Aerodynamics thread

[Quickie]

When we are talking about maximum speed at altitude we are always talking about "true airspeed". As you climb, your indicated airspeed goes down on your analog airspeed gauge, and so you have to calculate "true airspeed"...

True airspeed is the actual calibrated speed through the air, that's why speed records are always calculated by using two speed runs on a reciprocal heading, you break the timer on a heading outbound to begin that leg and break another timer at the end of that leg, you then turn around and break the timer as you begin the inbound leg and finish your run on the inbound leg by breaking the timer that started your outbound leg.

You then take those two numbers, add them together and divide by two... for your true airspeed

That way you need not be concerned about air pressure and its effects. Both recip and turbine engines both lose thrust as they climb into less dense air, the supercharger and turbo are able to increase available horsepower at sea level by compressing the mixture, although as you compress that mixture you increase the heat production, hence higher octanes and intercoolers are necessary to support combustion and avoid detonation..

All turbines lose thrust as they climb, as most poster's probably realize you can play with variable inlet design to achieve a "ram-jet" effect, effectively increasing the amount of thrust available at a given altitude

When we are talking about maximum speed at altitude we are always talking about "true airspeed". As you climb, your indicated airspeed goes down on your analog airspeed gauge, and so you have to calculate "true airspeed"...

True airspeed is the actual calibrated speed through the air, that's why speed records are always calculated by using two speed runs on a reciprocal heading, you break the timer on a heading outbound to begin that leg and break another timer at the end of that leg, you then turn around and break the timer as you begin the inbound leg and finish your run on the inbound leg by breaking the timer that started your outbound leg.

The above is basically 2 different ways of determining the airspeed (relative to the aircraft), albeit the second method being more accurate one, provided the air-stream condition are unchanging i.e. stable wind condition with low wind speed (relative to earth of course, as opposed to airspeed relative to aircraft).

So, your explanation don't go against the points I was trying to make. The Mach number I was referring to, is the same airspeed as the "true airspeed" you are trying to explain above, which should be the case when we're talking about an aircraft Mach Number to begin with.

That way you need not be concerned about air pressure and its effects.

It may not be a concern if you are mechanically measuring the airspeed as you describe above, but the air pressure/density does in the end determine the max airspeed that an aircraft is capable of at at some specific altitude.

 

[latenlazy]

The F-22 has a fixed inlet, whereas the J-20 and F-35 have DSIs. I'm not sure if DSIs can achieve the same efficiency at speed, and I'll happily admit I don't know enough about whether DSIs might have better supersonic performance at high altitudes (lower air pressure) and speeds.

A DSI is just a fixed inlet that replaces the diverter with a bump to handle boundary layer flow. Every argument for why a DSI can’t perform as well above a certain speed relative to a variable inlet also applies to a fixed inlet with a diverter plate. There are other implications in a comparison between a bump and a diverter, but in a comparison with variable inlets on limits to top speed the points against either are the same. My main point though is that the typical arguments evoked in a fixed vs variable inlet debate is an oversimplification of the issue.

 

[Inst]

@Richard Santos:

Boeing, the F-15E's manufacturer, seems to disagree with you.

Please, Log in or Register to view URLs content!

@SDWatcher

The trick seems to be that the J-20 actually reduces Coefficient of Drag as speed goes up, but wave drag increases. It's not a general property of aircraft, but is a specific properly of the J-20's design.

@Others

Well, I mean, what is exactly wrong with the J-20 as an interceptor-type air superiority fighter? I mean, if the J-20 doesn't have excellent subsonic maneuvering, who really cares? In WVR, you're just HOBS-bait. Keep the battle BVR or medium-ranged, scoot in, launch missiles, and scoot out, and does anyone really care that you can't beat an F-22 in a guns dogfight? I've said this a thousand times; if the J-20 is excellent as a high-speed BVR fighter, it can have a real advantage over the F-35 by launching missiles before the F-35 can achieve a missile lock within NEZ, firing missiles from within its own NEZ, and then slowing, turning, and fleeing before the enemy can retaliate.

 

[latenlazy]

@Richard Santos:

Boeing, the F-15E's manufacturer, seems to disagree with you.

Please, Log in or Register to view URLs content!

@SDWatcher

The trick seems to be that the J-20 actually reduces Coefficient of Drag as speed goes up, but wave drag increases. It's not a general property of aircraft, but is a specific properly of the J-20's design.

