Actually, I have a buddy in Denver who has been working for a few years on
such an aircraft but haven't heard from him for a while and he may have
given up. He's a Toyota master mechanic and was using a 200 hp MR-2 engine
driving a 28 inch dia fan. He elected to build an aircraft similar in
appearance to an F-5. His drive-shaft extends via a fairing into the
portion of the duct where the twin intakes meet to drive the rotor. He
built a full-size test item of the propulsion section and was finishing up
the engine mods the last time we talked.
I agree with Paul that the cooling
issues almost drive this type of aircraft. You really need a very high
power density engine which pretty much demands liquid cooling. My buddy
went with a pusher approach and a mid-engine. I haven't seen the details of
how he would cool it, but I suspect he's thought it out carefully. He's
aiming at about 250 mph and driving the fan about 7000 rpm. There are
bearing issues at these speeds and I don't know if he's had the numbers run
regarding vibration and resonance on the short drive shaft.
There is a
tendency for designers to assume that direct-drive is the way to go.
Interestingly, the majority of the "successful" ducted fan designs tend to
incorporate a gear box. They've learned that very high speed fans are
noisy. Use of a stator section can help with this. On the other hand, RFB
found at one point that their gearbox noise had become really annoying and
had to change boxes. Typical aircraft development glitches. Inlets are a
topic providing hours of "fun". A ducted or shrouded system doesn't need
the same kind of protruding inlet as a turbojet or fan. In throttled back
or engine out conditions, these types of inlet are actually a detriment. A
simple flush inlet (NOT NACA) is perfectly feasible.
One of the major
misconceptions is that a ducted inlet system is limited to the 8 deg ramp
limit typical of turbojets. There is strong evidence that Raymer's
observation that pusher aircraft can have up to 30 deg of after body angle
into a propeller without separation problems is correct. It turns out this
seems to apply to ducted or shrouded fans. You don't have the boundary
layer issues with the ducted/shrouded systems typical of turbojets. Your
inlet does not have to be raised to get past the boundary layer into laminar
air. The ducted/shrouded systems cheerfully suck in the boundary layer.
I would be fascinated to talk to the guy from Raytheon about his project.
Haven't heard of it before.
George W>
George,
Your friend's project sounds intriguing. This is very much along the
lines of what I am proposing.
But I think the rotary is a much better choice than a piston engine. It
has a much higher power to weight ratio, is smoother, and is better
suited to turbocompounding.
Of course you could use a normally aspirated engine and aim for lower
altitudes and speed. A two-seater with an Rx-8 engine for example. I
think 250 mph top speed could be within reach, with easy cruise at about
200 mph.
This is the kind of plane you might even build at home, provided the
concept is proven in flight testing and all the details of the fan and
duct have been worked out. I bet it would be an instant hit.
Regards,
Gordon.
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