Skydiver / Aeronautical Questions

Casey Karp · Sep 19, 2013

Got a couple of questions that are proving somewhat difficult to pin down in my Googling around. Hopefully youse can point me in the right directions.

1) Is there a good way to figure out how far a skydiver could land away from the point directly below where they jumped out of the plane? Clearly it's going to depend on how high up they are, what the winds are like, what kind of parachute they're using, and probably seventeen other things I haven't thought of, but -- assuming they don't have a chute designed for gliding -- what's a plausible rule of thumb?

2) What kind of options are there for a vehicle to remain stationary relative to a specific point in the air? In this case, I'm looking at whether something can stay relatively stationary near a specific spot at around 16,000 feet. I've established that a helicopter isn't an option*, but are there other possibilities? Maybe a lighter-than-air craft of some sort? For plot purposes, I don't much care whether the answer is "There aint no such beast" or "You could use a..."; I can write this either way, but I want to make sure I've justified the way I go with it. Again, plausibility is the key, not AbsoluteTruth(tm).

* I gather that the CH-47 Chinooks used for rescues on Mt. Denali have a high enough ceiling (18,000-19,000 feet), but can't hover particularly well above around 10,000 feet.

thothguard51 · Sep 19, 2013

Blimps...

cbenoi1 · Sep 19, 2013

http://en.wikipedia.org/wiki/High_altitude_military_parachuting

Define "relatively stationary". A small turbocharged piston aircraft (ex: Cessna Skylane) facing a very strong head wind will essentially be 'stationary' wrt ground. It would have to be a VERY strong headwind because stall speed increases with altitude, and the poor bugger would have to crank out piston-breaking RPM just to avoid stalling.

Also, you may want to explore human physiology above the 10,000 ft mark WRT your scenario. Hint: supplemental oxygen.

-cb

ClareGreen · Sep 19, 2013

Casey Karp said:
Got a couple of questions that are proving somewhat difficult to pin down in my Googling around. Hopefully youse can point me in the right directions.

<snip>

2) What kind of options are there for a vehicle to remain stationary relative to a specific point in the air? In this case, I'm looking at whether something can stay relatively stationary near a specific spot at around 16,000 feet. I've established that a helicopter isn't an option*, but are there other possibilities? Maybe a lighter-than-air craft of some sort? For plot purposes, I don't much care whether the answer is "There aint no such beast" or "You could use a..."; I can write this either way, but I want to make sure I've justified the way I go with it. Again, plausibility is the key, not AbsoluteTruth(tm).

* I gather that the CH-47 Chinooks used for rescues on Mt. Denali have a high enough ceiling (18,000-19,000 feet), but can't hover particularly well above around 10,000 feet.

Helicopters are actually plausible at that height. There's a set of photos online, taken by a chap who flies Eurocopters in the Himalayas and works at up to 22,000ft above sea level. Chinooks not so much, but smaller helicopters can manage to get over 16,000ft, though the all-up weight gets critical at the top end. PM if you want a pointer to where the pictures are - the photos are gorgeous, if nothing else.

The practical limit for airships would seem to be much lower, at about 8,000ft; they can go higher, but it starts to hurt. That's sourced from Wikipedia, though, so reality may vary.

Have you thought about the oldest form of flight, kites?

Casey Karp · Sep 19, 2013

Good point on the "relatively stationary". How about "stable enough that someone at the "specific spot" would have a 50/50 chance of hitting the aircraft with a hurled baseball"? (That's not what I'm trying to do, but it's close enough to get the idea across.) I should probably also include "for an indeterminate but extended period of time", since I'm not sure when the baseball will be thrown.

I don't think the Cessna in a headwind will work -- the blimp suggestion has more merit, but I can't find anything useful on the practical ceiling and stationkeeping abilities of blimps.

Casey Karp · Sep 19, 2013

ClareGreen said:
Have you thought about the oldest form of flight, kites?

Interesting notion. That's one hell of a long kite string, though.

I had thought about tethered balloons, but rejected it for the same reason.

cbenoi1 · Sep 19, 2013

> How about "stable enough that someone at the "specific spot"
> would have a 50/50 chance of hitting the aircraft with a hurled baseball"?

A circling drone?

-cb

Casey Karp · Sep 19, 2013

Hmm. I may have been too quick to dismiss tethered balloons. Record altitude for a kite is a bit below the 16,000 feet I'm looking for, but while searching for that, I found a Wikipedia article on a tethered radar system that hoists a one-ton payload to 15,000 feet. Grain of salt applies -- why a 25,000 foot cable if the maximum altitude is 15,000 feet -- and I'm still a bit dubious about stationkeeping ability (crosswinds over water, oh my!), but it at least seems plausible that it could work in the context I'm looking at.