@Others

Well, I mean, what is exactly wrong with the J-20 as an interceptor-type air superiority fighter? I mean, if the J-20 doesn't have excellent subsonic maneuvering, who really cares? In WVR, you're just HOBS-bait. Keep the battle BVR or medium-ranged, scoot in, launch missiles, and scoot out, and does anyone really care that you can't beat an F-22 in a guns dogfight? I've said this a thousand times; if the J-20 is excellent as a high-speed BVR fighter, it can have a real advantage over the F-35 by launching missiles before the F-35 can achieve a missile lock within NEZ, firing missiles from within its own NEZ, and then slowing, turning, and fleeing before the enemy can retaliate.

If it were the case that the J-20 doesn’t have good subsonic maneuverability then ¯\_(ツ)_/¯ , but we really don’t have any good basis to draw that conclusion. Our best piece of evidence to conclude anything at all is what a J-20 pilot said in an interview, which was that the J-20’s subsonic maneuverability is about as good as the F-16’s, but even then we don’t know what “subsonic maneuverability” specifically entails. The flight envelope between 0 and 600 mph covers a pretty wide range.

 

[Figaro]

If it were the case that the J-20 doesn’t have good subsonic maneuverability then ¯\_(ツ)_/¯ , but we really don’t have any good basis to draw that conclusion. Our best piece of evidence to conclude anything at all is what a J-20 pilot said in an interview, which was that the J-20’s subsonic maneuverability is about as good as the F-16’s, but even then we don’t know what “subsonic maneuverability” specifically entails. The flight envelope between 0 and 600 mph covers a pretty wide range.

Where did the J-20 pilot say that the subsonic maneuverability was about as good as that of the F-16? If I recall, he only said it was good or "不错", without any comparison to other foreign aircraft.

 

[latenlazy]

Where did the J-20 pilot say that the subsonic maneuverability was about as good as that of the F-16? If I recall, he only said it was good or "不错", without any comparison to other foreign aircraft.

Now that I think about it I might be miseremembering and it was actually comments from a forum post, not the pilot interview with the 不错 comment. I’ll try to do a search when I can to find the source.

 

[SDWatcher]

Unless the paper from Virginia Tech underestimated Total Drag by more than a factor of 2, their calculations suggest the J-20 may already supercruise with the AL-31.

Is there any reliable source claiming that the J-20 can't supercruise with the AL-31?

 

[latenlazy]

Where did the J-20 pilot say that the subsonic maneuverability was about as good as that of the F-16? If I recall, he only said it was good or "不错", without any comparison to other foreign aircraft.

Now that I think about it I might be miseremembering and it was actually comments from a forum post, not the pilot interview with the 不错 comment. I’ll try to do a search when I can to find the source.

Yep, just checked. I misattributed the original source of that information. It was from the Guancha article summarizing the supposed first exercises the J-20 conducted. The story came out quite a bit after those pilot interviews, but I guess it's been long enough that my memory compressed the two together.

Please, Log in or Register to view URLs content!

"先进的气动布局，让该机不开加力达到音速，超音速机动性能超越所有三代机，同时亚音速下，稳定盘旋率打平了“战斗机黑手党”最爱的F-16A（开个玩笑：歼-20堪称最强机炮格斗四代机，虽然它目前没有装炮）；先进的飞控系统，带来强大的大迎角控制性能，能在接近失速状态下做出匪夷所思的机动动作；先进的隐身设计，让该机雷达反射截面积达到F-22同等水平，在敌人的眼皮低下如入无人之境；先进的航电系统，让它成为空中的信息节点，成为未来空中战场的核心……"

Bolded section roughly translates to "can reach the speed of sound without afterburner, has supersonic maneuverability that surpasses every third (fourth outside of China) generation plane, while also has sustained turn rates equal to the F-16".

 

[manqiangrexue]

Is there any reliable source claiming that the J-20 can't supercruise with the AL-31?

Brat's imagination

 

[SamuraiBlue]

Basically drag and engine output both has a say in maximum speed.
Drag is resistance less means you can accelerate more with less energy but drag becomes exponentially with speed so you need a stronger engine to go beyond a certain limit.
Visa versa with engine power in which with more engine out put you can accelerate more in the same airframe but will reach a limit due to exponential amount of resistance due to drag with more speed..
Basically you need both to achieve super-cruise.