Personal backpack helicopter: Impossible?

Factual Questions
1

I was watching an episode of one of my favorite shows on tv called Mythbusters. In this show the Mythbusters were building out what was basically a personal rotary drive backpack flying device from plans they had bought on the internet. Their purpose was to determine if its even possible to build such a device and have it actually work.

I don’t have a link to the actual show, but here is a wiki link simply talking about the concept I’m getting at here which will be part of the GQ question I ask in a sec. The device in the Mythbuster show was different, as it actually had two tandem rotor blades, one on each side of the backpack (and above the person who is trying to fly), instead of the one blade shown in the wiki article. Its flight controls were basically just tilting of the rotor blades via hand control.

Anyway, in the show the Mythbusters followed the instructions they had purchased, except they bought a lighter but more powerful engine than the one the plans called for. They then got FAA approval to test fly the device (as some kind of experimental class aircraft).

However, the thing just didn’t have the lift necessary to even get the crash test dummy they initially tried off the ground. Even after tinkering with the thing they simply couldn’t make it work. And this from some guys who DID get a rocket to fire out of a civil war cannon and fly 500 yards…made of plumbing parts, wax and NO! Their conclusion on the backpack personal helicopter concept? Busted…it can’t be done.

My question is…are they right? Is it impossible with todays materials, engines, etc, to build such a craft that could lift a single person into the air and fly about? I’m not talking about a jet pack…we know those work (though they are dangerous as hell). I mean a rotary driven personal device that one could fit in a back pack and carry around…and then use to fly with. Busted? Plausable? Confirmed? :stuck_out_tongue:

-XT

2

I guess it depends on what you mean by “backpack”.

We know small one man helicopters are possible. So start with the smallest, lightest helicopter on the market. Start sawing off parts…the landing gear, the seat, the canopy, the gauges. Add straps so the pilot can dangle. Then see if one person can lift the thing onto their shoulders. Probably not.

And here’s a problem. From this site: http://www.nasm.si.edu/research/aero/aircraft/pentecos.htm

“Combined with the heavy weight of the contraption, the pilot could easily stumble, causing the blades to strike the ground and send potentially lethal splinters in all directions.”

Yep, that’s gonna be a problem.

3

http://www.peroxidepropulsion.com/images/backpackheli.jpg
http://www.skyventure.co.kr/newupfiles/20030402_4.jpg

4

I am not a helicoptor pilot (although we have at least one on the forum - paging Johnny LA, please come to the thread…) However…

Yes, you CAN get off the ground with a contraption via rotors. The problem is doing so reliably and with some reasonable safety, and, oh yes, getting back onto the ground in one piece. Rotorcraft have some inherent stability problems (which I’d much rather a fling-wing pilot discuss) that were touched upon with the reference to stumbling and showering splinters in all directions. Small helicopoters, like all small aircraft, are more affected by wind and gusts than large ones. Backpack powered parasails have a very low wind tolerance, as an example. People do fall down on launch/landing, which is referred to as “eating dirt” because that’s largely what happens. If you’re lucky. If you’re having a bad day you’ll eat teeth and facial bones as well, if you smash into the ground hard enough. The big difference is that a parasail collapses into soft fabric when you hit. A rotor will dig up great big divots and shatter, but before it does that it might well twist you around fast and hard enough to break important things like your spine and/or neck. Helicoper accidents are scary and ugly, regardless of size.

You’ve got three problems to solve.

  1. You need a small enough, but powerful enough, engine to get you off the ground. We have these now.
  2. A helicopter, by it’s nature, involves a LOT of moving parts going at high speed. You need very strong, very lightweight materials so you’lll be able to carry all this. We might have that now, I’m not sure either way
  3. You need to control all this. Helicopters are touchy at the best of times. Someone who was a qualified helicopter pilot would be needed as test pilot - I can’t fathom teaching yourself to fly one AND design/build one at the same time. Yes, Igor Sikorsky did just that - but he was an aircraft designing genius before that, he spent decades developing the prototype, and it’s a wonder he didn’t kill himself like a lot of other people did. I don’t think anyone has worked out a viable control system for a rotorcraft that small.

In sum - it should be possible but I view it as highly improbable.

And I’ll shut up now, because we’re about at my limit of rotorcraft knowledge.

5

I’d re-gear the driveshaft to provide a taller gear ratio to the rotors, giving them higher rpms, and add a small turbo to give the power needed to create those rpms.

6

If anyone is interested, I tracked down the Mythbusters episode…its episode 32:

-XT

7

I have two contributions to this thread. First is that I don’t think it’s quite correct to call thing the Mythbusters were trying to build a “helicopter,” because it made no attempt to use variable angles of attack on the blades to change direction or thrust as a true helicopter does.

Their device, which they had to modify significantly from the poorly conceived plans they bought over the Internets, used a lightweight engine to power two stubby props in ducted fan-style cowls that both increased lift and provided some protection. I think flight control was going to be handled simply by shifting the operator’s body.

So I think this makes moot many of Broomstick’s concerns.