Thanks for the rap upside the head regarding kites.

Edit: Plausibility trumps 100% accuracy here...

Any thoughts on the first question?

Drachen Jager · Sep 19, 2013

Casey Karp said:
I found a Wikipedia article on a tethered radar system that hoists a one-ton payload to 15,000 feet. Grain of salt applies -- why a 25,000 foot cable if the maximum altitude is 15,000 feet --

You think the cable would go straight up?

cbenoi1 · Sep 20, 2013

> Any thoughts on the first question?

http://en.wikipedia.org/wiki/High_altitude_military_parachuting

Check for HAHO.

-cb

King Neptune · Sep 20, 2013

Casey Karp said:
s.

1) Is there a good way to figure out how far a skydiver could land away from the point directly below where they jumped out of the plane? Clearly it's going to depend on how high up they are, what the winds are like, what kind of parachute they're using, and probably seventeen other things I haven't thought of, but -- assuming they don't have a chute designed for gliding -- what's a plausible rule of thumb?

This is almost unanswerable, because the factors you mentioned are highly variable. Even a parachute that isn't designed will allow quite a lot of horizontal travel. If you would allow wing suits, then 4.6 miles is the record. I couldn't find anything for regular parachutes, but there probably is something. My guess is that the horizontal distance would be less than the vertical, but there are parachutes that slow one to a quite low speed, and that would allow more horizontal travel. If you want a plausible idea, then keep the horizontal to no more than half of the vertical. That's just a plausible guess for relatively still air.

King Neptune · Sep 20, 2013

cbenoi1 said:
> Any thoughts on the first question?

http://en.wikipedia.org/wiki/High_altitude_military_parachuting

Check for HAHO.

-cb

Yes, thirty or more miles, but they use parachutes that are designed for horizontal travel.

Casey Karp · Sep 20, 2013

Drachen Jager said:
You think the cable would go straight up?

LOL. No, but I hadn't thought it would be that great an angle. My gut thought was that for 15K feet, 20K of cable would be sufficient. I probably had an unarticulated assumption that if the winds were high enough to require more than that, the balloon would be grounded.

Probably irrelevant for my story needs. The important part is that the balloons can get up to those heights; the amount of cable left on the spool is, at least at this point, one of those details the author needs to know, but the reader doesn't.

King Neptune said:
This is almost unanswerable, because the factors you mentioned are highly variable. {deletia} If you want a plausible idea, then keep the horizontal to no more than half of the vertical. That's just a plausible guess for relatively still air.

Darn. That checks nicely with my gut reaction, which (in view of the above) I'm now suspicious of. But since you got there independently, I suppose I can work with it. Time to go do a little handwaving, I guess.

Thanks, by the way, for the thought about wing suits. I had considered them, but they're just not going to work given where the story is going.

King Neptune said:
Yes, thirty or more miles, but they use parachutes that are designed for horizontal travel.

Heh. Yeah, the HALO approach is closer to what I had in mind, but looking at that link and a couple of others, I get the impression that one of the goals in HALO is to minimize the horizontal travel.

I think that "half of the vertical distance" rule will work for me, especially if I allow some leeway for some not-at-all still air.

Thanks, folks. This is a big help.

King Neptune · Sep 20, 2013

Casey Karp said:
Darn. That checks nicely with my gut reaction, which (in view of the above) I'm now suspicious of. But since you got there independently, I suppose I can work with it. Time to go do a little handwaving, I guess.

Thanks, by the way, for the thought about wing suits. I had considered them, but they're just not going to work given where the story is going.

Heh. Yeah, the HALO approach is closer to what I had in mind, but looking at that link and a couple of others, I get the impression that one of the goals in HALO is to minimize the horizontal travel.

I think that "half of the vertical distance" rule will work for me, especially if I allow some leeway for some not-at-all still air.

Thanks, folks. This is a big help.

Look at HAHO (High ALtitude High Opening) info. Those with directional parachutes can go thirty miles, but they use chutes disigned for it. Does your character have training and a decent chute?

"HAHO jumps also allow a longer travel distance due to increased under canopy time, allowing travelling distances of more than 40 miles (64 km).[3]
http://en.wikipedia.org/wiki/High_altitude_military_parachuting#HAHO_-_High_Altitude_High_Opening

I think it means that you can do whatever you want, as long as you don't stretch too much. If you want a character to make ten miles horizontal from 40,000 feet, then he'll have to have O2. Ten miles from 25,000 would just require luck with the winds.

