LO's F-15 vs. the F-15 Streak Eagle

[SwingKid]

41 s.

 

Agreed. So, I think Rhen maybe should subtract about 20 seconds from all of his "Dash-1" times in post #273. In that case, the Lock On F-15C may no longer appear to be "overpowered" at low altitude. :punch:

 

-SK

[Yo-Yo]

Agreed. So, I think Rhen maybe should subtract about 20 seconds from all of his "Dash-1" times in post #273. In that case, the Lock On F-15C may no longer appear to be "overpowered" at low altitude. :punch:

 

-SK

 

I am afraid it's not so overpowered. Here are the results of the test I performed. I recorded the track in 1.12 and it seems to be playable. If not so I describe the technics for everybody who wants to re-test.

1. Set initial mass to 17700 kg or 39000 lb.

2. Fly the plane and set the throttle to 100% then carefully advance it by little steps till AB is on. Keep in mind that there is a delay between the throttle movement and AB flame. When AB is on get the throttle back by the step you use. If AB flame is off you are home and dry. It is 100% MIL. Don't touch it! :)

Another way to set MIL is to switch off the throttle slider in OPTIONS window and use keyboard.

3. Record a track. In one second: Pause off, wheel brakes on, flaps down. As the thrust rizes release wheel brakes and rotate at 120 kts. Gear up, flaps up. Gain altitude to have 300 kts at 1000 ft. Then begin to climb gently to aperiodically stop the IAS at 350 kts and rotation simultaniously.

4. Maintain constant 350 kts slightly changing the pitch.

5. When you are over 25000 begin to decrease IAS controlling the pitch.

You must continiously decrease IAS to intersect the altitudes at the following IAS:

30 k - 324 kts

35 k - 288 kts

40 k - 257 kts

[Yo-Yo]

The results of the test.

 

1. Ground run. Dash 1 at these conditions gives 1800 ft but with the stick FULL AFT from the low speed till AoA 12 that is different with the test I performed (somebody can perform his own right test). My result is 1450 ft but the drag is lower because of low AoA at the end of the run before rotating.

 

2. 350 ktas at 1000 ft safe altitude at 45 s.

 

3. Look at the table. The most interesting and informative thing are not the curves itself but their slopes. If the slopes are the same the specific excess power (SEP, energy gain or altitude gain if the speed are equal for Dash 1 and LO) IS EQUAL or very close so I would not speak about "overpowered plane".

If we set equal pre-climb time (it can be different) i.e. eliminate it from the result we can see that they are match up to 30000 and then LO SEP begins to fall faster. The total time error is though about 15% (400 s vs 350 s if the red curve is moved up to match up to 30k ) and of course can be eliminated by the fine tune of the MIL thrust.

 

4. The overall distance to get 40 k is 72 nm for Dash 1 and 83 nm for LO that is about 15 %.

 

So the overall error not more than 15%... at the low margine of an engineering error :), not perfect but the FM was tuned when no full dash -1 was available.

[Yo-Yo]

The track. Must be playable... :)

F-15 39000 lb max climb to 40000.zip

[Pilotasso]

This is coool, its like a public DEV forum :D

[Fox One]

For comparison:

 

Lock On Su-27: ~18,600 kgf thrust

real Su-27: 25,000 kgf thrust

Lock On Su-27 underpowered by: 25.6%

 

Lock On MiG-29: ~14,000 kgf thrust

real MiG-29: 16,600 kgf thrust

Lock On MiG-29 underpowered by: 15.7%

 

No wonder, we have little problem accelerating vertically in Lock On's MiG-29...

 

-SK

 

 

You’re in big error here. AL-31F 12,500 Kgf thrust (that is bench thrust as everybody knows) won’t be the same with the engine installed in the fighter, not even at zero speed. There will be thrust losses caused by the air intakes ducts, and these are considerable. Let’s suppose that air intakes-caused thrust losses for Su-27 are similar (as fraction of bench thrust) with the air intakes of MiG-25 :)

 

Watch in the MiG-25RB practical aerodynamics manual, page 79 – it’s the air intakes-caused thrust losses. You can see, even at zero speed the thrust losses are as high as 26%! Watch at low speeds the lower curve – this shows how because the lower lip of the air intake is down (compared to “lip up” curve above) it decreses the thrust losses (by inhibiting turbulence forming that will lead to pressure losses in the intake ducts, and finally thrust losses). The lip will decrease thrust losses with more than 8%. Air intakes-caused thrust losses is a big deal indeed, somewhere in the manual it is said that if the plane is heavy (or is flying with only one engine) during landing approach and the pilot for whatever reason has to abort landing, and will power up but will have little excess thrust, to not forget that when will retract landing gear the lip will also go up and will decrease a bit the thrust, so is better to retract gear when higher-faster-safer. Also in the graph you can see how the bench thrust is almost reached at high subsonic speeds, and the way inlets starts producing thrust at high Mach.

