Spill Hole Specs

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accooper

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According to the Army a spill hole to be effective needs to be 20% of the total diameter. Do you guys feel this is right?

Andrew
 
I would research "Parachute Recovery Systems Design Manual" by Theo Knacke.

Greg
 
Quite possibly.

The purpose of a spill hole is to stabilize the parachute by preventing oscillations.

A 20% diameter hole only reduces the area of the parachute by 4% so it does not significantly alter the descent rate, but dumps enough air to prevent oscillations.

Area is proportional to diameter squared. Physical area is pi*(D/2)^2 = (pi.4)*1*D^2 and hole area is pi*(0.2D/2)^2 = (pi/4)*(0.04)*D^2 so the lost area ratio is 0.04/1 = 4%.

Bob
 
Quite possibly.

The purpose of a spill hole is to stabilize the parachute by preventing oscillations.

A 20% diameter hole only reduces the area of the parachute by 4% so it does not significantly alter the descent rate, but dumps enough air to prevent oscillations.

Area is proportional to diameter squared. Physical area is pi*(D/2)^2 = (pi.4)*1*D^2 and hole area is pi*(0.2D/2)^2 = (pi/4)*(0.04)*D^2 so the lost area ratio is 0.04/1 = 4%.

Bob

That makes sense when you put it like that.
 
Quite possibly.

The purpose of a spill hole is to stabilize the parachute by preventing oscillations.

A 20% diameter hole only reduces the area of the parachute by 4% so it does not significantly alter the descent rate, but dumps enough air to prevent oscillations.

Area is proportional to diameter squared. Physical area is pi*(D/2)^2 = (pi.4)*1*D^2 and hole area is pi*(0.2D/2)^2 = (pi/4)*(0.04)*D^2 so the lost area ratio is 0.04/1 = 4%.

Bob

Bob:
That's the formula I've used for years with plastic and Rip Stop nylon chutes.

Over time I've gradually reduced these rather large openings to a point today I'm using a 1-1/4" spill hole in 36" hemi's, down to 3/4" diameter holes in 12 to 24" Plastic or Rip Stop Nylon flat chutes. These reduced spill holes completely cancel out model oscillation swing during full open decent.
 
There is a large margin with spill hole area versus descent rate as shown in the table below.

Spill Hole Diameter
Reference Area
Reference Speed
0%
100%
100%
10%
99%
101%
20%
96%
102%
30%
91%
105%
40%
84%
109%
50%
75%
115%
60%
64%
125%
70%
51%
140%
80%
36%
167%
90%
19%
229%


I wonder if the larger spill holes area provides a propulsive thrust to counter the downwind drift of the chute in high winds that a smaller area would not as the mass flow would be proportional to the hole area.

Bob
 
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Bob,

Shouldn't the speed increase be as follows?

Spill Hole DiameterReference AreaSpeed Increase
0%100%0%
10%99%1%
20%96%2%
30%91%5%
40%84%9%
50%75%15%
60%64%25%
70%51%40%
80%36%67%
90%19%129%

Greg
 
I wonder if the larger spill holes area provides a propulsive thrust to counter the downwind drift of the chute in high winds that a smaller area would not as the mass flow would be proportional to the hole area.

Bob

As it was explained to me by an experience skydiver, and it took me a while to really grasp the implications, everything that happens to a parachute on the way down occurs with a zero mph base speed relative to the air mass. In other words if you have a ram air parasail chute that has a nominal 15 mph forward speed, that speed and direction is measured relative to the air mass the chute is in, not the ground. If you want to calculate the speed and direction on the ground, you have to add the movement of the air mass to the movement of the chute. If the design of the chute does not induce any motion relative to the air mass, the size of the spill hole will not affect the relative motion at all. The chute is going to have zero relative motion to the air mass and move relative to the ground at the same speed as the air mass.

To answer your question, no. A larger spill hole will not change the downwind drift of the chute in high winds, because as far as the chute is concerned, there is no wind. It might increase the drop rate causing it to land closer to the launch ponit since it is in the air less time, but there would be no aerodynamic affect the would make any difference.
 
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Bob,

Shouldn't the speed increase be as follows?

Spill Hole Diameter
Reference Area
Speed Increase
0%
100%
0%
10%
99%
1%
20%
96%
2%
30%
91%
5%
40%
84%
9%
50%
75%
15%
60%
64%
25%
70%
51%
40%
80%
36%
67%
90%
19%
129%

Greg
You are correct. I actually calculated the Reference Speed and I edited my column heading from Speed Increase to Reference Speed.

