Basejumper.com - archive

A friend and I were having a discussion on Density altitude and its affects on stall point in regards to deep brake settings...

what affect does a change in density altitude (i.e. Moab in the heat of summer at 5000' versus say Sea level in winter in the North Pole) have on stall point and dialed in deep brake settings?

Also has anyone noticed any affects on one of their canopies, say changing environments and having a canopy that opened perfectly in one environment, suddenly start backing up and stalling on opening in a different environment... with all other factors (line trim, wing loading, porosity ect), staying the same..

Thoughts?

thanks,

_justin

theoretically, stalls are ONLY a function of the Angle of Attack. exceed the critical angle, flow separates, and a stall occurs.

a fixed brake setting should thus NOT affect anything.

the main thing is that the speed will increase (to generate the same amount of lift).

it might be interesting to hear if anyone noted a difference.

taking your statement of :
" theoretically, stalls are ONLY a function of the Angle of Attack. exceed the critical angle, flow separates, ( <-- ... fill in ..... --> ) and a stall occurs."

Good statement but being that Slow speed of canopy fight in deep brakes settings. And sometimes pilots noticing the Canopy rides the edge of clean and dirty flight in it.
Because of the Ever present, influencing environment you always jump into. Maybe throw-Out the word ( theoretically ) & Add-In, in your own wording . Info pertaining to the:
Canopy speed, direction and it's flight speed threw the Relative air-flow speed and direction it (the canopy) is traveling in. Influencing the Canopy Inflation and flight quality. Into that statement ?
.

Like Ray said, you only have half of the concept of "critical angle of attack is ALWAYS the same for a given wing". That's true, at any speed, weight, wingloading, density altitude, etc. HOWEVER, all of those things change the path of the wing through the air, and the angle at which the wing meets the relative wind.

In slow flight, for example in deep brake settings immediately upon opening, the angle of attack is greater because the relative wind is coming from below the canopy to a greater degree than it is when in full flight. Add in that deforming the rear of the canopy by pulling down the tail changes the chord of the wing, and therefore the canopy is meeting the relative wind at a larger angle of attack than may be readily apparent, and the concept that a wing stalls at the same angle of attack regardless of other factors becomes worse than meaningless in this discussion, its misleading.

At a higher density altitude (read lower number of molecules supporting the canopy in the air), the canopy will have less lift, a steeper glide angle, a higher angle of attack, and therefore yes, will stall at a shallower brake setting than it would on a standard day.

Of course, I could be completely wrong.

I disagree. The only relevant parameters to stalling an airfoil are AoA and the particular airfoil design. The Coefficient of Lift (or Drag) is a dimensionless number. Thus will vary independently of any of your variables.

Or to save me typing...



I disagree. The canopy MUST generate the same amount of lift, once it reaches a steady-state (i.e. non-accelerating mode or full-flight). It might take more altitude to reach a steady glide, but when it does, the canopy will create the entire weight of the jumper + gear.


The glide angle is established by the line trim (and brake settings). If these do not change, then the canopy should fly through the air the same way. With a decreased air density, the speed must increase to generate the "missing" lift.


If the parachute travels down a fixed glide slope, the AoA can not change.


Thus I disagree with your theory.

Practically, jumpers rarely reach steady-state (i.e. full flight) with the brakes stowed. The thinner air should delay the opening time, thus allow the jumper to notice things they may not on a faster opening.

But is that what people notice?


edited to add:
practical experience should be given priority over theory. theory may not be correct, or poorly applied (including by me). each jumper should develop their own experience base and go from there.

Shit. Very bad wording on my part. I meant, "no matter what the speed, weight, etc." not "for a given speed, weight, etc."

I'll respond to the rest when I have a few more minutes, but for now, I'll leave it that I disagree with the rest.

Justin, i will simply reply to your question.



None.

AoA at stall speed is constant, regardless of air density.

True airspeed and whether you took a huge poop before your jump or not will have more effect.

of course i've never proven this for myself, but i never felt it was warranted.

Thanks Chuck!

Just trying to figure out why someone's canopy would start opening and flying backwards when it appears nothing has changed but the time of year, or temperature.

A wear issue I could understand, or lines being out of trim, or just becoming a fat bastard.....

_justin

I had typed up a really long response and then closed the window. So I will make this one shorter.

I think where we are in disagreement is how the canopy gains speed. You are imagining it nosing over like an airplane and keeping the same angle of attack. I am imagining the sink rate increasing while the chord of the wing remains in a constant relationship to the ground or the horizon (for lack of a better way to say that), causing a greater angle of attack.

Going way beyond what I know, I would GUESS that different canopies have different relationships between the position of the center of lift and the center of gravity which would affect which one of these more closely resembles what happens.

I'd be interested to know which is the case. Either way, I have exercised the gray matter a bit and thought a lot more about canopy flight today, so that can't be a bad thing.

The opening sequence if messy. The canopy starts off like a round. It just slows you down via drag. Eventually it will start to fly and become a wing.

Suspension lines:
A - shortest
D - longest

Thus, an unbraked canopy is in a slight dive. As it starts to generate lift like a wing, the lift also pulls the parachute forward. (The lift of the wing is actually inclined off of vertical.)

Deep brakes alter the camber of the wing, and increases the AoA. This will tilt the overall lift of the wing back toward vertical. Thus there will be minimal forward speed.

Tailwind?

Same magnitude and direction of wind on both jumps?

Strong(er) tailwind can cause opening backsurge, especially on vented canopies with good deep brake settings.

Yeah, I get the whole bottom skin then topskin inflation and the line trim idea. I've even noticed a time or two that the canopy tends to descend when in full flight.

Anyway,so here we are after opening but before releasing the brakes. The top skin is inflated and the wing is generating lift. The camber of the wing is increased by the deep brake setting. The forward speed is low. The canopy will attempt to generate lift through additional speed to compensate for the High DA. Doesn't slow forward speed + high angle of attack= increased vertical speed if the canopy is to remain stable? And doesn't the increase in vertical speed in relation to forward speed increase the angle of attack? Or do you think they increase proportionately to each other AND to drag?

Justin- Sorry for rambling on, but at this point I am convinced that the deep brake setting needs to be shallower for high DA, and getting my mind around what makes the gear work the way that it does is one of the things I find most useful about this site.

Don't confuse forward speed and airspeed (or groundspeed for that matter). The wind from falling IS the airspeed.

Draw a line from the airfoil leading edge (which is not really there on a parachute) to the trailing edge (where the brake lines attach). This is the chord line. With the brakes set, the tail is pulled down. This shifts the airfoil from being more nose down to more horizontal. That is what increases the AoA.

Draw a line perpendicular to the chord line. The "Total Aerodynamic Forces" will actually act aft of perpendicular. As brakes are pulled down, the chord line rotates, the AoA increases, and this "TAF" moves closer to vertical. We like this position since it minimizes forward drive.

On most jumps, the jumper will release the brakes, thus changing the configuration before the downward speed stabilizes at a given value (think a version of terminal velocity).

ALL speeds will be increased by the lower DA, and it will take longer to reach the increased speeds.

The slow initial airspeeds mean gusts, turbulence, thermals, tailwinds, etc. mess with everything.

I hope that helps, and hope I got it right!
(BTW, I can add numerous complications. Thus I prefer jumpers just know their own canopy, how it flies, etc. It's so much easier to predict a canopy from experience than theory!)