Jet-Man Yves Rossy: Best Engine,etc?

[sanman]

So by now, everybody's probably seen the news and videos of Yves Rossy, the Swiss pilot, jetting around over the Alps with wings strapped to his back, and 4 mini-turbojet engines attached to those wings. The entire apparatus is said to weigh 120 lbs.

Each of his 4 engines is a JetCat P200, with specs as follows:

Thrust at full power 50 Lbs / 220N
Weight incl starter 4.8 Lbs / 2.2Kg
Diameter 5.1 inches / 130mm
RPM Range 33,000 - 110,000
Exhaust temp 670C
Fuel consumption 25.37 oz min at full power
Fuel Jet Al, 1-k kerosene
Lubrication 5% oil mixed in fuel
Maintenance interval 50 hoursBut apparently his flight only lasted 5 - 10 minutes, due to fuel constraints.
I was wondering how this might be improved upon.

I was reading that turbofan engines typically offer superior fuel efficiency over turbojets, as well as quieter operation. Also, Yves had to wear thermal protection because of the heat from the jet exhaust, and hopefully turbofan exhaust is less hot. Another thing is that a turbofan could perhaps reverse thrust by flipping the blade angle, allowing aerobraking for landing without a chute.
But how do they fare on thrust-to-weight ratio?

I hunted around the next for the smallest turbofan engines I could find. One site I found was this one:

http://cpl.usc.edu/eschuste/

The other I found was this one:

http://www.globalsecurity.org/military/systems/aircraft/systems/f107.htm

So the smaller engine out of those two turbofans seems to have a higher thrust-to-weight ratio (granted, the first one isn't a military-certified product)

Suppose we assume that a 2-engine setup is the most desirable, so that if one fails then you still have the remaining one to fly with.
Then what would be the best type of engines for our winged man?

I'm thinking that you'd want each engine to be capable of generating 100 - 120 lbs of thrust, with a max weight of 10 lbs each. Is that doable for a turbofan?

 

[Greg Freeman]

It's been awhile since I've studied the subject, so don't take 100% of what I say to be valid.

JetCats, from what I know, are reasonably popular engines for RC aircraft, and thus have lots of hours on them, and I've heard of just about every variety of engines made by them except for turbofans. They make turboprops, helicopter setups, etc. but I don't know of a turbofan in production.

It might be possible to make an engine with that weight and thrust, but the design will have to be different. I was trying to imagine what would be so much different between the turbojet and the turbofan and why you don't see RC turbofans as much...I just realized it must be because of the vast difference in diameter of the compressor/turbine components and the fan components. If everything turns at the same RPM, your fan sections is going to have much higher stresses on it. To get around this, you'd need some kind of gearing system, which drops your efficiency a little, and adds more components and complexity. These little puppies are spinning sometimes faster than 100,000 RPM, and I'm unsure how close they are pushing the limits of the materials, and if they are using exotic stuff or not.

 

[Greg Freeman]

To add to that, I wonder if you could take their turboprop set up, change the gearing, and drive a ducted fan with it. I don't know if you'd gain much from that modification or not.

 

[sanman]

I was thinking that with 4 engines, it'd be best to give them a 4-cornered layout:

x x​

---------------

x x​

whereas with 3 engines you'd give them a triangular layout:

x​

----------------

x x​

Either of these layouts would enable the jet-wing to stably stand on end, to enable VTOL.

But I'd imagine that the 4-cornered layout would be better, since if one fails, then at least you have the remaining 3 to keep you stable for VTOL.

You'd of course need electronics to rapidly adjust the engine outputs to maintain balance and stability in VTOL.

 

[FredGarvin]

The limitation is not in the engine. It's in the fuel capacity. You are talking about huge issues with making a larger bypass ratio turbofan that small (trust me, I work with small engines). To keep the size manageable, you would need to shrink the core components. You reach a ton of limiting factors very quickly, namely blade tip clearances and bearing problems. Besides, the efficiency gains you would get probably wouldn't be measurable at that scale and you will not cool the exhaust gases appreciably. There are just some aspects you can't get around.

