Hawker-Siddeley HS-681

[Archibald]

Time to think about this victim of the 1965 Wilsonstorm (along the P.1154 and TSR.2...).
If I understand well... this plane was a classic cargo (high mounted wings, podded engines and the like). The brilliant idea had been to mount four Harrier engines to allow STOL or VTOL. (well, if I'm wrong, correct me, there's very few data available on the web about the HS-681!!).
This was far less complicated than the Do-31 for example!
Ideas are coming...RAF but also RN (it depend from the landing and take off distances). Country X and it's VTOL air force...USAF...
I was thinking : the X-32 used the F-135 engine, combined with a nozzle system similar to the Harrier.  What about a modernised HS-681 with those F-135 ? much more power...  

[Hobbes]

Yes, the plan was to use Pegasus engines. The models I've seen at Coventry show engine nacelles with two nozzles in the sides, and a single normal-looking exhaust at the rear. I assume this rear nozzle would have to rotate as well (using a system similar to the Yak-141 and JSF).  

[Geoff_B]

Air Enthusiast No.124 from July 2006 has a nice article on the HS681 written by Harrier off this website.

The HS-681 used its vectored thrust for STOL performance with teh vectoring nozzels intended to be used to deflect thrust to enable short landings and assit take off. The VTOL option required the addition podded banks of lift engines mounted on the wings, but the RAF decided to drop that part of the requirement before the design matured.

Think RR Medways were preferred over the Bristol Pegasus.

G

[jcf]

The brilliant idea had been to mount four Harrier engines to allow STOL or VTOL.

Daft is more like it, the useful load in VTOL mode of even a dedicated airframe like the Harrier is a fraction of the load the aircraft can carry in a more conventional take-off mode. VTOL capability in a large transport aircraft is pointless unless you are carrying a high-volume but light-weight cargo...say blocks of foam or ping-pong balls.  The fuel requirements to get the thing off the ground vertically would be grotesque. Even in STOL mode the fuel burn would be ridiculous...better to use smart and efficient aerodynamic design as opposed to inefficient brute-force jet lift.
Also while both JSF prototypes demonstrated a VTOL capability the production aircraft will actually be used as a STOVL aircraft...you could make an argument I suppose for a STOVL transport, but not for one the size of the HS681 and it wouldn't be a "heavy-lifter".

My thoughts on the HS 681 would be to redesign it into a really tough forward zone STOL aircraft. Keep the Pegasus (or Medway) engines, ditch the cold section swivelling nozzles, use a single limited-angle pitch-vectoring exhaust nozzle, and full-span blown flaps and ailerons fed by bleed air from the engine cold sections to get good short field performance. Yeah, I know the bleed air setup complicates the engine nacelles and pylons, but you could get an aerodynamically clean instalaltion with minimal moving parts.

Cheers, Jon  

[Lawman]

I have to agree, making a V/STOL transport is not as useful as at first look. In practice, a better option might have been to fit a rear loading ramp to the De Havilland Canada DHC-7, and perhaps some more efficient engines and high lift devices. For forward operations, I would stick with turboprops, probably with metal props (though heavier, they are much more resistant to damage on the ground). With suitable effort, even the excellent STOL performance of the dash-7 could be improved, and it would deliver better results than trying to shoehorn Pegasus engines into a transport, which would be very vulnerable to ground fire.  

[Archibald]

Wanted : some numbers about this aircraft (performances, dimensions).
Other question : what stage reached the project before its cancellation ? Was there an unfinished prototype ?  

[kitnut617]

This is it:

[rallymodeller]

Hmm. Rings a bell:

[Mossie]

Here you go Archie, HS.681, taken from Project Cancelled:


HS.681 with Medway Engines


HS.681 with four RB.175 Medways & eighteen (9 in each pod) RB.162 lift engines.


HS.681 with comet wing.  Last gasp to push the concept.


Rolls Royce RB.175, one option for the pod mounted lift engines

Project Cancelled lists some of the figures for OR.351 (of which the 681 was the main contender) & some for the 681 itself:

Length: 101ft
Height: 37ft 3in
Wingspan: 123ft 4in
Take off weight: 171,000lb (OR.351)
Range: 2000nm normal, 1000nm at max weight, 4200nm ferry with 1000lb payload. (OR.351)
Short take off distance: 50ft to 1,700ft (OR.351)
Cruise speed: 325knts
Passenger capacity: 60 fully equipped Paratroops
Medway engine thrust x4: 3100lb dry thrust, 17,300lb STOL thrust (initial), 18,290lb developed
Medway fuel consumption: 0.683 lb/hr/lb thrust
RB.175 or RB.162 engine thrust x18: 11,000lb
Total available STOL thrust:  267,000lb initial, 271,160lb developed.    :dum:

Compare this to the C-17, 168,800lb thrust available for a maximum take off weight of 580,000lb!  The HS.681 would have a huge footprint & would probably make a huge hole in the ground (remember what the Yak-141 did to the Farnborough runway).  You'd need special surfaces, limiting it to certain runways, which would take away the reason for it's existance.  Nice idea, just probably would never have made it past prototype stage.

Simon.

[Hobbes]

remember what the Yak-141 did to the Farnborough runway

I didn't remember, but from what I can find on the internet, the Yak used its afterburner to do a vertical takeoff. The HS-681 didn't have afterburners.  

[Mossie]

Thats a good point Hobbes.  The extra heat would have melted the tarmac, combined with the pressure.  Maybe not so much of a problem after all.

Simon.

[Archibald]

Thank you very much Simon-mossie!!
I understand there was various variants, but I'm more interested by the one of the "in flight picture" (without the pods and 18 lift-jets).
If I understand well, they had applied the Pegasus technology to the Medway, to achieve STOL take off? (and they added the 18 lift engines to obtain VTOL take off?)

[Mossie]

That sounds right!  The Medway was similiar in concept to the Pegasus, & the lift engine pods could be removed if VTOL was not required.

The Pegasus may well have powered the 681, there was a huge battle between Bristol & Rolls Royce as to who was to provide the powerplant.  It was by no means definatley the Medway, although Rolls Royce, who had there noses pushed out with Pegasus & Harrier pushed very strongly for this engine & it may well have ended up being a political decision.

Simon.

[kitnut617]

I don't think the 681 was ever designed to be a pure VTOL, it was for STOL.  Much like how Harriers do on short take-offs.  Nozzles pointed back for the initial thrust then nozzles swing down at a certain air (ground) speed

Edit: Oops,  Thorvic has already said this

This would make it tactically more advantageous as any short space would do.

Robert

[Geoff_B]

Harrier is the best one to discuss the HS-681 as he did the recent Air Enthusiast article and therefore has most of the relevant fact regarding the development and exolution of the 681 design. The Final version of the design being like the CGI artwork used in the article rather than the manufactures models that we know.

Archi might be interested in Armstrongs propossed testbed which used a Nord Nortalus fitted with the podded lift engines as illustrated in Project Cancelled