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#3882236 - 12/23/13 10:54 AM flight physics: sustained turns and corner speed  
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I never had the most encompassing education in physics sadly but the following questions bugged my mind for a little while.

Ok, there's the concept of corner speed whixh should exist for any given airframe (f16, f15, spitfire, mustang).
When youre talking about the ww2 era prop aircraft though, the conversation goes to the direction that the corner speed basically doesnt matter, because when these spitfires and mustangs are flying corner speed turns, they would be unable to turn sustainably, at corner speed.
So by definition they would be making an instantenious turn as opposed to sustained turn. This would indeed be disasterous for a turning fighter to be doing it like this (you would lose altitude when making the turns).

Why would ww2 aircraft be unable to sustain corner speed in turns? I remember that there was one equation which described the turning perfformance of a given aircraft, at some given speed, at some given g-load.

But from what i remember the problem with prop fighters in this equation was that they were incapable of pulling enough Gs, to turn at their own respective corner speed. Basically in other words, they weree also incapable of keeping airspeed high enough during those turns at max Gs.

Does a modern fighter jet like F-15 or F-22 have enoguh power though? Enough power so rhat in theory, if the pilot wanted to, he could keep making horizontal sustained turns (without altitude loss), at the corner speed of an F-15 /F-22, until your jet fuel runs out of the aircraft?

I realize that it's a silly question, but what is the crucial difference, regarding turns at corner speed, in a comparison between let's say p51 mustang and F-15.

Forgive me the spelling errors im writing this on my phone in the canary islands on vacation. Gonna go play golf soon :-)

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#3882310 - 12/23/13 01:42 PM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Specific power for most propeller driven aircraft is only sufficient for a relatively shallow climb (under 30 degrees for most cases for best rate). Some of the more powerful jets are capable of climbing vertically, and even accelerating during the climb at low altitudes.

While the propeller driven aircraft have better wingloadings in most cases, and propulsion systems favouring lower speed flight (thrust increases with reducing speed), this isn't sufficient to make up for the overall lower power, nor does it help at all with sustaining speed in high-speed, high drag conditions such as turns at higher than corner speed.

Modern jets have poorer relative performance at slow speed (thrust nearly constant with changes in velocity) so their low speed performance is mushier and less responsive than a 'similar' propeller aircraft, but their performance at or above a typical corner speed is much superior, and maximum speeds tend to be significantly higher.

Constant speed/variable pitch propellers do increase the useful range of speeds for maximum power from the propeller, allowing it to approximate the ideal conditions, but the fixed pitch propellers of earlier aircraft have much more restrictive rpm/speed combinations and tend to lose performance away from their ideal speed/power settings.

Corner speed and instantaneous performance is vitally important for defensive performance, and for pressing for a snapshot, but in many cases cannot be sustained. It is indeed sustained performance that dictates the frequency of offensive positions and possible shot opportunities rather than the peak turn capacity. Never hurts to be better at both though...

#3882330 - 12/23/13 02:01 PM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Originally Posted By: Laurwin

Why would ww2 aircraft be unable to sustain corner speed in turns?


The concept of corner speed (or corner velocity) is often misunderstood. The definition is simple. For a given altitude, weight and configuration, all aircraft have a limiting g load. Corner speed is the minimum speed at which the limiting g can be reached (not sustained, just reached).

Back pressure on the stick produces g. This also produces an increase in lift due to increased angle of attack (AOA). But there is a limit to AOA, and that is called the stall.

For a given altitude, speed, weight, and flap setting, an aircraft is capable of a certain g loading before the AOA reaches the stall point. Starting from a slow speed, g loading capability increases as speed increases...but only up to a point. And that point is when the speed produces a g loading that is equal to the max allowable (sometimes called placard g). That is the max permissible g. Going faster may allow more g to be pulled but at the expense of breaking the airplane.

OK...the lift that pulling on the pole produces also produces an increase in drag...more g, more drag. The engine has to produce enough thrust to offset that drag, otherwise the airplane stops accelerating and slows down. WW2 aircraft piston engines and post-WW2 jet engines could not produce the needed thrust to offset the drag, consequently aircraft of these times could not sustain corner velocity. It wasn't until the introduction of aircraft with high thrust capability that sustained high g loads were possible.

Quote:
I realize that it's a silly question, but what is the crucial difference, regarding turns at corner speed, in a comparison between let's say p51 mustang and F-15.


