Forum: Technology

Although in most cases thrust being ported to various points on the aircraft- e.g. wingtips or midwing ala F-35B, would not generate anywhere near the moment that aerodynamic surfaces would, there are a few specialized cases where they are very helpful.

The first and most obvious is at low speeds where the aerodynamic surfaces can't impart much force. That's why they're on the Harrier and F-35B, but this could also apply in low speed maneuvering. The second is at high alpha where the flow over the regular surfaces is drastically reduced or when the surface is totally blanked. If you look at the design of the YF-23 you'll see that Northrop/MDD went to great pains and clever design to compensate for this phenomena. The third case is post stall maneuvering, where the force of the control surface could be reduced or in some cases actually result in the reverse of control input.

The biggest gain would be in roll, pitch and to a lesser extent yaw being easier to handle with conventional tail thrust vectoring. But the bottom line question that always applies remains: Would incorporating such a system be worth the complexity, weight and cost? Except for the case of a STOVL aircraft , the answer seems to be "no".

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aaam wrote:

Although in most cases thrust being ported to various points on the aircraft- e.g. wingtips or midwing ala F-35B, would not generate anywhere near the moment that aerodynamic surfaces would, there are a few specialized cases where they are very helpful.

The first and most obvious is at low speeds where the aerodynamic surfaces can't impart much force. That's why they're on the Harrier and F-35B, but this could also apply in low speed maneuvering. The second is at high alpha where the flow over the regular surfaces is drastically reduced or when the surface is totally blanked. If you look at the design of the YF-23 you'll see that Northrop/MDD went to great pains and clever design to compensate for this phenomena. The third case is post stall maneuvering, where the force of the control surface could be reduced or in some cases actually result in the reverse of control input.

The biggest gain would be in roll, pitch and to a lesser extent yaw being easier to handle with conventional tail thrust vectoring. But the bottom line question that always applies remains: Would incorporating such a system be worth the complexity, weight and cost? Except for the case of a STOVL aircraft , the answer seems to be "no".

You forget to mention operation at high altitude where air is thin.

Neno wrote:

You forget to mention operation at high altitude where air is thin.

Actually, I didn't. It is true that the air is thin at high altitudes, but that is partly compensated for by the fact that the plane is flying much faster, like SR-71, F-22, etc. which produces more airflow, or has a large span like the U-2 and Global Hawk with proportionally large control surfaces.

More importantly, though, at high altitude, that thin air also means that the engine is producing only a fraction of its sea level thrust (it also means less drag, so that less thrust is enough as long as you stay in envelope). Less thrust means that those diverted thrust ports are also putting out proportionally less force to maneuver the aircraft, so they're penalized just as much.

As an aside, that's why every so often the concept of a rocket added to a fighter is proposed. Examples include the F8U III, F-15 and F-20, as well as the British SR 177, among others. The idea was that even though the rocket only produced far less power than the jet engines on the aircraft, unlike them it maintained the same thrust regardless of altitude. This meant that at high altitude the rocket was putting out as much thrust as the jets were (in that thin air). it was like getting an additional engine at much less weight. Thing was, the benefit really came into its own only at high altitudes, and wasn't considered worth the complexity.

Anyway, back to the topic and hand, again, it just isn't considered worth the cost in complexity, weight and money.

I can see thrust ports on the nose, and tail providing excellent lateral nose control. That's to say IF you could get sufficient thrust to overcome the mass/drag of the aircraft in a timely enough manner.

The shape of the aircraft, and its center of gravity would also matter. Just for a what if, say you could turn the aircraft at a 90 degree angle in a second , or so, in level flight at high speed, momentum would be carrying your plane sideways.

If the shape of the plane had too much drag from the angle it's now moving, the braking effect from the fuselage, and vertical control surfaces of a conventional fighter would be a killer.

If the shape allowed you to slice through the air at that angle while thrust from the rear of the craft got you moving forward again, then it could be useful. You could make sharper turns at high speed without banking, for example. You would be sliding in an arc.

We probably couldn't do anything like that without great expense, but I can dream.

Last edited by aaam on Sep 16, 2011 - 04:29 AM; edited 1 time in total

epicvalor wrote:

I can see thrust ports on the nose, and tail providing excellent lateral nose control. That's to say IF you could get sufficient thrust to overcome the mass/drag of the aircraft in a timely enough manner.

The shape of the aircraft, and its center of gravity would also matter. Just for a what if, say you could turn the aircraft at a 90 degree angle in a second , or so, in level flight at high speed, momentum would be carrying your plane sideways.

If the shape of the plane had too much drag from the angle it's now moving, the braking effect from the fuselage, and vertical control surfaces of a conventional fighter would be a killer.

If the shape allowed you to slice through the air at that angle while thrust from the rear of the craft got you moving forward again, then it could be useful. You could make sharper turns at high speed without banking, for example. You would be sliding in an arc.