Bob
 
As it was explained to me by an experience skydiver, and it took me a while to really grasp the implications, everything that happens to a parachute on the way down occurs with a zero mph base speed relative to the air mass. In other words if you have a ram air parasail chute that has a nominal 15 mph forward speed, that speed and direction is measured relative to the air mass the chute is in, not the ground. If you want to calculate the speed and direction on the ground, you have to add the movement of the air mass to the movement of the chute. If the design of the chute does not induce any motion relative to the air mass, the size of the spill hole will not affect the relative motion at all. The chute is going to have zero relative motion to the air mass and move relative to the ground at the same speed as the air mass.

To answer your question, no. A larger spill hole will not change the downwind drift of the chute in high winds, because as far as the chute is concerned, there is no wind. It might increase the drop rate causing it to land closer to the launch ponit since it is in the air less time, but there would be no aerodynamic affect the would make any difference.
I fully understand that a parachute will move with the wind. My speculation is that in a wind the angle of attack of the chute is not 0, and the chute might generate some lift and move into the apparent wind. As the pressure under the canopy will be slightly higher than above the canopy and by adding a spill hole to the canopy, there could be a small thrust vector opposing the direction of the apparent wind due to the accelerated mass flowing through the spill hole. As you increase the hole area, you release more mass and should obtain more thrust. This additional effect might be too small to measure, but the downwind drift will be decreased slightly due to the shorter in-air time due to the slightly higher descent velocity.

Bob
 
I fully understand that a parachute will move with the wind. My speculation is that in a wind the angle of attack of the chute is not 0, and the chute might generate some lift and move into the apparent wind. <snip>

There is no "apparent wind" as far as the parachute canopy is concerned. There is only motion relative to the ground - of which the parachute has zero 'knowledge' of. That motion is simply a function of descent rate and the velocity of the air mass the parachute is descending in. If the air mass was moving at 50mph *relative to the ground* or was dead calm, the parachute sees the same thing: zero velocity.

[edit & expansion]

I want to elaborate a little on this aspect of this discussion and put my own reasoning out here - in hopes of helping those (not necessarily yourself, Bob) who find some aspects of this bizarre (as it surely can be): To be sure, it is counter-intuitive to say, "Parachutes don't drift - the air mass just moves" (as we're almost saying). Look at it this way: if the parachute/payload 'drifts' (i.e. has a velocity relative to the ground) LESS (by even the slightest amount) than the velocity of the air mass, then there must be some 'force input' to retard that motion relative to the air mass. OK, so what is it? (is it dragging some kind of 'anchor' or something?) There isn't any. If it 'drifts' at a rate GREATER than the velocity of the air mass, there is some other 'force input' to provide that additional velocity. OK, so what is *that*? Again, there isn't any. (gliding behavior discussed in a moment). The only 'equilibrium state' for this canopy descending in a moving air mass is exactly in accord with that air mass's velocity - otherwise, one has to explain where that 'delta V' is coming from.

Gliding? OK, sure, canopies can 'glide' (most particularly the cellular canopies), but even circular canopies can show that tendency (the Para-Commander particularly comes to mind). Will this increase (or decrease) drift? Unlikely, as (again) the canopy has *zero* knowledge that the air mass is moving in any particular direction at any particular velocity (how could it possibly know which direction is 'downwind'?) Thus, 'gliding' is entirely in a random direction and whatever portion is 'downwind' will cancel with whatever portion is 'upwind' -- net result: no change in distance covered *relative to the ground*. IF there was any gliding, I just can't see that the final touchdown point would change one iota from the exact same descent with a non-gliding canopy.

One way to help visualize the canopy in relation to the air mass it is in would be to tie on a 20 (or 50) foot piece of surveyor's flagging tape to the rocket -- to be ejected with the main canopy. Disregarding the smallish fluttering (from the actual descent velocity), I would venture to posit that the tape would hang straight down (and, no, I haven't tried it). Maybe this would graphically illustrate the situation this discussion is dealing with.

Back on topic ---- as far as the canopies I've made (see other threads), I've generally always used from 0.5% to 1% of the canopy AREA for my spill holes --- heavily leaning to the 1% figure (i.e. 10% of the canopy diameter (and, yes, I would call it "nominal diameter" Do)).
[end edit]

-- john.
 
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There is no "apparent wind" as far as the parachute canopy is concerned.

That is the concept I had an issue understanding until I had a long conversation with a skydiver. Wind is a concept that only applies to someone on the ground or if you are referencing air movement relative to the ground.

When a chute is in the air, it is "in the air" and the only air movement relative to the chute is what flows past it because of the vertical drop. There is NO wind affecting the chute in any way.
 
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