Harrier and Sea Harrier

[dy031101]

Just spotted this over on secret projects.

What if, to avoid inflicting undue vibrations upon the rear gunner, we use two smaller engines in pods attached to the underside of the wings instead of one big engine in the fuselage?

Hey, maybe the fuselage space freed by doing this could be used for more fuel or even an internal bomb bay......

And if it sounds practical, would the under-fuselage lift improvement devices still be placed the same way?

[rickshaw]

And when one engine fails?

[dy031101]

And when one engine fails?

Soviet/Russian STOVL aircraft were designed to automatically eject the crew in case of any engine failure...... of course, I don't know the magnitude of the problem compared to that of a Harrier when its single engine fails......

[rickshaw]

And when one engine fails?

Soviet/Russian STOVL aircraft were designed to automatically eject the crew in case of any engine failure...... of course, I don't know the magnitude of the problem compared to that of a Harrier when its single engine fails......

A single engine out on a multi-engine STOVL aircraft in the hover invariably results in the automatic loss of the aircraft.  As you note, the Yak series automatically ejected the pilot.  They had a horrendously high loss rate as a  consequence - any slight loss of power and BOOM!  Out they went.  Problem was the asymmetrical thrust.  With multiple engines, its much more likely than _one_ engine.  With a single engine design, you'll still lose the aircraft if the engine fails _but_ you won't suffer the problems of asymmetrical thrust.  Also, single engines tend to be more reliable than multiples.

[PR19_Kit]

Wasn't there a version of the P1154 with two engines, in which the rear exhausts crossed-over each other?

The idea being that if one engine died the thrust wasn't asymmetrical even though the overall thrust was halved. Quite how they planned to engineer it beats me, the large diameter hot plumbing at the rear of the fuselage would have been mind-boggling!

[Weaver]

Wasn't there a version of the P1154 with two engines, in which the rear exhausts crossed-over each other?

The idea being that if one engine died the thrust wasn't asymmetrical even though the overall thrust was halved. Quite how they planned to engineer it beats me, the large diameter hot plumbing at the rear of the fuselage would have been mind-boggling!

Yes, that was Rolls-Royce "spoiler" offering (designed to queer the pitch for Bristols) where they offered to supply handed Speys with cross-over ducting within 18 months from go-ahead. I don't know what was more unbelievable: that proposal or the fact that the RN took it seriously.... RR may go on about how proud they are of the Pegasus these days, but back before they absorbed Bristol Engines, they had every reason to kick it, since a Mirage IIIV buy would have nine Rolls-Royce engines per aircraft  , wheras a Harrier buy would have none.

In all fairness though, the cross-over ducting proposal was probably practical if you could find the space for it (and two Spey jetpipes are rather more compact than one BS.100 one). If the aircraft had lost one engine in the hover, it would have landed heavily, but still on it's wheels because it wouldn't have lost attitude control. Hard to see how you'd do it with engines in pods under the wings (as was suggested) though.

[rickshaw]

Wasn't there a version of the P1154 with two engines, in which the rear exhausts crossed-over each other?

The idea being that if one engine died the thrust wasn't asymmetrical even though the overall thrust was halved. Quite how they planned to engineer it beats me, the large diameter hot plumbing at the rear of the fuselage would have been mind-boggling!

I read the history of RR once where it was mentioned.  It had a diagram, which had pipes going up and over one engine to the exhausts on that side and pipes going under the engine on that to the exhausts on the original side.  It could have been done but I don't think anybody sane would have wanted to fly in it.  It would have weighed a ton and would have flown like a dog in all likelihood with loads of thrust lost in the ducting.

[Weaver]

And when one engine fails?

Soviet/Russian STOVL aircraft were designed to automatically eject the crew in case of any engine failure...... of course, I don't know the magnitude of the problem compared to that of a Harrier when its single engine fails......

A single engine out on a multi-engine STOVL aircraft in the hover invariably results in the automatic loss of the aircraft.  As you note, the Yak series automatically ejected the pilot.  They had a horrendously high loss rate as a  consequence - any slight loss of power and BOOM!  Out they went.  Problem was the asymmetrical thrust.  With multiple engines, its much more likely than _one_ engine.  With a single engine design, you'll still lose the aircraft if the engine fails _but_ you won't suffer the problems of asymmetrical thrust.  Also, single engines tend to be more reliable than multiples.

Indeed. Multiple engines bering multiple sets of failure probabilities to the party, but on a conventional aircraft they also bring redundancy, and therefore safety, since the aircraft can continue to fly if one of those probabilities becomes actual. The same is not true for a hovering VTOL aircraft though. Thrust is so critical during hover than the loss of any engine will cause the aircraft to rapidly descend and hit the ground, but at least if that thrust loss is evenly distributed, then it will land on it's wheels. If the thrust loss is asymetric though, it'll land on some other part of it's structure, and that's never going to end well. A Harrier that loses all it's hovering thrust has a pretty good chance of surviving the experience, but a Forger that loses just one lift engine will hit the ground nose-first with dire consequences. Thus the multiple engines don't add any safety (in fact, they make it worse), but they still bring their collected failure probabilities to the party, thus making that worse accident more likely!

In all fairness to Yak, Forgers may have had a horrendous loss rate but they had a zero fatality rate, because that automatic ejection system worked really well. It sensed height, roll and pitch, and their respective rates of change, together with engine behaviour, and made a pretty sophisticated calculation as to whether things were going well or not. In spite of it's overall better loss-rate, there are some Harrier pilots who wouldn't have died had such a system been fitted to their aircraft.

[GTX]

There are ways to get around the problem of engine cutout with multi-engine VTOL.  Here's one proposal:



Note the use of a Pegasus with only two nozzles - something people often forget is possible:



Regards,

Greg

[GTX]

Here's one for the experts out there - what is it (not a Harrier)?



Regards,

Greg

[PR19_Kit]

Doesn't look Hawker-ish at all, and the fin has a certain French air about it? Does it come from the Dassault stable?

[GTX]

Does it come from the Dassault stable?

Maybe...me thinks you might know already .

Regards,

Greg

[GTX]

Dassault Cavalier?

You lot are too good!  It is the Dassault MD.610 "Cavalier" which planned on using the engine like the Harrier:



You can even get a model of it ...if you want to play with Unicraft.

Or one could kitbash a Harrier into one..maybe with the tail from an A4 Skyhawk.

Regards,

Greg

[GTX]

And I did offer a kitbash alternative(probably the better choice)....a small challenge for you!

Regards,

Greg

[Weaver]

It doesn't even look like a goodfit in Unicraft's own photos......

Nice find Greg!

Note the use of a Pegasus with only two nozzles - something people often forget is possible:

One of my ideas is for a Harrier-style aircraft with two such engine side-by-side, with the nozzles in the middle. The cold pipes would be angled inward and downwards and meet in an "oval box" shaped nozzle on the lowe centreline, while the hot pipes would go inward and upwards to feed a similar hot nozzle on the upper centreline. The structure aft of that would be twin booms behind each engine, while the structure between the engines would be a wedge-shaped fuel tank. The nozzles wouldn't mix flow from the two engines (too many cross-flow issues in the event of one failing)  but the left and right nozzles would be so close together that if one engine did fail, the asymeteric lift would be within the limits of the roll-control RCS system to deal with it.