We probably couldn't do anything like that without great expense, but I can dream.

With that much yaw, you'd run into two problems. First, controllability, and second, the loss of energy from the kind of drag you mention.

In any case, this points up the benefit. vs. cost problem. Let's say you solve all that and you start your really tight flat turn. The missile tracking you makes a 40+ g turn, cuts the corner and nails you anyway. Or, let's say you want to go offensive on that bandit sitting off your starboard wing. While you're slewing your nose around for your off axis lockup and shot, he turns his head and looks at you while wearing his HMS and hoses off a missile which makes a 60+ g turn off the rail and still nails you. .

It's all a question of what's your gain vs what's your pain.

epicvalor - i personally hear where you're trying to go on this. Being the 'mad scientist' too, I must admit I've also pondered this 'airframe thrust porting' possibility, or even some form of JTOL-assisted thrusting for emergency ACM, etc.

Yet, the actual 'port-thrusting' concept we're talking about would probably be best at very high altitudes, where the air is thinner - e.g. > 60,000'. So perhaps indeed, a 7th gen aircraft operating at over 75k' would potentially benefit from this design spec.

In the meanwhile, perhaps one could better ponder a conventional mix of center-of-lift, control-surface performance, and TVC manipulation as the optimal design combination?

geogen wrote:

epicvalor - i personally hear where you're trying to go on this. Being the 'mad scientist' too, I must admit I've also pondered this 'airframe thrust porting' possibility, or even some form of JTOL-assisted thrusting for emergency ACM, etc.

Yet, the actual 'port-thrusting' concept we're talking about would probably be best at very high altitudes, where the air is thinner - e.g. > 60,000'. So perhaps indeed, a 7th gen aircraft operating at over 75k' would potentially benefit from this design spec.

In the meanwhile, perhaps one could better ponder a conventional mix of center-of-lift, control-surface performance, and TVC manipulation as the optimal design combination?

Now, the question comes up---why would you want to do this (unless you're assuming some form of non-trainable directed energy weapon)?

Sorry... why would someone want to do what? Some form of futuristic 7th gen design study over the next-decade? Or an evaluation of an improved control-surface performing, plus TVC-manipulating design in the mid-term??

I guess it really doesn't matter much with the advanced targeting of the F-22, but it's the thought of being able to do something at mach+ that a conventional fighter can't do that is fun.

Would it be useful? I suppose if you were trailing some other plane that could out turn you in a dogfight while banking, if you were close enough, and all you had left were guns, maybe it would let you get a shot if you can't get inside his turn. That's a lot of what-ifs, heh.

Heck, I don't know. I've never flown a real fighter. It just seemed like more ways you can use to point the nose of your plane, and at harder angles, can't be a bad thing.

epicvalor wrote:

I guess it really doesn't matter much with the advanced targeting of the F-22, but it's the thought of being able to do something at mach+ that a conventional fighter can't do that is fun.

Would it be useful? I suppose if you were trailing some other plane that could out turn you in a dogfight while banking, if you were close enough, and all you had left were guns, maybe it would let you get a shot if you can't get inside his turn. That's a lot of what-ifs, heh.

Heck, I don't know. I've never flown a real fighter. It just seemed like more ways you can use to point the nose of your plane, and at harder angles, can't be a bad thing.

Or rather than worrying about those sorts of issues, you could just go the route that the F-35 has, with the ability to engage targets within a 360 deg sphere around the plane, along with reasonably high agility too.

I guess so, unless you believe some of the reports of exercises with F-22/35 going against other planes in a visual fight, and losing badly. Whether or not those events really happened, I couldn't tell you.

I really hope that's just false internet BS. Else, it's a sad statement for US air superiority.

Sorry, which reports (that have access to the F-22/35 specs) have concluded that they lost "badly"?

Last edited by aaam on Sep 17, 2011 - 01:40 AM; edited 2 times in total

geogen wrote:

Sorry... why would someone want to do what? Some form of futuristic 7th gen design study over the next-decade? Or an evaluation of an improved control-surface performing, plus TVC-manipulating design in the mid-term??

Fly sideways, or at least with extreme yaw, at 75,000 ft..

Last edited by epicvalor on Sep 16, 2011 - 10:45 PM; edited 1 time in total

It was some aussie, or british news article I read a couple of years ago, maybe less.

Of course, after just now reading some of the euro forums trying to find that article, the euro fighter is the best in the world according to them. They had lots of quotes from supposed articles, but only one link to an actual news organization, the BBC. Which really didn't say much.

http://news.bbc.co.uk/2/hi/business/1818077.stm near the bottom

I'll take that as more internet fodder from those who need to feel better about themselves.

Why would you want to fly sideways at 75k?

Forum: Technology

General design question