If you think you have fuel concerns now, you might as well forget an F107. Their 160+ pounds and we run them on JP-10. The average joe can not buy JP-10. Also, strapping two F107's to your body would probably tear you apart and they would be so heavy, the basic premise that Yves is operating under would no longer apply.

He needs to examine better ways to carry fuel. That's where the current technology makes you look.

 

[sanman]

Hi, I just mentioned the F107 for reference. Clearly its thrust and weight are overkill. But I'd point out that the sports industry has often used cutting-edge technology to deliver leading-edge performance that people are willing to pay a pretty penny for.

If someone can just establish a basic platform to get the foot in the door and establish a market, then the leisure/hobbyist world will come running, and their money will drive the innovation and evolution of the platform.

Don't underestimate the power of male teenage hormones to move massive mountains of money -- look at internet porn and GTA-IV. Think of this jet-wing thing as the next GTA-IV.

 

[NateTG]

In the US the whole FAR 103 thing massively favors stuff that can't go more than 45 knots, which also makes for easier fuel economy and range.

Beyond that, pulse jets may be practical for an application like that since they're light, simple, and cheap.

 

[minger]

The difference between turbojets and turbofans are bypass flow. In a turbojet engine, all of the flow is directed through the combustion area. In a turbofan, a portion of the flow is directed around the combustion area.

http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html

The rest of the incoming air passes through the fan and bypasses, or goes around the engine, just like the air through a propeller. The air that goes through the fan has a velocity that is slightly increased from free stream. So a turbofan gets some of its thrust from the core and some of its thrust from the fan.

And agreed with Fred (as usual). You can get fuel economy close to a turboprop by having large bypass ratios, which is way too difficult on a small jet like that.

 

[sanman]

Here's an article:

http://machinedesign.com/ContentItem/61407/TomThumbturbinespowerradiocontrolledjets.aspx

Smaller engines also mean smaller Reynolds numbers, even if airflow is the same. In other words, the air seems thinner or less viscous in smaller engines. To deal with this, the engines employ centrifugal compressors, often using off-the-shelf centrifugal compressor wheels, such as those in automotive turbochargers, rather than axial compressors, such as those found on full-size jets. Lower Reynolds numbers and reduced size also mean turbine blades are fewer and larger, relative to engine diameter.

Since the F107 and PW610 are each about 12" (30cm) in diameter, then why is it outrageous to consider a turbofan with half that diameter? 6" (15cm) would be a reasonable size for personal-sized wing.

 

[sanman]

What about a Wankel/rotary-piston engine vs a turbofan?

http://www.defensenews.com/osd_story.php?sh=VSDS&i=3386638

Which has the better power-to-weight ratio? Which has better fuel efficiency?

 

[sanman]

What about a small-sized version of the geared turbofan?

http://frontierindia.net/geared-turbofan-engine-demonstrates-alternative-fuel-capabilities

The Geared Turbofan engine targets double-digit reductions in fuel consumption, engine noise, environmental emissions and maintenance costs.

In a Geared Turbofan engine, a state-of-the-art gear system allows the engine’s fan to operate independent of the low-pressure compressor and turbine, resulting in greater fuel efficiency and a slower fan speed for less noise.

 

[sanman]

Here's a nice article from latest May2008 edition of Mechanical Engineering Magazine:

http://www.memagazine.org/contents/current/features/changing/changing.html"

Potentially radical advances in gas turbines came in all shapes and sizes in 2007.

Just read this and drool:

Engine compressors and turbines run most efficiently at higher rpm, while fans operate best at lower speeds. The unique feature of the geared turbofan engine is a fan hub-mounted epicyclic, or planetary, gearing system that drives the fan at lower speeds, permitting higher bypass ratios. By using a three-to-one gearing system, the GTF fan speed is cut by one-third, allowing for much less fan noise and higher bypass ratios—8:1 to 11:1. With such a high bypass ratio, fuel consumption goes down. A gearbox adds weight to the engine, but this is counterbalanced by the need for fewer engine airfoils, since engine components can now run at more efficient speeds that aren't limited by fan aerodynamics or stress limits.