 

In the same book, page 63, R-15B-300 thrust, military 7,500, max AB 11,200. So we have 15,000 and 22,400. On page 267, in the text – takeoff thrust military 12,200, max AB 17,600. So the average takeoff thrust is 81% of bench thrust in military and 78% in max AB.

 

In LockOn takeoff performance for supersonic a/c is already outstanding, no need to increase thrust here. The thrust is low only at high altitude. In MiG-29 manual it says AB takeoff will take 7-8 seconds. I think you can do that easily in LockOn.

 

MiG-23ML/UB practical aerodynamics manual, page 345. Takeoff forces distribution. It is in that manual, R-35-300 engine is 13,000 Kgf thrust. All you can count in the graph at zero speed is 10,500. That is 80% of bench thrust.

 

Coincidence? No. It is SAFE to assume that in takeoff condition supersonic fighter a/c have about 80% of the bench thrust. So the Su-27 will have about 20 tons takeoff thrust. There are also ejector nozzle flaps losses (airframe-mounted on MiG-23). Anybody not convinced yet is invited to have a more in-depth lecture of those two books, air intakes chapters.

[GGTharos]

Very good information Fox One! :)

[Yo-Yo]

Thank you FoxOne! You have done my work explaining things SK must know... :) I already have no vital forces to do it...

[D-Scythe]

You’re in big error here. AL-31F 12,500 Kgf thrust (that is bench thrust as everybody knows) won’t be the same with the engine installed in the fighter, not even at zero speed. There will be thrust losses caused by the air intakes ducts, and these are considerable. Let’s suppose that air intakes-caused thrust losses for Su-27 are similar (as fraction of bench thrust) with the air intakes of MiG-25 :)

 

Watch in the MiG-25RB practical aerodynamics manual, page 79 – it’s the air intakes-caused thrust losses. You can see, even at zero speed the thrust losses are as high as 26%! Watch at low speeds the lower curve – this shows how because the lower lip of the air intake is down (compared to “lip up” curve above) it decreses the thrust losses (by inhibiting turbulence forming that will lead to pressure losses in the intake ducts, and finally thrust losses). The lip will decrease thrust losses with more than 8%. Air intakes-caused thrust losses is a big deal indeed, somewhere in the manual it is said that if the plane is heavy (or is flying with only one engine) during landing approach and the pilot for whatever reason has to abort landing, and will power up but will have little excess thrust, to not forget that when will retract landing gear the lip will also go up and will decrease a bit the thrust, so is better to retract gear when higher-faster-safer. Also in the graph you can see how the bench thrust is almost reached at high subsonic speeds, and the way inlets starts producing thrust at high Mach.

 

In the same book, page 63, R-15B-300 thrust, military 7,500, max AB 11,200. So we have 15,000 and 22,400. On page 267, in the text – takeoff thrust military 12,200, max AB 17,600. So the average takeoff thrust is 81% of bench thrust in military and 78% in max AB.

 

In LockOn takeoff performance for supersonic a/c is already outstanding, no need to increase thrust here. The thrust is low only at high altitude. In MiG-29 manual it says AB takeoff will take 7-8 seconds. I think you can do that easily in LockOn.

 

MiG-23ML/UB practical aerodynamics manual, page 345. Takeoff forces distribution. It is in that manual, R-35-300 engine is 13,000 Kgf thrust. All you can count in the graph at zero speed is 10,500. That is 80% of bench thrust.

 

Coincidence? No. It is SAFE to assume that in takeoff condition supersonic fighter a/c have about 80% of the bench thrust. So the Su-27 will have about 20 tons takeoff thrust. There are also ejector nozzle flaps losses (airframe-mounted on MiG-23). Anybody not convinced yet is invited to have a more in-depth lecture of those two books, air intakes chapters.

 

Wasn't SK's point that the F-15 should be able to accelerate vertically in Lock On (because it can IRL) but cannot? Therefore, any losses in thrust have already been accounted for, simply because the real F-15 can perform such a feat.

 

Furthermore, on the other hand, an engine may produce more thrust than its bench numbers indicate depending on the portion of the flight envelope being traversed (as I'm sure you're aware of). I think the basic point is that the F-15 doesn't seem to ever produce more than 41 000 lbs of thrust in the portion of the flight envelope associated with taking off in the various fashions.

 

And that's clearly false.

[GGTharos]

'Clearly' according to whom?

If US engines are typical and similar to Russian engines in terms of thrust loss once put in a plane, then the F-15 should be limited to about 38000lbs of thrust on takeoff and that might be generous. ;)

[D-Scythe]

'Clearly' according to whom?

