'Jet Deflection' for longer-range fast cruise as per Chris Gibson & Tony Buttler?

[Owens Z]

I was re-reading my copy of the fine book British Secret Projects: Hypersonics, Ramjets & Missiles by the noted Chris Gibson and Tony Buttler (Midland, 2007) recently. On pages 160 through 162, the authors discuss British (paper) studies from the 1960's about weight and drag reduction by jet deflection for a highly supersonic or hypersonic aircraft. That is, a high-speed aircraft with thrust vectoring nozzles (or thrust-vectoring paddles abaft the nozzle a la X-31) would vector the engine thrust somewhat downward for the entire trip; this added lift from the thrust allowing the aircraft to be designed with a substantially smaller wing, and therefore a lighter wing that gives less drag. Thus a jet-deflection aircraft would, the studies concluded, have a longer range than a conventional aircraft.

Previously the only times I have ever heard about jet deflection are for its brief use to enhance a fighter plane's agility in combat, or brief use to slow a plane's takeoff and landing speeds, to land safely on a very short runway (or vertically), so this was news to me. The authors discuss a Gloster Meteor successfully refitted in 1953 with deflection gear, and jet deflection for the unbuilt Saunders Roe P.177 and Westland M.148, but those were all to help land more slowly. I have of course heard about pivoting nacelles, vectoring nozzles, upper surface blowing, blown flaps, etc., to help landings and takeoffs, but never about continually deflected thrust to extend range in high-speed cruise.

I suspect that the Brits' math was off somewhere, and that the costs of a continually downward thrust would outweigh the benefits of a smaller, lighter, less-draggy wing (or, as the authors mention, a less-draggy wing incidence). To my knowledge, no vehicle has since been designed for jet deflection cruise. But have any of you heard of this interesting matter outside of the 2007 Gibson/Buttler book? What do you all think about jet deflection for long-range high-speed cruise being designed today from the start into, say, a new supersonic business jet? Or a hypersonic reconnaissance plane? Would it indeed result in longer range? Please comment.

 

[sienar]

Quick search on AIAA find this; https://arc.aiaa.org/doi/10.2514/3.43815

Which is about hypersonic thrust deflection for cruise.

 

[CJGibson]

Beat me to it.

Thanks

Chris

 

[red admiral]

There's also jet deflection on other slower aircraft, e.g. Jaguar has a permanent downward cant on the nozzles. There's usually a net benefit from what I've seen but it depends on a number of things e.g. trimming the aircraft over the range of speeds

 

[CJGibson]

It's really about marginal gains. If a component of lift comes from deflected thrust, the size of the wing can be reduced. Reduce the size of the wing and you reduce the weight of the airframe. Reduce the weight of the airframe, you reduce the amount of no doubt exotic materials required. Reduce the amount of exotic materials and you reduce costs and what kills aircraft programmes at the end of the day? Cost.

As Red Admiral says, mounting engines, or rather adjusting thrust lines is a common method applied to aircraft and off the top of my head I can think of two examples - Short Bros Tucano and Mil Mi-24 Hind.

Chris

PS to quote the RAF pilot in Masters of the Air, it's maths - there's more than one. I can't see the engineers at Bristol Siddeley, Hawker Siddeley and the RAE getting their sums wrong. Actually I can, but that's another story.

 

[Mike Pryce]

There's also jet deflection on other slower aircraft, e.g. Jaguar has a permanent downward cant on the nozzles. There's usually a net benefit from what I've seen but it depends on a number of things e.g. trimming the aircraft over the range of speeds

Phantom too. 11 degrees of permanent 'vectored thrust' at all times that gives 'free lift'. Similar to how the Harrier stays up when accelerating away from a hover.

 

[Scott Kenny]

PS to quote the RAF pilot in Masters of the Air, it's maths - there's more than one.

'Murrican English versus the Queen's. "Mathematics" gets shortened to "math" no s in the US.

I can't see the engineers at Bristol Siddeley, Hawker Siddeley and the RAE getting their sums wrong. Actually I can, but that's another story.

I mean, it happens. Dropped decimal places seems to be a real killer, witness the Sanger Silbervogel.

 

[Owens Z]

Thank you for the referral to this 1967 paper by Krase, "Thrust deflection for cruise." I am not allowed to read the paper online after the first page without paying a fee, but it sounds interesting, so I will look up the back issue of the Journal of Aircraft at the local public library. This paper must be roughly from the same time period as the British studies mentioned in the 2007 Gibson/Buttler book.

Setting jet thrust lines, and propeller shafts, and rocket thrust lines on the Space Shuttle, to be slightly off the axis of the fuselage has indeed been commonplace, to get the thrust vector to pass closer to the center of mass and/or for other reasons. (The McDonnell-Douglas Phantom was designed with a permanent 11-degree deflection? So great an angle is interesting, if true. I will read further.) But with respect, that is not what we are discussing in this thread. The 1960's paper studies promised significant gains in cruising range for highly supersonic and hypersonic aircraft by using deflected thrust. The line drawing on page 162 shows a jet deflection angle of 30 degrees for such a vehicle while it cruises (for an hour or more?). Let's stay on target here, people. A Phantom or Jaguar could be highly supersonic for only a few minutes. Now, I have seen fewer hypersonic aircraft than Mr Gibson has. But there have been many published studies of supersonic cruise aircraft since the 1960's. For example, the several serious supersonic business jet projects following the Sukhoi/Gulfstream design of 1989. To my recollection, none of these SSBJs mention deflected thrust, although the X-31 demonstrated this as an add-on to an existing jet engine thirty-four years ago, and although a lighter airframe with greater range would be a godsend. Why haven't the purported benefits of variable thrust deflection for high-speed cruise (not for combat agility, or STOL) ever been acted upon in the past half-century? One possible explanation, not the only one, is that the British studies and Professor Krase were mistaken.

