Under-modelled power

Um, I really don't think there was a Dash 100 tweaked to produce 29K lbs of thrust. I know that there are the Dash 220E and the Dash 220E+, derived from the 100 and 220 turbofans with -229 tech, but I don't think either of these engines were retrofitted to F-15s in the late 1980s when the F-15E program was underway. Reason for this is because the update incorporated IPE engine technology into the older engines, but the Dash 229 didn't exist back then.

Just looked it up, there was a test F-15D fitted with a pair of uprated -100s. They were uprated to 110% and had a life of around 150 hours. -100s uprated by 110% would give around 26,200lbs each, not quite the 29k I was stating.

Note that in this track, the 15 (clean w/50% fuel) can climb straight up after take off but it doesn't accelerate. In fact it does the opposite.

I'm perfectly willing to accept that the engines don't produce full thrust at take-off speed, however.

Check this little jewel out.

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

It's a java applet and it has the F100 engine, but not the old -100 version.

Thanks for that link. Just loaded up the F100 model and it produces over 33k thrust in AB. Must be based on the new F100-PW-232, if it is the 229 then the 229s in service must be slightly derated to give longer life.

Explain in simple terms why any free object weighing 35,000 lbs (a clean F-15C w/50% fuel load) can't be accelerated straight up by 47,000+ lbs of thrust (2xF100-PW-100 afterburning turdofans).

In order to get off the ground, an F-15C needs to be doing roughly 300 km/h.

Very roughly speaking, we can estimate the drag by assuming a section-referenced coefficient Cd = 0.15, a section area 8 sq. m, to get:

Fd = 0.15 * 0.5 * 1.225 * (300/3.6)^2 * 8 * 2.2

~= 11000 lbf drag

If the engines are producing 47000 lbf thrust to lift a 35000 lb aircraft AND overcome 11000 lbf drag, that only leaves about 1000 lbf excess thrust for acceleration.

As the aircraft gains speed, the drag increases with the square of the speed, and as the aircraft increases altitude, the engine thrust decreases.

So, while these are very rough numbers, you can see how any vertical acceleration is likely to be a very transient phenomenon, that only happens for a moment near the ground, at very low speed.

For comparison, consider this video of an F-15C "Viking takeoff" from Boeing's site:

http://www.boeing.com/defense-space/military/f15/video/15cvklbk.mov

http://www.boeing.com/companyoffices/gallery/video/index.html

Notice that he appears to be flying at a 45-degree climb angle, maintaining a constant speed, exactly according to the performance climb profile described at the top of the chart.

EDIT: note, I think there is an error in my drag calculation somewhere, it seems too high, but the general idea is still true

-SK

An empty F-15C is 28,000lbs + 13,000lbs full internal fuel, thats around 41,000lbs basic takeoff weight. A pair of 220Es will give just under 47,000lbs thrust while the -100s will give around 48,500lbs.

SwingKid, I'm glad you agree that the F-15 should accelerate straight up. Now, as to your drag calculations, using this

and converting your figures into the correct units I get a force of 1147.1 lbs.

300 kph = 273.3 ft/sec

p=0.002378 slugs per cubic foot

8 square meters = 86.111 sq feet

Cd = .15

D = .5*.002378 * 273.3^2 * 86.111 * .15 = 1147.1

Now, would you care to contend that the F-15's engines rapidly lose thrust at altitudes under 10,000 feet or are you willing to admit that the power available to the F-15 in LOMAC is considerably lacking compared to its real life counterpart?

He's not necessarily wrong.

The A-10's engine upgrade for exmaple doesn't bring it increased overall thrust. It increases thrust AT ALTITUDE ... so yes, the F-15's engine -may-be losing a lot of thrust with altitude in a similar fashion.

The trouble is of course that what we really need are the figures for the 220 and 220E at altitude ...

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

Plug in the thrust you want and adjust for speed and altitude.

He's not necessarily wrong.

