Ducted fan aircraft

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Doggzilla

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If there is a difference in efficiency between the blade types I assume its because of the central hub. With a small hub the center of the blades is not moving nearly as quickly as the edges and will produce lower pressure. So the pressure is more equal along the blades if a large hub is used.

I know turbine engines have this issue at high altitude. The high pressure air from the edges can "unstart" and fire through the lower pressure center of the blades. This is why high altitude engines have a very large hub. Compare the Concorde's Pegasus to the RB on the Tornado. The RB is supposedly a better engine but will unstart at high altitude if the RPMs are too low because of the length of the blades and how small the hub is. Causes a very substantial pressure difference between the base and end of the blades.
 

Malish

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Hey Malish, what are you getting for your static thrust?

I was checking out the Martin Jetpack after they recently went out of business and was surprised to see their ducted fans produced the same thrust as a prop for the same HP. Over 700lbs with 200hp, the same as a 200hp Cessna with prop.
We're have static thrust now is about 800lbs at 350hp, but we're working on the fan blades to improve the thrust. Martin Jetpack has fan design for more static thrust, then for speed. In our case, fan designed for speed, not for static thrust + we're have long ducts which reduce fan efficiency also. But this is trade off we must take to have fighter jet looking aircraft.
 

pictsidhe

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We're have static thrust now is about 800lbs at 350hp, but we're working on the fan blades to improve the thrust. Martin Jetpack has fan design for more static thrust, then for speed. In our case, fan designed for speed, not for static thrust + we're have long ducts which reduce fan efficiency also. But this is trade off we must take to have fighter jet looking aircraft.
I'm really, really impressed at the way you've balanced art and engineering on this aircraft. As you say, ducts are not good for piston engines. I'd already noticed that you've managed to keep them fairly short. You've also given it a halfway decent aspect ratio. Despite these differing quite a long way from typical fighter jets, this thing still manages to look just like a fighter jet. My hat is off to you.
 

Doggzilla

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We're have static thrust now is about 800lbs at 350hp, but we're working on the fan blades to improve the thrust. Martin Jetpack has fan design for more static thrust, then for speed. In our case, fan designed for speed, not for static thrust + we're have long ducts which reduce fan efficiency also. But this is trade off we must take to have fighter jet looking aircraft.
It may just be a size limitation, since they are very close in cross section.

According to the prop calculators I can find online it only takes about 20% increase in cross section to double thrust.

Since cross section is squared, it takes a 41% increase to double the cross section.

But since tip Velocity increases as well, doubling of performance occurs at around 20% increase in cross section instead of 41%. The effects are cumulative.

To test this you may even be able to use the same blades by mounting them on a wider hub. This should keep complexity down.

The alternative is to gear the existing blades 20% higher, because increase in velocity of blades increases pressure by the cube. But of course this risks failure of blades.

It’s at least worth a try on a test stand.
 

Doggzilla

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Pulled up the fantrainer info and it has about 1.85 feet of prop arc area for every 100hp. The prop is 3’ 11” with 650hp. That’s almost exactly 12 feet of arc area.

At 350 hp that means it should have at least 2.8 feet cross section with a single prop to get 6.5 feet of arc area.

For twin props it should be roughly 2.1 feet across each for 350hp.

Not including the changes to rpm required to keep the same tip speed and pressure.
 

Malish

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Have you thought about a turbo-diesel so it can run Jet-A?
Because Ducted fan in our airplane turned at hi RPM(7000+), it's need the engine that could operate at hi RPM without big problems - diesel engines can't do that. That is why we use hi performance GM LS6 gasoline engine - it's can easily run at 5000 RPM(for T/O power) and 4000 RPM at cruise.
 

Malish

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Pulled up the fantrainer info and it has about 1.85 feet of prop arc area for every 100hp. The prop is 3’ 11” with 650hp. That’s almost exactly 12 feet of arc area.

At 350 hp that means it should have at least 2.8 feet cross section with a single prop to get 6.5 feet of arc area.

For twin props it should be roughly 2.1 feet across each for 350hp.

Not including the changes to rpm required to keep the same tip speed and pressure.
That what we have in our airplane fan's - 69cm across! And this fan tip speed stay just below speed of sound at 7100 fan RPM(at 5000 engine RPM).
 
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Doggzilla

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The Audi 350hp diesel on the Raptor would likely work. Not much heavier than a standard Lycoming.

Would make a good luxury option, as it would increase the range 30% and also probably add 100+ knots due to high altitude performance. Maybe more.

The diesel 172 gained 30% range but kept the same top speed even with a 17% reduction in power, because it can hold that power as altitude increases. The performance increase for turbo diesels can be surprising.

Probably the easiest major upgrade, because no modification to blades or ducts would be required.
 

pictsidhe

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The Audi V6 in the Raptor is a 230hp engine. In his infinite wisdom, Peter Muller has decided that it will be just fine putting out 350hp for hour after hour in a plane. Not everyone agrees with him.
Saying that, I thibk BMW makes a suitable TDi that won't need running way past its design power. Audi will be playing catch up, if they haven't already.
Doesn't the Dreamer have a gearbox that could be used to increase revs?
 

Doggzilla

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It’s 286hp limited by emissions and not engine performance. The emissions system has an over temp limiter to protect the platinum catalysts in the exhaust after treatment system. Aircraft do not have this issue.

And no, it will not be running at a full 350 hp for hours, that is the takeoff power.

It’s okay for you guys to write constructive criticism, but just flinging mud constantly does not help.
 

Speedboat100

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Because Ducted fan in our airplane turned at hi RPM(7000+), it's need the engine that could operate at hi RPM without big problems - diesel engines can't do that. That is why we use hi performance GM LS6 gasoline engine - it's can easily run at 5000 RPM(for T/O power) and 4000 RPM at cruise.
That is my dream engine !
 
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