 

[F119Doctor]

From what I am seeing in this thread is the idea of deflecting the thrust line downward for additional lift.

However, when an aircraft goes supersonic, the center of lift moves aft relative to the center of gravity, resulting in a nose down pitching moment that has to be countered by a tail down trim load on the horizontal stabilizer (or a nose up trim load on a canard), significantly increasing trim drag. Staticly unstable aircraft minimize this supersonic trim drag increase, but it still occurs.

With vectoring thrust at the aft end of the aircraft, a slight upward thrust deflection can balance the nose down moment and reduce trim drag with significantly reducing the forward thrust component.

 

[NUSNA_Moebius]

With vectoring thrust at the aft end of the aircraft, a slight upward thrust deflection can balance the nose down moment and reduce trim drag with significantly reducing the forward thrust component.

Sounds like in this case it’d be better to use an aft-centric CG relative to center of lift so you can deflect down and contribute a bit to lift instead. What’s the F-22’s case?

 

[F119Doctor]

Most modern fighter aircraft from the F-16 and beyond with fly by wire flight controls are statically unstable (CG aft of CL) or neutral stable (CG at CL) while subsonic, and become stable (CG forward of CL) when supersonic as the CL moves aft. The classic center of lift movement is from 1/4 of the wing chord subsonic to 1/2 of the wing chord supersonic. Swept / delta wings and body lift complicate the analysis, but the trend is the same. I don't know if anyone has moved the CG so far aft as to be neutral or unstable at supersonic speeds.

 

[red admiral]

With vectoring thrust at the aft end of the aircraft,

For these old high speed concepts then they are quite neat as the thrust gets vectored very close to the CG so the moment imparted will be small.

 

[Owens Z]

Quick search on AIAA find this; https://arc.aiaa.org/doi/10.2514/3.43815

Which is about hypersonic thrust deflection for cruise.

At my public library I retrieved the 1967 issue of Journal of Aircraft (Vol 4 No 2) that includes this article, "Thrust Deflection for Cruise" by WH Krase of The Rand Corporation in California, and I made a photocopy of the 2.5-page article for my reference. Until the very last paragraph, the article deals with hypersonic air-breathing vehicles, and the purported benefit to those of increased range (about a third more, so quite valuable if true) by continuous deflection downwards of the vehicle's jet thrust, thus allowing a smaller lifting surface of less weight and drag. Substantial downward deflection, but not as substantial as the 30 degrees shown in the line drawing on page 162 of the 2007 Gibson/Buttler book, which must be based on British studies from around that same time. WH Krase's text and graphs are obviously entirely theoretical, as presumably were the British studies. Mr Gibson has seen more hypersonic air-breathing vehicles than I have, so I am unable to follow up to determine whether thrust deflection for longer cruise range actually works. (I considered checking any thrust deflection built into the X-43 and X-51, but rejected that as inapplicable after recalling that those were under power for only a few seconds.)

After all the hypersonic info, the article's last paragraph reads, "Even at supersonic-transport cruise conditions, with L/D approximately equal to 8 and Fg/Fr approximately equal to 1.2, a thrust decrease and range increase of about 4% is possible with a deflection angle of 7.1 degrees. The increased altitude would also tend to decrease the sonic boom somewhat."

4% is a marginal gain, as Mr Gibson wrote above, but would nevertheless be useful for a supersonic business jet. Sienar, is it telling that your search could find no papers about this subject after the Krase article, which is older than I am? Is it telling that, to my knowledge, the SSBJs conceived from the 1989 Sukhoi/Gulfstream to the present, which would cruise at Mach 2 or 2.5 for an hour or more, and fight for anything to reduce their sonic boom footprint, pay no attention to thrust deflection for increased range? I think a reasonable conclusion is to mark this subject as "Unproven", at best.

 

[DWG]

4% is a marginal gain

Promise the airlines a 4% increase in range for the same fuel burn and you'll be trampled in the rush....

 

[Scott Kenny]

Promise the airlines a 4% increase in range for the same fuel burn and you'll be trampled in the rush....

Seriously.

Airlines are extremely sensitive to fuel burn.

 

[Owens Z]

Promise the airlines a 4% increase in range for the same fuel burn and you'll be trampled in the rush....

Seriously.

Airlines are extremely sensitive to fuel burn.

Your observation, while correct, is not relevant to this thread's subject. I had pointed out that if jet deflection for a high-supersonic-cruise aircraft worked even a little, then the resulting lighter airframe with greater range would be a godsend. The fact that, to my knowledge, none of the several supersonic business jet proposals from 1989 to the present consider jet deflection seems to speak for itself.

 

[DWG]

Your observation, while correct, is not relevant to this thread's subject.

What percentage of improvement in fuel burn is significant is directly relevant to whether diverted thrust should be pursued. However, as pointed out by Chris Gibson, the gain might instead be used to reduce wing area, total vehicle size or vehicle cost, so will not necessarily show in a particular figure.