alters the direction of flight (making it more horizontal). This tends to decrease the angle of attack

{The angle of attack increases in the level-off process aka "round-out", unless the pilot lets off the brakes. BSG}

but it also causes the lift and drag vectors (and the T.A.F.) to rotate clockwise. The canopy/pilot system must also rotate clockwise to keep the center of mass colinear with the T.A.F. This tends to increase the angle of attack again. Thus, once initiated, the roundout is a self-sustaining process. There are two other processes, however, which will oppose the roundout. (1) The component of gravity which opposes lift will increase. This will tend to eliminate the unbalanced lift needed for the roundout to continue. (2) The component of gravity which opposes drag will decrease. This will cause the airspeed to decrease which also tends to eliminate the unbalanced lift.

(3) Inertia: The mass does not want to change direction. This increases the Apparent Weight of the suspended load, increasing the line tension to the lines attached to the front risers. The lines to the rear risers tend to soften or go limp. BSG}

The pilot may need to apply some toggle input to complete the roundout. As he pulls down on the toggles the tail of the canopy deflects downwards. This inherently increases the angle of attack of the wing (similar to lowering the flaps on an airplane). In addition the new shape decreases the lift to drag ratio of the canopy. In other words the T.A.F. rotates clockwise. The canopy/pilot system also rotates clockwise to keep the center of mass colinear with the T.A.F. This rotation further increases the angle of attack. The higher angle of attack provides the extra lift needed to complete the roundout.

{Actually, the dominant force in a brake flare is drag. The increase in tail input increases the drag of the canopy itself, which offsets the balance of drag between the canopy and the jumper. This allows the suspended weight to move forward relative to the canopy, thus increasing the angle of attack, increasing the lift of the system. Comparing the flaps on an airplane to the tail deflection of brake input on a ram air canopy is a messy road to travel on. Since the weight is essentially inside the wing of an airplane, the pendular stability is not the same. It is true that lift drag thrust and weight are comparable on the two systems, but the suspension of the weight so far below the wing profoundly changes the dynamics of the system. In fact, flaps tend to cause an airplane to decrease its pitch angle with the induction of drag. BSG}

At the end of the roundout and the beginning of the surf (horizontal flight) the lift force must be equal to the gravity force. If there is too much lift at this point the canopy will continue past horizontal flight and transition to ascending flight. The pilot can ease up on the toggles (decreasing the angle of attack) to correct for this. During the surf all of gravity opposes lift and none of it opposes drag. With nothing to oppose drag the airspeed will steadily decrease. As the airspeed decreases the pilot steadily pulls down on the toggles (increasing the angle of attack) in order to maintain constant lift. Before reaching the stalling angle of attack the pilot touches down and completes the landing.

I think you did a great job with the anaylsis. Keep on thinking about it. This is how we take the next steps in the evolution. We think.

Keep up the good work! BSG

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[swoopdoggy 0]

hay
why don't take backriserz in stead of toggles!
so you don't lose your speed

live to fly, fly till death

[Fast 0]

Interesting post resurected from long ago...

It's a shame that Dan wasn't around the weekend you were at our DZ Brian.


Dan you had any changes to this line of thinking, in the last oh,.. few years? Since you have undoubtedly learned a lot in that time.

~D
Where troubles melt like lemon drops Away above the chimney tops That's where you'll find me.
Swooping is taking one last poke at the bear before escaping it's cave - davelepka

[raymod2 1]

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Actually, the dominant force in a brake flare is drag. The increase in tail input increases the drag of the canopy itself, which offsets the balance of drag between the canopy and the jumper.

@Brian: Thank you for your comments. Conventional wisdom (and all papers I have read on the subject) indicate that the ratio of canopy drag to pilot drag (cd/pd) is how tail deflection alters angle of attack. I am challenging conventional wisdom and I submit that it is actually the ratio of canopy lift to canopy drag (l/d) that causes the change in angle of attack.

Consider the first hypothesis (cd/pd). This ratio can be changed by altering either the numerator or the denominator. If you cut pilot drag in half you should get the same result as doubling canopy drag. Experience shows that this is not the case. Large changes in pilot drag have a negligible effect on angle of attack while small changes in canopy drag produce large changes in angle of attack.

Also consider the fact that the center of mass for the canopy/pilot system is somewhere inside the pilot's body. Therefore changing pilot drag will produce no moments (which require a force acting perpendicular to the radius of the center of mass). With no moments acting on the system it will not rotate. You could eliminate pilot drag completely (and cd/pd would approach infinity) and there would be little or no change in angle of attack.

Now consider my analysis (l/d). When the canopy/pilot system is in equilibrium the TAF is colinear with the center of mass. Tail deflection increases both lift and drag of the canopy but it increases drag more. Therefore l/d decreases. l/d determines the direction of the TAF vector. Since the TAF acts on the canopy and not on the center of mass, changing the direction of the TAF vector will cause it to no longer be colinear with the center of mass (producing an unbalanced moment). The canopy/pilot system will seek to eliminate that moment by rotating until the center of mass is back in line with the TAF. This, I propose, is the dominant principle by which tail deflection alters angle of attack.

[BrianSGermain 1]

Interesting thoughts. This is going to be something that canopy pilots continue to discuss around campfires worldwide as long as skydiving continues. It is unprovable except for those fluent in the language of physics, so I wonder if perhaps we are getting too heady for the rest of the crowd.

The bottom line is, when you apply the brakes fast enough to make the pitch move, the canopy will rotate about that axis. If it does so in a short enough timeframe, the parachute will achieve adequate lift to stop descending, or even climb. If the process occurs too slowly, by the time the angle of attack increases, the airspeed is sufficiently diminished so the canopy does not level off.

Any deeper anaylsis will drive us to pontificate our navels...not that I have any problem with that, but we need to keep the big picture in mind. The basic understand of how a canopy levels off prevents collision with the planet. It is not really necessary to go much deeper than that.

Physics is fun, but it can lead us away from skydiving...that's the whole point, right? :)

By the way, your analysis is very well thought out. Impressive...

Blue Skies,
+

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