If US engines are typical and similar to Russian engines in terms of thrust loss once put in a plane, then the F-15 should be limited to about 38000lbs of thrust on takeoff and that might be generous. ;)

 

Um, we weren't talking about thrust being produced at static/take-off. This is after the F-15 has picked up some speed and is pulling vertical? Completely different part of the flight envelope. More speed, more thrust.

 

And, clearly, since we know for a fact that an F-15 can accelerate in the vertical, it must produce more lbs of thrust than its own lbs of weight in order to achieve such a feat. Doesn't mean that Fox was wrong and the F100-PW-220 doesn't produce less than its bench thrust when the F-15 is taking off.

[GGTharos]

More speed, more drag. The F-15 follows the -1 relatively closely at low altitude. It might be a little underpowered, but so little you likely wouldn't notice.

 

The F-15 DOES accelerate straight up, and all your have to do to -see- that is monitor the TAS in external view. Oh, and your F-15 needs to be rather light ... keep in mind the Streak had a 1.4+ (which means probably less than 1.5) TWR ratio while uber-light and with its engines tweaked to provide a minimum of extra 4000lbs of thrust.

 

That puts the ballpark numbers in the correct place to say the Streak generated 42000lbs of thrust.

[D-Scythe]

More speed, more drag.

 

At this portion of the flight envelope (ala Viking take off)? Any drag induced by increased speed is negligible.

 

And no, I have not been able to get my Eagle to accelerate straight up - or any other energy intensive manuevers (like entering a loop at 180 kts and coming out the top faster than when I started). Of course, I didn't go all out and reduce my fuel load to 50lbs. Can you post a track?

[GGTharos]

I suggest you read what has been said about engines, engine drag, and the ram-air effect in this thread again, *very* carefully, and then you'll get the idea of accelerating straight up from 180kts right out of your head.

[D-Scythe]

I suggest you read what has been said about engines, engine drag, and the ram-air effect in this thread again, *very* carefully, and then you'll get the idea of accelerating straight up from 180kts right out of your head.

 

Well...according to Swingkid's sources, the F-15 can enter a loop at 150 kts (sorry, I remembered it wrong, but nonetheless, it'z not possible in Lock On - well, it wasn't when I tried it, but it was a very rushed job :( ).

 

http://forum.lockon.ru/showpost.php?p=320302&postcount=28

 

Regardless, this is irrelevent to our discussion. Your point that more speed = more drag is clearly false, since once again, drag induced by increased speeds in this portion of the flight envelope is largely negligible. More speed = more thrust (due to ram air effect) which is why the F100 is NOT limited to 80% of its bench thrust like Fox was saying.

[GGTharos]

Let me quote Fox One for you then:

 

In LockOn takeoff performance for supersonic a/c is already outstanding, no need to increase thrust here. The thrust is low only at high altitude. In MiG-29 manual it says AB takeoff will take 7-8 seconds. I think you can do that easily in LockOn.

 

And while more speed = more thrust, more speed also = more drag. Depending on the speed you're flying at, the extra thrust may or may not overcome the extra drag. This is precicely why you have 'economy cruise' settings for each aircraft, most efficient climb profile, most efficient range, or most efficient endurance, etc.

[D-Scythe]

Let me quote Fox One for you then...

 

And again, we AREN'T talking about the take off. IIRC, the F-15 accelerates for a while on the deck AFTER take off before it pulls up and accelerates in the vertical. The portion of the flight envelope I'm talking about is COMPLETELY different from Fox's - in his argument, increased thrust generated from increased speed is basically a non factor, since we're only talking from 0 kts to take off airspeed (less than 200 kts any way you look at it).

 

My point is that the F-15 cannot accelerate vertically no matter what the airspeed.

 

And while more speed = more thrust, more speed also = more drag. Depending on the speed you're flying at, the extra thrust may or may not overcome the extra drag.

 

@ subsonic, sea level? You honestly think the extra thrust generated will not be enough to overcome any induced drag caused by the increased airspeed?

 

We're not talking Mach 1.8 here. Parasitic drag is a non-factor.

 

This is precicely why you have 'economy cruise' settings for each aircraft, most efficient climb profile, most efficient range, or most efficient endurance, etc.

 

How did you make that jump in logic? Fuel economy has NOTHING to do with the argument of thrust/ram-air vs. drag. There are a billion other factors to consider for fuel economy.

[GGTharos]

Try a pull up from 450kts or greater. TAS goes up, watch the external.

