4.33 F-15 ?
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…it went from a near zero-airspeed tail-slide to about 20-30K MSL…I’d call that accelerating. Then it slowed down and spent a bit of time on it’s back before returning to show center. I had better eyes back in the late 70s, and I could also see the pilot’s orange flight suit under the canopy.
With you naked eye you can’t see F-15 above 20k from the ground. Also even with -229 engine with very low fuel (~4k lb) F-15E maybe can climb to 20k but I’m 100% sure it cant 30k. At low alt for a time it can accelerate a very bit but above 13-14k with M0.4 speed the thrust curves said what will happen. (-229 has likley similar curves to -100 just a bit upscaled.)
You also forget the drag. Not only gravity force and thrust act on airframe… -
…it went from a near zero-airspeed tail-slide to about 20-30K MSL…I’d call that accelerating. Then it slowed down and spent a bit of time on it’s back before returning to show center. I had better eyes back in the late 70s, and I could also see the pilot’s orange flight suit under the canopy.
Nothing to do with better eyes……you cannot tell the climb angle, altitude, rate of climb, velocity, if it was accelerating or not, etc etc from looking from a certain angle on the ground…I realise some think you can…
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TBH it is next to impossible to build an FM based on impressions like what you saw in a meeting. What would be needed is hard data in terms of aero coefs and thrust, or EM diagrams. Not the easiest thing to come by
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Hi Folks,
Nothing you guys didn’t already know - but - here’s the response from the F-15 pilot when I asked him about vertical acceleration and time to climb….
Yep, I'm an Eagle driver. Been flying that beautiful beast since 2001\. Lots of people ask about the thrust to weight and vertical acceleration. The thrust to weight of a "demo clean" Eagle is greater than 1:1\. That jet doesn't have pylons or external tanks on it. It is a configuration that we could get into in combat, but it's never happened. The short story is that you are not going to accelerate in the vertical because you'd need 1:1 PLUS 9.8m/s/s to overcome gravity so if you had a 56k# airplane and you had 56,500# of thrust, you have greater than 1:1, but you aren't going to go straight up like a Saturn 5 rocket. You have induced drag from creating lift, you have induced drag because you are going through a fluid (wind resistance) and you might have engines that have been slightly tuned back to get better life out of them. Climbing to the flight levels all depends on how high you are going and how fast you need to be there. We normally take off in our training configuration using afterburner till around 300KIAS, then accelerate to 350KIAS and climb at 350 until around 31k' when we switch to .90M and climb at that airspeed. Typical loads and temps you'll probably see just upward of 6k'/min climb below 10k' and then between 4-5k'/min until the low 20's. If on the other hand you leave the afterburners cooking, you are about 50-60 degrees nose high at 350KIAS and you are climbing somewhere in the neighborhood of 15k'/min. But, as with anything in aviation, it all depends. Hope that answers your questions.
Regards,
Scott -
Thats the long and the short of it. TWR slightly more than 1 does not work. You need thrust greater than mass to stay where you are. You then need additional thrust to at least equal the drag at the desired speed. You need enough to overcome the decrease in thrust as you climb. And to get a decent acceleration, you need at least another set on top… if you want to accelerate in the vertical, you want TWR greater than 2 at sea level. And if you have a jet that can do that, you basically dont need aerodynamics, either.
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Thats the long and the short of it. TWR slightly more than 1 does not work. You need thrust greater than mass to stay where you are. You then need additional thrust to at least equal the drag at the desired speed. You need enough to overcome the decrease in thrust as you climb. And to get a decent acceleration, you need at least another set on top… if you want to accelerate in the vertical, you want TWR greater than 2 at sea level. And if you have a jet that can do that, you basically dont need aerodynamics, either.
Yes. Most people somehow forgets the drag in vertical climbing which acts just way as in level flight…
And this is why a totally bad indication of total flight perfomance only the t/w ratio because in defines only the climbling capability above a certin pitch rate where the force of gravity dominates over the drag. -
Yes. Most people somehow forgets the drag in vertical climbing which acts just way as in level flight…
And this is why a totally bad indication of total flight perfomance only the t/w ratio because in defines only the climbling capability above a certin pitch rate where the force of gravity dominates over the drag.As the force of gravity only dominates over the force of drag at low speeds, its a bad indication in general.
Rockets accelerate slowly in the vertical with a TWR of 1.5:1. Fighter jets with a nominal TWR of 1.1 at sea level do not accelerate in the vertical, typically. Its not the simplest physics problem.
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With you naked eye you can’t see F-15 above 20k from the ground. Also even with -229 engine with very low fuel (~4k lb) F-15E maybe can climb to 20k but I’m 100% sure it cant 30k. At low alt for a time it can accelerate a very bit but above 13-14k with M0.4 speed the thrust curves said what will happen. (-229 has likley similar curves to -100 just a bit upscaled.)
You also forget the drag. Not only gravity force and thrust act on airframe……sure I can…I see them all the time. In fact, I can spot an F-16 at about ten miles, on a clear day…I have 20/15 vision.
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As the force of gravity only dominates over the force of drag at low speeds, its a bad indication in general.
Rockets accelerate slowly in the vertical with a TWR of 1.5:1. Fighter jets with a nominal TWR of 1.1 at sea level do not accelerate in the vertical, typically. Its not the simplest physics problem.
…all I know is that to go from 0 to x, you HAVE to accelerate…to some steady state. And I know an F-15 can accelerate in the vertical, at some gross weights…usually when they are clean. And the GE powered ones can REALLY scoot.
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…it went from a near zero-airspeed tail-slide to about 20-30K MSL
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Gents
The Time to Climb profile flown by the Streak Eagle may be of interest :
“In early 1975, flying out of Grand Forks Air Force Base in North Dakota, an F-15A known as Streak Eagle set many time-to-climb world records. Between Jan. 16 and Feb. 1, 1975, the Streak Eagle broke eight time-to-climb world records. It reached an altitude of 98,425 feet just 3 minutes, 27.8 seconds from brake release at takeoff and coasted to nearly 103,000 feet before descending”
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…all I can say is you shoulda been there…I was impressed enough to join the Company.
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Yup, I saw a 15 at an airshow do a shallow takeoff and just beyond the edge of the runway it pulled to vertical and climbed straight up in a slow roll completely out of sight. Awesome. Only using the binocs could I spot it after that till it came back down.
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I general they are limited to 60 degree climbs in controlled airspace to preclude “aerobatic” maneuver…they used to clear the STL traffic area straight up because it was the fastest way out of the TCA (anybody care to guess how old I am?..), but I think they eventually got restricted to 60 degrees sometime after I left the area. They still hot-foot it out of there though to get out of the way of the airliner traffic…there’s a viewing lot at the west end of the airfield where folks gather and watch.