Forum: General F-35 Forum

sewerrat wrote:

cola wrote:

There was a smart guy from US, that once said; "...double your speed and you can consider yourself having half the RCS (not literally ofc), from what you had at original speed."

...and to get down to the F-35's RCS, you'd have to fly even much, much faster than the X-15. A RCS reduction of 0.5 is meaningless and useless. Even the mildly-stealthy Mach 3+ cruising SR-71 was shot many, many times. If it's crusie speed was only < Mach 2 and 60k feet(like a Pak Fa), it'd likely have had more than 'some' losses. No figures publicly exist, but I'd be willing to wager a dollar or two than the F-117 was detected, and targeted, and shot at much, much less times than the SR-71. I wonder how much more the F-35 would cost if its airframe was built to spend all its time at Mach 1.8. That kind of performance comes at a cost - literally a $ cost.

What if F-35 faces an opponent capable of both supersonic and stealthy in future?
For example for USN, who has no F-22 but F-35C to face such a challenge.

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johnwill wrote:

I posted that information on March 29 at 0534. The source is me, as I was team leader of the flight test that measured the wing, fuselage and tail loads during F-16 FSD flight test in 1977. At the design point (0.95 mach, 10,000 ft, 9g) total wing lift was 110,000 lb, tail lift was 0, and total lift was 202,500 lb (9g x 22,500 lb gross weight). So, fuselage lift is 92,500 lb or 45% of the total. All that information is in the flight loads test final report (which I wrote), 16PS011.

johnwill,

OT, but why does the lift test flight refer to the speed in mach and not in KIAS or KCAS?

Last edited by wrightwing on Apr 02, 2012 - 12:34 AM; edited 1 time in total

megasun wrote:

What if F-35 faces an opponent capable of both supersonic and stealthy in future?
For example for USN, who has no F-22 but F-35C to face such a challenge.

You mean post PAK FA/J-20? By that time, you'll likely see F-35 successors arriving in service

By the way, I presume you meant supercruise, and not supersonic? The F-35 is stealthy and supersonic. You have to remember, that even if a foe is capable of flying at M1.5(or higher) on dry thrust, it will only do so when that speed is required. Even the F-22 spends most of its time subsonic. The more important thing, is who can spot the other aircraft first, and position themselves most advantageously. It's highly likely, that the PAK FA/J-20, will have RCS figures several orders of magnitude higher than the F-35, negating whatever speed advantage, that they might have.

Mach number is a better indication of the flight conditions when you fly near trans/supersonic. Mach .95 looks like Mach .95 no matter where the speed of sound is, but 700 knots can look different depending on the speed of sound, which changes with temperature. Typically, you design for Mach number rather than speed for this reason.

megasun wrote:

sewerrat wrote:

cola wrote:

There was a smart guy from US, that once said; "...double your speed and you can consider yourself having half the RCS (not literally ofc), from what you had at original speed."

...and to get down to the F-35's RCS, you'd have to fly even much, much faster than the X-15. A RCS reduction of 0.5 is meaningless and useless. Even the mildly-stealthy Mach 3+ cruising SR-71 was shot many, many times. If it's crusie speed was only < Mach 2 and 60k feet(like a Pak Fa), it'd likely have had more than 'some' losses. No figures publicly exist, but I'd be willing to wager a dollar or two than the F-117 was detected, and targeted, and shot at much, much less times than the SR-71. I wonder how much more the F-35 would cost if its airframe was built to spend all its time at Mach 1.8. That kind of performance comes at a cost - literally a $ cost.

What if F-35 faces an opponent capable of both supersonic and stealthy in future?
For example for USN, who has no F-22 but F-35C to face such a challenge.

You can canvas the internet to find the answer to this question. It has to do with the whole system, not one component.

exorcet wrote:

Mach number is a better indication of the flight conditions when you fly near trans/supersonic. Mach .95 looks like Mach .95 no matter where the speed of sound is, but 700 knots can look different depending on the speed of sound, which changes with temperature. Typically, you design for Mach number rather than speed for this reason.

Correct. And 700 kcas can be mach 1.05 or mach 2 or more or anywhere in between, depending on altitude. In addition, pressures are a function of airspeed (kcas), but pressure distributions vary with mach number. It actually takes both (airspeed, mach) to fully define a load condition.

Here's an example. 500kcas at 10,000 ft is .89 mach. 500 kcas at 15000 ft is .96 mach, radically different load distributions, but both are 500 kcas.

One thing to remember, in engineering terms, airspeed always means either calibrated, indicated, or equivalent, never true. Navigators use true airspeed.

sewerrat wrote:

No figures publicly exist, but I'd be willing to wager a dollar or two than the F-117 was detected, and targeted, and shot at much, much less times than the SR-71.

Public record states zero SR71 shot down.
F117 not so, even though it was much less fired upon.

Quote:

I wonder how much more the F-35 would cost if its airframe was built to spend all its time at Mach 1.8. That kind of performance comes at a cost - literally a $ cost.

Nothing, because that can't be done...and you don't have to wonder.
Take a look at F22.

haavarla wrote:

On the Flanker vs F-15 low speed handeling, the Flanker gain the edge from 450kn and down, as the speed drop lower than 450kn the Flanker edge over the F-15 rise higher.

What do you mean by "handling"?
How do you qualify and quantify that?

shingen wrote:

Why did they leave it off?