Casey Karp · Sep 20, 2013

I guess I wasn't clear enough. Sorry, I'm not really used to talking about works in progress, so I didn't really set up the problem correctly.

What I'm working on involves someone falling. No parachute, but possibly spent some time in one of those vertical wind tunnels, so he knows at least theoretically about spreading out and steering. Assuming still winds and all that, how far from the vertical is it reasonable to think he could get?

I kind of like that "half the vertical distance" idea. That's only about a 25% angle off the vertical, so it seems like it could work. Am I wildly off-base, or is that even slightly plausible?

Drachen Jager · Sep 20, 2013

Oh, well why didn't you say so?

A quick bit of google-fu and, presto!
http://en.wikipedia.org/wiki/Tracking_(freeflying)

"It is claimed that good trackers can cover nearly as much ground as the distance they fall, approaching a glide ratio of 1:1. It is known that the fall rate of a skydiver in an efficient track is significantly lower than that of one falling in a traditional face-to-earth position; the former reaching speeds as low as 90 mph, the latter averaging around the 120 mph mark."

debirlfan · Sep 20, 2013

I'm not sure what the victim (?) is wearing when he falls, but keep in mind that even a standard parachute outfit has some webbing that's going to help with steering - if your falling individual is wearing street clothes, they won't have that and regardless of position probably won't track as far.

King Neptune · Sep 21, 2013

Casey Karp said:
I guess I wasn't clear enough. Sorry, I'm not really used to talking about works in progress, so I didn't really set up the problem correctly.

What I'm working on involves someone falling. No parachute, but possibly spent some time in one of those vertical wind tunnels, so he knows at least theoretically about spreading out and steering. Assuming still winds and all that, how far from the vertical is it reasonable to think he could get?

I kind of like that "half the vertical distance" idea. That's only about a 25% angle off the vertical, so it seems like it could work. Am I wildly off-base, or is that even slightly plausible?

"No parachute" makes that half the vertical believable as the max. Is the character going to survive? SOmeone trained in falling can jump out of a car at highway speeds and urvive hitting the pavement with no major injuries.

Drachen Jager · Sep 21, 2013

King Neptune said:
"No parachute" makes that half the vertical believable as the max. Is the character going to survive? SOmeone trained in falling can jump out of a car at highway speeds and urvive hitting the pavement with no major injuries.

This isn't the pavement though. This is someone jumping out of a car into a brick wall as the ground is at ninety degrees (or forty-five at best) to the direction of travel, not flat as it would be with jumping from a car.

I presume the OP has other plans for how the character is to survive the suicide jump.

King Neptune · Sep 21, 2013

Drachen Jager said:
This isn't the pavement though. This is someone jumping out of a car into a brick wall as the ground is at ninety degrees (or forty-five at best) to the direction of travel, not flat as it would be with jumping from a car.

I presume the OP has other plans for how the character is to survive the suicide jump.

People have survived such falls. It isn't an experience that I would like to have.

Casey Karp · Sep 21, 2013

Drachen Jager said:
Oh, well why didn't you say so?

A quick bit of google-fu and, presto!
http://en.wikipedia.org/wiki/Tracking_(freeflying)

Because I suck at sharing anything related to a work in progress. Picture me wrapped around my keyboard, snapping at anyone who approaches, and growling "Mine, mine!" Not that "mine" is well-suited to growling, but you get my drift.

Well, hell. Right there in plain sight. No wonder I couldn't find it. Srsly, thanks. (It pays to learn the language of the country you're exploring, doesn't it?)

King Neptune said:
Is the character going to survive?

Drachen Jager said:
I presume the OP has other plans for how the character is to survive the suicide jump.

Survive? Will you accept "Yes and no"?

King Neptune said:
People have survived such falls. It isn't an experience that I would like to have.

Not one I want to experience, other than in fiction. Yeah, at least three reasonably-well-authenticated cases since WWII of people surviving falls in excess of 15,000 feet without a parachute. Takes skill and a lot of luck.

Thanks again, folks. You really are awesome.

Skydiver / Aeronautical Questions

Casey Karp

thothguard51

A Gentleman of a refined age...

cbenoi1

ClareGreen

Onwards, ever onwards

Casey Karp

Casey Karp

cbenoi1

Casey Karp

Drachen Jager

Professor of applied misanthropy

cbenoi1

King Neptune

King Neptune

Casey Karp

King Neptune

Casey Karp

Drachen Jager

Professor of applied misanthropy

debirlfan

King Neptune

Drachen Jager

Professor of applied misanthropy

King Neptune

Casey Karp