question came up in the AMA forum


Does anyone think or know whether a jet model, not a craft specifically design to do so, can surpass 275 mph terminal velocity... without power?

By terminal velocity do you mean a straight down dive or just flat out speed?

If you meant in a dive then yes. You just add enough weight so that the weight of the model equals the drag of the airframe at 275 or 300 or whatever. Mind you it may not be able to fly level on it's own at that weight.

And why power off?

Extra weight will have zero effect on velocity in a gravity powered fall. Check your physics text.

Larry

(~ means varies with below)

Drag ~ V^2

Gravitational Force = G * Mearth * Mobject / r^2 = g * Mobject

Terminal velocity is when, Drag = Gravitational force, so...

V^2 ~ Mobject
V ~ sqrt(Mobject)

.... terminal velocity varies with the sqrt of mass.

It also varies with drag coefficient and air density.

Larry, you're dead wrong. A higher weight will increase the terminal velocity. You're thinking of acceleration. More weight will not change the acceleration.

yes

consider that it should be able to fly well also...not a rock with something that kinda looks like wings



Why...just for the heck of it.

A 109 did'nt want a Jug on his a## in a long dive.

If it has to be able to fly in an acceptable manner under it's own power then that may change things. The maximum weight will then be set by many factors based on teh available power, size of the model, flight parameters that the owner/pilot is willing to tolerate and a few others.

Bottom line is that the airframe would have to be small enough and clean enough to have the same amount of drag force at that speed as what the weight of the model is. There's just too many unknowns to say yes or no for a practical example. But my gut reaction is to say no. This is based on the speed record set by a glider back in the 70's in Austria. The airframe was as clean as you're going to find and the record was set at about the 240 mph range. Given that the timing run through the traps was required to be basically level I have no doubt that the pre trap dive speed was well up there. Possibly in your 275 mph range. But this was a very clean design with an arrow like fuselage and very thin wings that were molded very accuratley. A typical jet style model with the usual visual styling items such as air intakes and canopies and gear doors and other "stuff" on it will have a much higher drag coefficient so it would require quite a bit of weight to counter the drag of the airframe.

PS: Larry, I hope you realize that you are mixing up acceleration with terminal velocity. Two different things. Even Leonardo Davinci was lucky that his little experiment at the Tower of Pisa used two items that had enough density that they did not reach a fair portion of their terminal velocity value before they hit dirt or he would have been quite embarrased. A rock and a feather have the same INITIAL acceleration but their terminal velocities are quite a bit different thanks to air drag.

Larry,

Weight absolutely makes a difference in the freefall speed of any object here on planet Earth.

I've done quite a bit of skydiving, and it's obvious in freefall. That's why jumpsuits are designed the way they are, to aid in helping us all fly at the same speed, without using drastically different body positions.

Shape, drag, etc., all come into play, but weight is a part of the equation.

While I was in the Army, I asked several paratroopers what terminal velocity of a human being in free-fall is. None of them knew and they all looked at me like I was retarded for even asking. A friend of mine who was in the University of Florida Aerospace program many years ago told me that it's about 110 mph for an average size man.

I've also wondered how much a person could steer himself with his body. For example, let's say you just happen to fall out of an airplane and you're in your three piece suit. You see a lake about a quarter mile away and your altitude is around 2 miles or so. Would it be possible to steer yourself to the lake and possibly dive in and survive? I would think that the cliff divers come close to terminal velocity, so assuming you could make it to the lake a person could theoretically survive, right?

That would be sick. A 10,000 ft swan dive.

That's because Airborne paratroopers don't freefall, they jump and their parachute opens immediately with a static line. Find a special ops guy and they may have the military freefall experience. Skydivers in normal jump suits can cover gound at a 1:1 glide ratio. Skydivers in "birdman"suits can do 3:1 or better.