My other point is that I think that Adam and Jamie’s uncharacteristic failure to make the machine work was largely a function of the time and budget constraints of the show. They were very creative with the modifications to the plans they bought, and they seemed quite close to making it work. But they ran out of time. I hope they go back and try that myth again, because if they’ve kept the hardware, I think they could probably make it work with some more tinkering.

8

Bit of an understatement. I’m not a pilot either but suffice it to say that powered rotorcraft (as opposed to say gyros) have nothing but stability problems. You hear the phrase inherently unstable so much it loses meaning.

Think of aircraft like water craft. A fixed-wing plane like a Cessna is comparable to a simple rowboat. It just floats. As long as you stay within reason it doesn’t make much difference if its got a 5HP or a 50HP motor, or if its going 3mph or 30mph, or if its got one person or 5 people in it, or if the people are sitting still or moving around. Its ability to stay afloat is not at all challenged by any combination of these factors. That’s what inherently stable means.

A helicopter is more like standing on the water on two very narrow kayaks. Its inherently unstable. Meaning that just remaining afloat takes constant and precise adjustments and feedback. And as things like speed, weight, and vibration increase, the inherent instability increases almost geometrically!

Anyway, consequently you can only make a stable rotorcraft so small before it becomes woefully impractical. IOW, making it smaller doesn’t reduce its instability so it still has to be incredibly mechanically complex. More so in fact, to be really small.

So no matter how you design it, you get a one person vehicle that’s still inherently unstable (and compared to everything else, unsafe) and it still costs $30,000 like this goofy thing–

9

Yep, helicopters are inherently unstable. Stability has been discussed here before, so in a nutshell: Positive stability is the tendency to experience lessening oscillations after a disruption until the aircraft returns to its stable state. Neutral stability is the tendency for the aircraft to maintain its new equilibrium. Negative stability is the tendency for the oscillations after a disruption to grow larger. Many of us built free-flight model aircraft. These have positive stability that allows them to fly without intervention and, given a suitable area, to land unaided. Most GA aircraft are similar. By contrast, a helicopter requires constant, minute control inputs in order to fly. Their inherent instability is what makes them so maneuverable, much like moder jet fighters are very maneuverable but require computers as in interface between the pilot and the controls.

Okay, backpack helicopters. There are several single-seat helicopter designs. Most have little in the way of reserve power. It’s been pointed out to me on these boards that the Mini-500’s engine is working far harder than it was designed to, resulting in failures. So you need an engine powerful enough to swing the rotor system without working so hard that it fails. In addition to the engine you need a transmission. You also need swashplates so that you can vary the pitch of the rotor blades collectively and cyclicly. And the transmission needs a freewheeling unit, by the way, in case of an engine failure. Things are starting to get heavy.

Now how do you counteract torque? There are a couple of ways. The most common is to have an anti-torque rotor. Another way is to have contrarotating rotors on the same shaft, or with twin rotors rotating in opposite directions. Another way is to use tip jets, so you only have to counteract friction – which may be do-able with a fin.

The Hiller HJ-1 Hornet was incredibly noisy under power and had an unacceptable sink rate in autorotation. And I doubt a ramjet-powered backpack helicopter would have enough fuel for useful flights. Twin-blade helicopters need to be fairly large compared to a single-rotor system. Too big for a backpack heli. Kaman makes a twin rotor where the rotor blades intermesh. But since they are inclined to the sides, they’d have to be on high masts for them to work on a backpack and provide enough ground clearance at the tips.

The wiki illustration (I admit I haven’t read the article) has contrarotating rotors to counteract torque. Okay. As long as the pilot doesn’t stumble, the blades won’t hit the ground. Some sort of landing frame would be in order.

And there’s the trusty tail rotor. This is by far the most popular configuration.

But with a tail rotor you need a tail boom. If you have a tail boom, you need a counterweight in front of the rotor mast to balance the structure. A ‘backpack’ helicopter in this configuration may end up looking like a pantomime horse!

So:
[ul][li]Tail rotor design ends up looking like a regular helicopter;[/li][li]Tip-jet design is too noisy and requires too much fuel, plus it needs a fin/rudder;[/li][li]Twin-rotor design needs a wide structure to separate the blades, or a tall structure to prevent the tips of intermeshing blades from hitting the ground. Looking more like another version of the pantomime horse.[/ul][/li]All of the above make one ask, ‘What’s the point?’ If you’re going to have a structure, why not a proper one? But what about the contrarotating design? It seems to me (and I’m not an aeronautical engineer) that it might work. If you could get a powerplant that’s small enough and light enough and powerful enough to spin 15 or 20 feet of rotor at such a speed that will alow them to lift themselves and the transmission with freewheeling unit and swashplates and control linkages and fuel tank and pilot. Oh, and a support structure so that the pilot doesn’t have to carry the whole weight on his shoulders. Landings would of course have to be zero froward speed to reduce the chance of tripping.

So I don’t see ‘backpack helicopters’ being at all practical. You either have to have an airframe anyway, or you won’t be able to get a strong reliable engine that a man can carry on his back.

Tangentally, there was also the Hiller VZ-1 Pawnee – sort of a Dick Tracy flying platform. The third generation one had a seat and helicopter controls (the first two were operated by a standing pilot who shifted his weight to steer) and apparently flew well enough. But they were too small and slow for military use.