Don't fly a blimp [at low altitude] in Oklahoma [or Texas?]

Marvin Polzien, intrepid blimp maker, was sniped in flight.
http://www.kten.com/Global/story.asp?S=11781376

This is not a small problem. If you're not certified ( read that - Ultralight) you're going to be flying where this can happen. The problem is if someone is redneck enough to do this, they're probably too supid to know it's against federal law to shoot at aircraft of ANY kind.

I am a little skeptical, "many of the holes were found on the bottom of the blimp'? That doesn't make sense. The bullets would rip through the top of the envelope too, leaving exit holes that aren't mentioned. Maybe the holes were caused in the crash. 

If the holes were at the top of their parabolic arc they would come in and go out near the bottom. I assume the shooter is at the side. For a practical calculation what % lift loss would have to happen to bring you down? This determines the height of the bullets through the bag that would cause an emergency landing.

If you loose pressure on a small craft, even aero lift won't bail you out.

If the bullets are low on energy, that means the shooter must have accounted for the drop when aiming. That seems unlikely considering that a 22 can fly more than a mile. I also think it would take a long time to lose a significant amount of gas through 22 caliber holes. I'm not saying that he didn't get shot at, but I am skeptical. Besides, how many times did he shoot? Most 22s only hold 10-15 rounds and you have to expect he missed a few times. He would have had to hit the blimp at least 11 times to account for all the holes. 

In the news story, Marvin didn't state that he heard the shots. That's why I suspected a long shot. Secondly, it's hard for me to imaging an adult being this criminal. A bunch of kids, maybe.

The German airships were routinely shot at over the Saraha during the non-war years. Since the mutiple gas cell design accounted for the possibility of some being lost and due to the size of those airships, it wasn't as big a deal as with a small ship. Also altitude and the quality of the guns being used then weighed in their favor.

With millions of Bambi hunters around, we ain't so lucky.

With motors going, Marvin could not hear shots. A soft fabric at low tension, like his ship was, will absorb impact from bullets. Really low tension fabric can actually catch bullets with no damage. Going up, 22 rounds may have only enough energy to penetrate one layer. He may have found some rounds inside the envelope. It would be nice if we could get Marvin into the group.

When I was in the Air Force the F-16s and A-10s would land with bullet marks (no real damage) on a fairly consistent basis according to my crew chief roommate.  Mostly when they were on straight, slow, and low slots during landing, takeoffs, or gun range exercises is what what was presumed.  And adults were caught on rare occasions, such as when one decided to take potshots at some pilots that were being driven across base during night exercises.  

A moron with a gun will shoot at anything that he hasn't shot at before.  

But let us not forget that the shots were not what brought him down. I've read his statement on the news that a motor broke its mounting, causing loss of firm control. He still had altitude control. One video I saw clearly showed a side motor blowing propwash upward against the billowing envelope, ie, he was trying to get down fast, rather than let the wind float him away from a landing on the highway.

His envelope was not too firm in the videos and he surely did lose some tiny amount of pressure from the bullet holes. Pressure started out low, I'll wager. Maybe two to five psi max?  If he was an actual ultralight blimp then he started out with no more than 250 cubic meters of gas and the total area of the ten .22" diameter holes, 2.2", especially on the bottom of the envelope where the pressure is less, it might take a few days to empty the blimp.

<<If you loose pressure on a small craft, even aero lift won't bail you out.>>>

Not so on EVERY small airship. A lifting body dirigible will still have aerodynamic lift after losing lift gas pressure. This is an impressive safety feature of dirigible lifting bodies and which we must emphasize and be very sure that it is true and that it works.  It makes us safer and much more desirable than crazy tinfoil airplanes which frequently drop out of the sky with lethal results.



Keep in mind it ain't the pressure, it's the air displacement volume that gives blimps their positive buoyancy. So, when the pressure slowly drops to zero after the bag has been holed, the bag, for a while, is still full of lift gas. It will descend slowly and safely as the gas slowly fizzles out the hole. 

  Except in the case of a hydrogen fire, which will not happen in a properly designed vessel, there is no POP, you're toast!, type of event possible in a blimp or in a dirigible. They go down slow. It's a great safety assurance and recommendation for this type of transportation.

A lifting body airship is even safer because the aerodynamic lift of the envelope is attached to the frames, whether or not there is lifting gas inside the envelope. 

You could instantly, totally remove the lift bags and the dirigible still has aerodynamic surfaces which provide lift and afford control as it descends. 