Academically, no difference. In the practical sense, the P-51 cannot sustain corner speed because it doesn't have engine thrust to do that. The F-15 may, given a particular altitude and gross weight/configuration.

Finally, keep in mind that corner speed is not a tactic...it is a math value that can be used to compare turn capabilities of aircraft and little else. This concept was often used back in the day when turning fights were the typical yardstick by which relative performance was the issue. That may not be the case today.

#3882469 - 12/23/13 05:16 PM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Ok, here's the question though.

-Can an F-15 actually keep turning, indefinitely (until jet fuel runs out)...

-pulling max allowable Gs (I suppose that would be 9 Gs for a G-suited pilot, this is sustained turning at max Gs)

-at corner speed of F-15. (at sealevel for example, or at some other altitude if sealevel is not possible)

-This is done, with full internal gas tank, and full gun ammunition load, at the very least. (with or without droptanks)

-afterburner, is it required?

(basically just going around in a merry circle, until fuel runs out, without losing altitude, in horizontal turning?)

-From what I understood so far, it actually can! (because it has enough thrust in the engines, F-15 also has quite good thrust to weight ratio too? )

-Is the thrust to weight ratio also important characteristic in this kind of example?

#3882567 - 12/23/13 07:52 PM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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The F-15 g-max is 7g, not 9g. It is possible to over-g in dynamic manoeuvres, but this is generally frowned upon as it shortens airframe life and can cause instant or delayed catastrophic failures (which can be other than airframe strength issues (instrument weight and mounting strength, connectors/fuel line strength, seat bearings etc can all fail at a specific 'g' regardless of strong reserves of airframe strength (e.g. a light weight and an unused capability to carry heavy payloads at high 'g')).

At low weight, clean and at low altitude it has sufficient thrust to sustain a tight turn, possibly including 7g at a higher speed. Whether it is possible to do at the maximum controlled AOA is another matter - I suspect that drag may give a sustained turn performance peak at 7g some velocity above the 'corner', with a lower AOA and less separation/induced drag.

The idea of a sustained 'high-g' turn is wrong in principle anyway. It requires too much from the pilot and will reduce his capability to perform adequately under the oxygen deprivation in the brain/eyes resulting from excessive g sustained too long. It also 'pins' the aircraft in the same area of the sky for too long.

Better is a series of shorter 'hard turns' at around 4-5g, with short separation between, and the occasional use of an offensive or defensive break turn (max-g) to prevent or complete an shot opportunity. Energy management, keeping speed near that for peak 'sustained' turn capability, and above 'peak instantaneous' or corner speed is critical. Excessive speed reduces turn performance dramatically, while insufficient speed reduces the ability to extend, and simplifies any unseen threat's engagement.
Optimum 'turn-performance' approximates an 'egg' shape, with loose turns to rebuild energy, and tighter turns to trade energy for angles ~ alternative strategies use lower g and the vertical component to reduce (horizontal) turn radius.

It would be worth finding a copy of Fighter Tactics: Combat and Manoeuvring - Robert Shaw, which covers the concept of performance and the utilisation of this in the real world for similar and dissimilar ACM, and multiple aircraft engagements (generally much less manoeuvring and more speed conservation and look-out).

#3882598 - 12/23/13 08:47 PM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Originally Posted By: Laurwin
-Can an F-15 actually keep turning, indefinitely (until jet fuel runs out)...

-pulling max allowable Gs (I suppose that would be 9 Gs for a G-suited pilot, this is sustained turning at max Gs)


Nope. It can only do so at weights that you would probably not run into in a real fight often, and then only near or at SL.

Quote:
-at corner speed of F-15. (at sealevel for example, or at some other altitude if sealevel is not possible)

-This is done, with full internal gas tank, and full gun ammunition load, at the very least. (with or without droptanks)


Not possible.

Quote:
-afterburner, is it required?


Yep.

Quote:
-From what I understood so far, it actually can! (because it has enough thrust in the engines, F-15 also has quite good thrust to weight ratio too? )


No, it cannot.

Quote:
-Is the thrust to weight ratio also important characteristic in this kind of example?


Yes it is.


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#3882600 - 12/23/13 08:48 PM Re: flight physics: sustained turns and corner speed [Re: Lieste]  
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It is 9g for any F-15A equipped with OWS. F-15Cs are 9g, but the pilot's maneuver limit (ie. when the OWS screams at you) is dynamic and based on weight, speed and altitude. So it can be 9g in some regimes, less in others.

