Hybrid propulsion aircraft: Fireball, Darkshark, B-36D, XP-81, ...

ysi_maniac · September 02, 2016, 07:05:56 PM

First a question. In the case that jet engine is just a booster, in cruise flight, is/are the jet/s powered on in slow motion or powered off?
In last case, would it be convinient to fair over intake to avoid drag?

TomZ · September 03, 2016, 01:00:37 AM

On the wiki page on the FR-1 Fireball it says this: "The turbojet's air intakes were moved from the wing roots to the fuselage in front of the wing; they were covered by electrically powered doors to lessen drag when the aircraft was flying only on its piston engine"
So the jet engine was off.

TomZ

NARSES2 · September 03, 2016, 05:18:59 AM

Interesting Tom. I've built a couple of Fireballs and didn't know that. Live and learn

kitnut617 · September 03, 2016, 05:55:43 AM

Actually, the fuselage inlets were only trialed on two experimental versions of the Fireball (actually one flying variant and one mockup). First use was in the XFR-4, which had a lengthened rear fuselage to house the Westinghouse J34 with 4200 lb of thrust. The inlets had sliding doors to seal the intake when the jet wasn't used. The installation was flight tested and found to be 100mph faster than the standard FR-1(which was only 419mph). The second use was on the variant specifically aimed at the USAF and was designated the XF2R-2. However only one mockup was built and it never flew.

Referenced from the Squadron/Signal Mini in Action book, FR-1 Fireball

sandiego89 · September 03, 2016, 08:35:44 AM

The booster jets on the B-36 (nearly identical to those on the B-47) had doors that would be closed and keep out nearly all air from entering the intake when the jets were off. A small amount of air was allowed to enter to keep the compressor slowly turning so the engine did not freeze up, but the engines were "off". The P2-V (and others) had spring loaded intake doors that would close off the intake when the engine was off.

These jets had a high fuel consumption and would only be used for brief periods. Perhaps surprisingly the jets used the same fuel as the pistons.

Open and closed intakes on the B-36, from "Magnesium Overcast" page 133.

-Dave

ysi_maniac · September 03, 2016, 10:56:33 AM

Thanks for your help!

ysi_maniac · September 03, 2016, 10:38:56 PM

And, in the other hand, is it possible that the jet is not a booster? I mean, is it possible to balance thrust between propelers and jets to reach best performance?

sandiego89 · September 04, 2016, 07:25:44 AM

Quote from: ysi_maniac on September 03, 2016, 10:38:56 PM
And, in the other hand, is it possible that the jet is not a booster? I mean, is it possible to balance thrust between propelers and jets to reach best performance?

Not sure what you mean by performance, but I don't think so. The booster jets were needed on several aircraft when high overall thrust was needed such as: takeoff, climb, reaching max altitude, dash speed, to allow piston tankers to pass fuel to faster jet aircraft, enhanced safety and thrust for go-arounds and in firefighting operations.... So the jets enhanced performance, and some aircraft really needed the extra thrust especially when operating at higher weights. The jets were not needed in cruise.

But the extra thrust came at the cost of fuel consumption. Massive difference in fuel burn rates.

I can not think of a blended arrangement where both pistons and jets were kept running all the time in normal operations. Perhaps in some C-119 firebombing mission close to home base is the most likely example I could think off.

ysi_maniac · September 04, 2016, 08:37:25 PM

Quote from: sandiego89 on September 04, 2016, 07:25:44 AM
Quote from: ysi_maniac on September 03, 2016, 10:38:56 PM
And, in the other hand, is it possible that the jet is not a booster? I mean, is it possible to balance thrust between propelers and jets to reach best performance?

Not sure what you mean by performance, ...

Efficiency.

zenrat · September 05, 2016, 02:10:02 AM

Quote from: sandiego89 on September 04, 2016, 07:25:44 AM
...I can not think of a blended arrangement where both pistons and jets were kept running all the time in normal operations...

The only one I can think of is the B-25 in Sucker Punch which has one piston engine and one jet.
But then anything is possible with CGI.

Dizzyfugu · September 05, 2016, 05:51:35 AM

I am also not aware of any aircraft/project that ran with both means of propulsion all the time. The Martin P4M Mercator and the North American AJ Savage come to my mind, but both used the jet engines as boosters. Turboprops would be the combination/compromise of both concepts, though, and the Tu-95 has been highly successful with this technology - until today.
There were turboprop/jet combos, too, though, like the XP-81 or the Breguet Vultur - but again the jet was only a booster for start and combat.

Anyway, efficiency is the key. Early jet engines were thirsty and did not offer much power, while the pistion engine/propeller offered more range and reliability.