     This image shows why I'm going rigid. Loss of pressure can mean loss of control. If a rigid loses buoyancy, you are now a glider. A good lifting body shape can glide a long way to a safe landing. With a motor, you're an airplane. Keep on trucking to the next gas station.
     On a good note, my plastic arrived, and I've got a gore design started on my work table. I used a geometry trick to draw an ellipse, should be ok for a gore. I'll do a paper one first to check it out. The plastic is thick enough for a full size ship, going to be way too heavy for the model to ever lift. It doesn't seem to come thinner for models.
    I'll check the specs, this stuff is strong as steel to feel it. I have a plan for forming the ends and gluing them on.

Didn't that blimp have a catastrophic envelope failure? This wasn't a slow leak IIRC. A seam failed or something like that. 

If that is the case, the answer to that problem is not to have a catastrophic envelope failure. A rigid airship can have a catastrophic structural failure as happened with the R38. Any design can fail. 

There seems to be a perception that you have to loose all your lift gas before you're in serious flight safety trouble. No. It is good to do the weight vs % lift gas left calculation to see what your terminal velocity will be when you kiss dirt.

Another comment declared that you could make up lift loss by aerodynamics. If you had enough aerodynamics to make up for lift loss you'd have an airplane and wouldn't need to putz with lift bags in the first place. If you have 20% aero and 80% gas lift, then the damage depends on whether the pressure in the bag was what gave you your aero shape or not. If not then all of the framework has to be strong enough to hold shape and provide lift with buoyancy and without it.

If a rigid ship, you're closer to the situation outlined in my first statement.

The smaller you make a ship, the more susceptible you are to narrow margins.  Physics can only do so much and hope does not widen margins.

We aren't crossing the Swiss Alps or the Atlantic. Slow failure should not be our concern. Only catastrophic or rapid failure should matter to us. We only need enough time to put down or bail out. If we crash, we only lose a $10k craft rather than a $200k Cessna. 

A slow bleed out due to hypothetical bullet holes is not worth much of our design time. We need to make something that works before we make it bullet proof, if it isn't naturally so. 

We need to say: we need X minuets to put the craft down on the ground with 20 .22 caliber holes in the envelope. Does our design pass this test? If yes, then we are good. If not, then we need to include a parachute. 

BTW many ultralights and small planes have integrated parachutes.

Not too long ago, I saw a video of a base jumper swinging on a rope from some canyon wall. He had the fastest opening parachute I ever saw. Haven't been able to find the video since. I've always wanted some safety device for low altitude escape, this looked the best yet. A Batman cape type thing is cool to think about, but probably not practical. Probably better to plan on riding the ship down, that would be slower and safer than free fall.

I was looking for base jumping videos. This a is crazy one that I found. I read that base jumping chutes are around $1200-1500. That isn't super expensive, but it is significant. You can deploy them with a static line or by pulling a pilot chute from your pocket. 

 
A slow bleed out due to hypothetical bullet holes is not worth much of our design time. We need to make something that works before we make it bullet proof, if it isn't naturally so. 
We need to say: we need X minuets to put the craft down on the ground with 20 .22 caliber holes in the envelope. Does our design pass this test? If yes, then we are good. If not, then we need to include a parachute. 
BTW many ultralights and small planes have integrated parachutes.

-dude6935

Totally agree. The only caveat is the weight of these parachutes and how close to the ground you are when you have to pull them. The usual distance to get to terminal velocity is 110 ft. The lowest ballistic parachute jump I've heard of was at 150 feet.

The weight of the parachute should not count toward the ultralight weight limit because it is an emergency safety device. I was thinking more of a parachute for the whole craft rather than for the pilot. Although you could go either way.

Those holes could have been a shotgun blast, number one shot, a single round, not much penetrating strength at 500 feet up.    It is a good argument for using the 3/4 inch hollow polycarbonate decking from Coroplast.com, you may have to google it.   Imagine covering it with a few layers of Kevlar or ballistic nylon, or better yet, ballistic foam like they use in helicopters.

   It does make sense to use a ridged frame for several reasons, one is aerodynamics of a ridged body are more consistent than for a balloon like blimp.  as well as other shapes are made available that a balloon cannot easily form like wing surfaces and nacelles.   Also it makes it possible to mount motors in more places and spread the thrust around more evenly and gain better speed.

Those holes could have been a shotgun blast, number one shot, a single round, not much penetrating strength at 500 feet up.    It is a good argument for using the 3/4 inch hollow polycarbonate decking from Coroplast.com, you may have to google it.   Imagine covering it with a few layers of Kevlar or ballistic nylon, or better yet, ballistic foam like they use in helicopters.

-inventive47

This would need to be weight evaluated for applicability.