It hadn't been invented yet. A programmed LEF requires something like a fly by wire computer to control it, which the F-15 did not have. I don't know if they considered it or not. Their solution, very good for the time, was a cambered leading edge, more so at the outer span than inboard. It's effective, but still a compromise.

The programmed LEF is heavier, more expensive, and requires more maintenance. Evidently, it is worth the cost and weight. Engineering a programmed LEF is not simple, as on the F-16 about 30% of wing lift is generated by the LEF.

What activated and controlled LEF before that, AoA, or speed?

megasun wrote:

sewerrat wrote:

cola wrote:

There was a smart guy from US, that once said; "...double your speed and you can consider yourself having half the RCS (not literally ofc), from what you had at original speed."

...and to get down to the F-35's RCS, you'd have to fly even much, much faster than the X-15. A RCS reduction of 0.5 is meaningless and useless. Even the mildly-stealthy Mach 3+ cruising SR-71 was shot many, many times. If it's crusie speed was only < Mach 2 and 60k feet(like a Pak Fa), it'd likely have had more than 'some' losses. No figures publicly exist, but I'd be willing to wager a dollar or two than the F-117 was detected, and targeted, and shot at much, much less times than the SR-71. I wonder how much more the F-35 would cost if its airframe was built to spend all its time at Mach 1.8. That kind of performance comes at a cost - literally a $ cost.

What if F-35 faces an opponent capable of both supersonic and stealthy in future?
For example for USN, who has no F-22 but F-35C to face such a challenge.

There is no such airplane in existence. We've seen the Pak Fa, and its only a somewhat LO aircraft. The plane the Chinese are testing, I also think its in the same category as the Pak Fa as just having reduced oberservables. There is literally nothing else on the horizon. Hell, we have to wait and see if those other 2 aircraft ever make it into production. By the time they do, if they do, we'll have hundreds of -35s in service, and we'll have enough time in them to work out tactics for dealing with advanced adversary aircraft by training against F-22s. We'll also have new variants of AAMs... By the time Russians and Chinese are able to introduce their aircraft, we'll probably have new variants of the -35 in various stages of development: a 50,000lb thrust USAF F-35D perhaps?

We in the US are in good standing for next long while assuming the F-35 isn't cancelled or cut to the bone and we end up with something like 600 spread across the USAF and USN supplemented by new build F-16s and F-18s.

Quote:

What do you mean by "handling"?
How do you qualify and quantify that?

As in Lateral stability in high apha. The engines airflow restriction(or the lack of it)! Its a large fighter, the weight follows it, its still very manuvereble, that would be Agility.

Anyway, here is the post from the book F-15 engaged: http://www.f-16.net/f-16_forum_viewtopic-t-18963.html

haavarla wrote:

As in Lateral stability in high apha. The engines airflow restriction(or the lack of it)! Its a large fighter, the weight follows it, its still very manuvereble, that would be Agility

Does the Su-27 flight manual say the vanilla flanker becomes unstable beyond 24 degrees alpha (20 degrees with weapons loaded) and is limitted to 8G's maximum below mach 0.85 and 7G's max from 0.85 to 1.25?

shingen wrote:

What activated and controlled LEF before that, AoA, or speed?

Before that, only a few fighters had LEF or something similar called slats. LEF rotate around a fixed hinge line and slats extend, leaving a gap for airflow between the slat and the wing. You can see slats on most commercial airliners used only for takeoff and landing. They were not programmed at all, simply extended or retracted, no intermediate positions. Some were manually activated, some automatic. I think the F-5 and F-104 had manually activated LEF and some models of the F-86 had automatically extending slats, activated by air pressure above a certain AoA. Left and right were not linked together, so sometimes they extended at different times, immediately causing an uncommanded roll.

There may be other airplanes with unscheduled LEF, I simply don't know about them. F-111, F-14, Tornado, and possibly others had TO and landing slats.

Lateral stability at high AoA has been mentioned as part of "handling". I think the writer has the right idea, but the terminology is wrong. Lateral Stability is stability around the roll axis, which normally is not a concern. I think the writer really means Directional Stability, or stability around the yaw axis. Directional stability suffers at high AoA due to the fuselage masking or blocking airflow past the vertical tail or tails. Directional stability is what keeps the pointy end in front. Loss of directional stability is what causes spins.

Here is my definition of good handling. The airplane responds to all-axis control inputs in a predictable, linear fashion, with sufficient rate to accomplish the pilot's intent.

Last edited by spazsinbad on Apr 02, 2012 - 07:53 AM; edited 1 time in total

The venerable A-4 Skyhawk has leading edge slats that extend fully when on the ground at less than 90 knots or so airspeed. Gravity and lack of airspeed below about 230 knots cause them to extend incrementally. At Optimum Angle of Attack (for carrier or field landing) the slats are extended about one third to one half down (from fully up for normal flight). As 'johnwill' says the A-4 was notorious for uncommanded differential slat extension - thankfully usually at low airspeed and low G - but depending on circumstances the 'flick roll' (remember the A-4 had a maximum roll rate of 720 degrees per second at 250 knots best roll speed with a maximum on one roll only allowed) could be 'dramatic' or catastrophic.

In landing configuration with slats out the A-4 was very stable however.

Forum: General F-35 Forum

JSF High Wing Loading: why?