As to whether any jet model could do 275 straight down, power off - not likely. Intakes go from being a part of the power system to being little parachutes of their own. Most turbine models have better than a 1:1 thrust ratio and might approach that speed, meaning the drag at max level speed would be more than their weight. So, a model that does 250 flat-out would do something less in a vertical power-off dive

Nope...

Skydivers have a very bad glide ratio, (about 1:1 max with a standard jump suit) and you definitely don't want to land on any surface, without a parachute. You could do it, but only once! I've daydreamed about jumping out of a crippled plane and heading for a lake, and I suppose anything is worth a try, if the option is certain death, but...

You're right about terminal velocity... Around 120 mph, depending on the skydiver's weight, size, body position, jumpsuit type, etc.. The "envelope" is approximately 90 or 100 mph at the absolute slowest, to nearly 300 mph for the "skydance" types in tight-fitting jumpsuits, in a head down dive.

Skydivers stick pretty close to the "32 ft. per second, per second" acceleration rate, and so the typical skydiver jumping out of a slow Cessna will take 3 or more seconds to get to terminal velocity. Cliff divers and other high divers going into water don't come close to terminal velocity, and also are at about the edge of survivability. You hit the water going 100 mph, it's not going to be pretty. But the thing is, you'd be going faster. "Terminal velocity" is typically measured by skydivers falling "face to earth", which is not how you'd want to land in water. When you straighted out into a head first or feet first position, you'd accelerate to as high as about 180 mph, even in a pretty big jump suit. In a clunky sort of way, it's like other airfoils we discuss. A pretty big skydiver in a pretty big and flappy jump suit is flying with a lot of drag, increased by the turbulent airflow around the flappy jump suit. The "skydance" types, typically skinny folks in skin-tight jump suits and full-enclosure helmets, are a lot more streamlined, and can fly faster. Just like an airplane, more airspeed gives more control.

If you ever have done a formation skydive, and have seen another skydiver whiz by in freefall, as you open your parachute, you know that landing in water will probably not affect the outcome. (150 feet per second to quite a bit faster! )

An interesting theory has been proposed more than once...
As tightly-grouped formations of skydivers grow in size, they actually decelerate, due to the drag. It's been conceptualized that if 100 or so skydivers formed a delta-shaped formation, and if all 100 of them were able to think and act in unison, to dive down, gain some speed, and then make the formation "flare", they might be able to actually land such a formation, if they all wore roller skates, (on their knees and hands, since you couldn't "stand up" ) and let the nose drop gently. If I remember right, I think the landing speed would've been around 90 mph forward, with something like a 20 ft. per second descent rate. (which is rough!) The catch is that if pretty much any of the 100 skydivers screwed up, (especially if it was low) the entire formation could collapse, or simply not maneuver correctly enough to survive. I hope very much that no one ever really tries this. (!)

(Photo of yours truly in freefall, a couple of seconds after exit from a Cessna 182)

Thanks for the info, Mike.

BTW - In that pic you look like a big Thanksgiving Day balloon on the ground.

It's hard to soar like an eagle when you look like a turkey...

You know... here in florida we have these vultures that I've always loved to watch. One day I was looking at one and told someone I was with to take a look. He said, "That's a vulture." An ugly bird that looks really beautiful in flight.

Hmm... this post would have made more sense if I had completed my thought. Until the guy told me they were vultures, I always thought they were hawks.

Evidently, a guy named Baumgartner managed to freefall at near a 1:4 "glide" ratio and cross the English Channel. Can't verify that though.

I knew an instructor pilot whose student was thrown from a PT 19 and survived-- the guy came down on a barn and his trajectory matched the slope of the roof. When it's not your time, it's not your time!

Baumgartner didn't really freefall in the normal sense... he had a small composite wing strapped to his back to improve his glide ratio a bit. http://www.felixbaumgartner.com/

-David

I COULD DO THAT! ! ! !


<....as I pull out my old Bird of Time glider and ponder how to bungie cord it to my back.....>

What Felix did takes some serious guts but that little wing is cheating. At what point does it metamorph from being a winged suit to becomeing a small hang glider?