[SwingKid]

Coincidence? No. It is SAFE to assume that in takeoff condition supersonic fighter a/c have about 80% of the bench thrust. So the Su-27 will have about 20 tons takeoff thrust. There are also ejector nozzle flaps losses (airframe-mounted on MiG-23). Anybody not convinced yet is invited to have a more in-depth lecture of those two books, air intakes chapters.

 

Intersting stuff, thanks.

 

Since Yo-Yo has apparently decided that it is no longer stupid to draw conclusions from "totally different aircraft" - like your MiG-23 and -25 data towards the Su-27 - I wonder if it wouldn't be anymore too off-topic if, after 33 pages of discussion, we finally consider the Streak Eagle:

 

http://www.ecf.utoronto.ca/~pavacic/lomac/f15climb07.jpg

 

I invite you (or anyone) to estimate, whatever way you like, what minimum force is required to accelerate a 28,000+ lb mass through the speed of sound in 23 seconds?

 

-SK

[GGTharos]

Some 45000lbs of thrust. Math error somewhere in there.

[SwingKid]

Some 45000lbs of thrust. Math error somewhere in there.

 

I calculate about 42,000 lbf, assuming no "ram loss", no installation loss, no aerodynamic drag, no gravity in an 80-degree-climb, no transonic shock wave, no induced drag from a 5 G pull-up, no wheel friction or any other source of resistance. All of these need to be added into the total, to yield the Streak Eagle's total bench thrust of about 52,000 lbf. Therefore, ALL of these sources of resistance put together seem to average only about (52,000-42,000=) 10,000 lbf.

 

So, if ALL of the resistance comes from installation loss alone, and all other friction during flight is zero, then yes - we can estimate from this data that the installation loss is around 20%.

 

Right or wrong, I'm just putting that out there, as something to consider.

 

-SK

[Rhen]

Rhen - where are these numbers coming from? Is this what you measured in Lock On?

 

In Lock On, my 40,000 lb F-15C takes only 37 seconds to reach 350 kts under mil power. By my estimate, the real jet should take only 30 seconds. This seems to be giving us disagreement in our other numbers, when measuring from brake release.

 

-SK

 

Yes, these numbers are from LOFC. Everytime I fly a 40,000Lb aircraft I get within 4 sec of a minute when doing a MIL power takeoff. AGAIN!!!:P I must emphasize that you must PERFORM THE DASH-1 CLIMB PROCEDURE FOR THE APPROPRIATE THROTTLE SETTING!!!:smilewink:

 

The reason why you're getting 37 sec, is because you're holding the aircraft on the deck and not establishing a 10degree nose up rotation attitude and maintaining until 350KCAS.

 

What we're trying to do here is act like test pilots. We're comparing our flight test data against a true benchmark - the Dash-1 data - which is based itself on flight test data. The only way to validate any data obtained in our experiment is to compare equivalent aircraft with equivalent performance (that's what we're trying to establish, right? Whether the LOMAC F-15 is equivalent to the real thing.) by measuring data using equivalent technique.

 

There's one other thing that hasn't been established yet - the validation of the measuring equipment. To obtain correct data, we must all agree on the measuring instrument. I've been using the HUD airspeed and Mach # indications. IRL, the info on the HUD comes from the ADC, so they're KCAS. What I don't understand is what Yo-Yo's using to determine the intersection of 350KCAS and M0.9. From what I observed, he seems to be TLAR'ing to hit a precomputed KCAS at 5000ft altitude increments, since it looks a little fast.

 

I've attached a track for your convenience. All timing is made by hitting "Ctrl+Backspace" and recording brake release time and time to each altitude, then subtracting brake release time from that.

 

http://forum.lockon.ru/attachment.php?attachmentid=10771&d=1178246839

[Rhen]

For some reason, I'm having problems attaching the attachment to the post. 2nd try.

F-15 4040 1.zip

[S77th-GOYA]

IRL, the info on the HUD comes from the ADC, so they're KCAS. What I don't understand is what Yo-Yo's using to determine the intersection of 350KCAS and M0.9.

 

Thanks for the answer on what the HUD should display. The manual and Yo-Yo are saying it is KIAS, but Yo-Yo is using 350 knots on the HUD to do his climb profile.

 

I still have no answer as to why Yo-Yo is using a 39,000lbs aircraft for comparison to a 40,000lbs Dash-1.

[Rhen]

Thanks for the answer on what the HUD should display. The manual and Yo-Yo are saying it is KIAS, but Yo-Yo is using 350 knots on the HUD to do his climb profile.

 

I still have no answer as to why Yo-Yo is using a 39,000lbs aircraft for comparison to a 40,000lbs Dash-1.

 

It's because there's other charts that do 38k, 39k, 42k, but no 40k, like the acceleration charts. But regardless, the LOMAC F-15 should be tested over multiple gross weights to determine any performance discrepancies.