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TheDrop
13th May 2002, 10:55
I don't know if I dreamt this or what, but suddenly it appeared to me as an idea to "take a twin turboprop and mount a jet engine in the tail". I am just a pilot, not an engineer, so maybe some clever head here can either tell me that is has already been done, or why it has never been tried.

I think the advantages of jet and turboprop engines could both be derived of such a mix, making it a more (fuel) efficient, economical and flexible aircraft.

For certification purposes it should be called a jet aircraft (with regard to legislative matters), or the most limiting of the two from case to case.

Advantages of the turboprop

Better braking capability in the air (discing the props instead of using spoilers)
Closer to "instant power" than a jet (go-around, windshear etc)
Better fuel performance at low levels
Shorter take-off and landing distances (but that is probably mainly due to wing construction of the TP)

Advantages of the jet

Better fuel economy on cruise at higher levels
Faster speed at higher levels
Higher service altitude

Advantages to the combination

Three engined - no ETOPS hazzle
Combined fuel economy and top speed/performance
Two cheaper turboprop units, yet having jet performance

Disadvantages

If the jet pushes the aircraft faster at higher speeds, would the props then be of any use, or just making drag and extra weight, or could the right prop with the right pitch still be effective enough to be of any use?

Well, just a thought ...

Alpine Flyer
13th May 2002, 12:06
This has been tried before but obviously wasn't a huge success. Most combinations used jet power for take-off augmentation and emergency power only.

The airplane used in ConAir (some US military transport) had jet engines (with closeable air intakes) in pylons under the wing and there was a bomber before that as well (B-36?)

A civil airplane using the concept was the Hustler (http://www.k12.nf.ca/sptech/aviation/passenger/GulfstreamAmericanHustler500.html) which probably fell victim to the 1980s crisis of general aviation.

As for the merits, there is probably too high a penalty for carrying around useless equipment most of the time, and a jet engine cold-soaked at altitude for several hours might also be hard to start (with heavy wear) as a go-around backup.

There seems to be some merit to the KISS principle and many of the obviously beneficial setups seem not to work in real life.

PorcoRosso
15th May 2002, 16:11
As in most hybrid project you will end up rather with a combination of all disadvantages than with good points.
As a matter of fact the C-123 was using the Jet to increase Take-off and climb perfs.
I think, as you suggested, the prop will generate more drag than thrust beyond a given speed... which will invariably increase your fuel consumption.

The Aircraft used in Conair (what a crappy movie !) was a C-123 Provider. You could have mentionned "Air America" as another famous operator . Who was flying Provider from during the 60's in Asia, til the mid 80's in South America, and probably in some other places I don't want to know about ....
Amazingly before being fitted with 2 turbojet, the C-123 was a classic piston twin . Therefore we can consider it was a succesfull hybrid (it's rarely the case in technology history )
What people don't know about the C-123 ( I know , I disgress ) is that, before being fitted with piston engine, it was fitted with ...nothing. Originally, it was designed as a glider !
The glider had so good flying characteristics that it was decided to turn it into an aircraft .

cwatters
15th May 2002, 17:56
Using jet engines to push aircraft around on the ground is also
inefficient - but aparently not as inefficient as carrying gas or
electric motors to drive the wheels directly.

ironbutt57
16th May 2002, 17:41
P2V Neptune another..C-119 a recip with a single jet on a pylon overhead....closest thing nowadays was boeing's idea few yrs back to out fit a large 777 with a "thruster APU" mmmm....

Weight and Balance
30th May 2002, 01:04
Many of the examples quoted were originally piston powered aircraft, that ran into gross weight creep and tropical operations. Hence the designers were desperate for more power, regardless of little details like efficiency.

The only other turboprop jet combos I can recall were the original Breguet Alize, which quickly dropped the jet, and the Japanese built Neptune that used T64s and small home grown jet engines (a fuel sellerís dream, no doubt).

One final rambling: the downfall of the Hustler was a combination of really bad low speed handling and the all-too-common North American phenomena of an airplane company run by people who were more interested in raising money that building airplanes or running companies, and who had no hesitations about leaving the poor wage slave engineers in the lurch (huff huff). Been there, done that.

Stan Evil
30th May 2002, 21:16
Just for completeness, the Avro Shackleton Mk 3 Phase 3 (?) had four RR Griffons and 2 Bristol Siddeley Vipers. The Vipers were in the back of the outboard nacelles, were just used on take off and burnt AVGAS. They used to burn out pretty quickly.

Ranger One
31st May 2002, 03:34
They may have burnt out pretty quick Mr. Evil, but I can think of one occasion when a Shack *just* made it back to nearest land with multiple dead Griffons, courtesy of a Viper that kept going for an inordinate amount of time... IIRC one of the larger issues was the fuel pump suffering from AVGAS, considerably less lubricative than the stuff it was designed to pump...

Now if we're going to get into boosters, what about the 'plastic fantastic' (AKA RB 162)

R1

Metro man
1st Jun 2002, 10:03
The Antanov 26 (Russian F27) has a turbo jet mounted at the rear of one of it's engines ,which is used as an APU and for additional thrust on take off.It can even taxi with it ! Interesting to watch them moving around with both props stationary.However it is extreamly noisy ,worse than a non hush kitted F28.