A combat-loaded F-15 will pull 7g all day long as sea level, but typical fights see you 4-5g for 10 sec, 7-8 for 5 sec, rinse, lather, repeat. This is done to adjust the turn circle, not just because the pilot can't take it. His life is on the line, he'll take it.

Having said all that, you're right, it's not about sitting around on the max-g speed and turning at max g ... it's all about BFM.

Originally Posted By: Lieste
The F-15 g-max is 7g, not 9g.

Last edited by GrayGhost; 12/23/13 08:57 PM.

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#3882739 - 12/24/13 12:36 AM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Originally Posted By: Laurwin
Ok, here's the question though.

-Can an F-15 actually keep turning, indefinitely (until jet fuel runs out)...

-pulling max allowable Gs (I suppose that would be 9 Gs for a G-suited pilot, this is sustained turning at max Gs)

-at corner speed of F-15. (at sealevel for example, or at some other altitude if sealevel is not possible)

-This is done, with full internal gas tank, and full gun ammunition load, at the very least. (with or without droptanks)

-afterburner, is it required?

(basically just going around in a merry circle, until fuel runs out, without losing altitude, in horizontal turning?)

-From what I understood so far, it actually can! (because it has enough thrust in the engines, F-15 also has quite good thrust to weight ratio too? )

-Is the thrust to weight ratio also important characteristic in this kind of example?


What happens when you pull G in a turn is you bleed airspeed off but your ideal corner speed for the F-15 is 440-550kts. Yes afterburner is required if you start your turn at lower speed, at higher speed you would add afterburner to maintain speed otherwise the speed decays. One way to help is to lower the nose while opening the afterburner if at low speeds to maintain that energy. F-16s corner speed is 330-440kts at 9Gs CAT I if its CAT III then its much less G(forgotten how much that you can pull), CAT III is with external stores which limits how much G you can pull without breaking something.

In the event of bleeding to much speed you can unload and try to build up energy(accelerate) and then start turning again. immelman



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#3882741 - 12/24/13 12:39 AM Re: flight physics: sustained turns and corner speed [Re: GrayGhost]  
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Originally Posted By: GrayGhost
It is 9g for any F-15A equipped with OWS. F-15Cs are 9g, but the pilot's maneuver limit (ie. when the OWS screams at you) is dynamic and based on weight, speed and altitude. So it can be 9g in some regimes, less in others.

A combat-loaded F-15 will pull 7g all day long as sea level, but typical fights see you 4-5g for 10 sec, 7-8 for 5 sec, rinse, lather, repeat. This is done to adjust the turn circle, not just because the pilot can't take it. His life is on the line, he'll take it.

Having said all that, you're right, it's not about sitting around on the max-g speed and turning at max g ... it's all about BFM.

Originally Posted By: Lieste
The F-15 g-max is 7g, not 9g.


I wonder how much G a brand new F-15 can pull because I always thought that it was more than that.



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#3882773 - 12/24/13 01:28 AM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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The limit is 9g, after that the jet must undergo over g inspection. Eagles have pulled 12g on real combat situations without ill effects on the airframe other than reducing airframe lifetime.

As for the corner, it can be as low as 350 - the answer is always "it depends".

And as someone above pointed out - Lieste I believe, you're usually not doing yourself any favors by riding the bottom end of corner.


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#3885798 - 12/29/13 08:11 PM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Thank you for good answers to clear up this question for me.
This was for me, a question from the perspective of flight physics, not really about the specific combat tactics.

But I obviously decided to post it in this forum, because I figured this subforum about air combat tactics would get the most views and possibly the most knowledgeble answers.

#3885939 - 12/30/13 12:45 AM Re: flight physics: sustained turns and corner speed [Re: Laurwin]  
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Were we able to answer your questions?

If not, please try again!

#3886489 - 12/30/13 10:18 PM Re: flight physics: sustained turns and corner speed [Re: GrayGhost]  
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Originally Posted By: GrayGhost
The limit is 9g, after that the jet must undergo over g inspection. Eagles have pulled 12g on real combat situations without ill effects on the airframe other than reducing airframe lifetime.

As for the corner, it can be as low as 350 - the answer is always "it depends".

And as someone above pointed out - Lieste I believe, you're usually not doing yourself any favors by riding the bottom end of corner.


EDs about to release the AFM for the F-15C in DCS and it will have some interesting low speed capabilities not yet modelled in a sim. So I'm looking forward to taking that up for a spin against some migs.



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