 

The A-10's engine upgrade for exmaple doesn't bring it increased overall thrust. It increases thrust AT ALTITUDE ... so yes, the F-15's engine -may-be losing a lot of thrust with altitude in a similar fashion.

That's a laughable leap in logic. The Ford Escort has been moddified to give it better highway gas mileage so the Volkswagon GTI -may- get terrible highway gas mileage in a similar fashion.

The LOMAC F-15 doesn't have enough thrust to vertically accelerate at 3000 feet.

He's not necessarily wrong.

 

The A-10's engine upgrade for exmaple doesn't bring it increased overall thrust. It increases thrust AT ALTITUDE ... so yes, the F-15's engine -may-be losing a lot of thrust with altitude in a similar fashion.

 

The trouble is of course that what we really need are the figures for the 220 and 220E at altitude ...

You can also check Maverick's and Raptor's HFFM work with Falcon 4.0. I believed that they had access to some very detailed sources for their project.

Thanks for that link. Just loaded up the F100 model and it produces over 33k thrust in AB. Must be based on the new F100-PW-232, if it is the 229 then the 229s in service must be slightly derated to give longer life.

Um, not exactly. The figures for thrust quoted for engines are usually in a specific scenario - the amount of thrust produced by engines are highly dependent on factors like Mach and altitude. An F100-PW-220 can thus produce 104 kN of thrust in one regime and just as easily produce 130 kN of thrust in another.

Now, as to your drag calculations...

I get a force of 1147.1 lbs.

Agreed, I forgot a unit coversion to divide by (9.8 N/kgf), my estimate should be the same as yours. Thanks for the correction.

Now, would you care to contend that the F-15's engines rapidly lose thrust at altitudes under 10,000 feet or are you willing to admit that the power available to the F-15 in LOMAC is considerably lacking compared to its real life counterpart?

Good question. I admit I don't know what the exact reduction of thrust with altitude is but the applet you linked is very interesting!

Now let's condsider 350 kts. The drag should be proportional to the square of the speed, so at 350 kts, the drag should be about 4.5 times what it was at 300 km/h - around 5100 lbs. Ok?

So, let's take a 35000 lb F-15, add 5100 lbs drag = ~40000 lbs.

So in maximum thrust with 47000 lbs, at what climb angle will the F-15 pilot be able to "maintain 350 KCAS," according to the instructions for a performance climb?

It will be impossible - even flying vertically, he'd just accelerate with 7000 lbs excess thrust. In order to "maintain" such a speed, there must be some climb angle at which the F-15C will cease to accelerate, when it's flying 350 kts.

So, I think that in addition to the engine thrust loss with altitude, I've just simply underestimate the Cd or aircraft area somehow, our new drag figure must be too low now.

I admit I'm not sure myself, in fact I brought this topic up for discussion elsewhere recently convinced that the Eagle should accelerate in the vertical. I've only recently become more skeptical. Thanks for the help with the math, it's much appreciated.

-SK

Don't forget the thrust goes up with speed and the 47000 lbs I used was conservative even for stationary.

Ok, I just ran some more tests, and...

I'm confused.

I don't see any evidence, that Lock On's F-15C engines ever produce more than 41000 lbf thrust, if that.

On the other hand, the time-to-height test matched very well.

What's going on?

-SK

I'm guessing it's not based on a real world atmosphere/gravity model. Either that or the F-15's Cl might be way too high. Maybe a stall speed test?

I'm afraid Lock On has nothing to do with it. Simply, the Dash-1 manual seems to be - lying!?

Take a look at the area circled in red in this distance-to-climb chart:

If there was any weight for which the aircraft could climb vertically at 350 kts, there should be a horizontal section of the curve here, to indicate altitude gained for zero distance traveled. There isn't one. The best it says you can do is about 5000' per 1 nm horizontal distance, all the way down to sea level. That can't be right, can it? How on earth can a 30,000 lb F-15 experience more than 17,000 lbs drag at 350 kts? What am I missing?

Anyway, Lock On's F-15C matches this chart fairly well, but the chart seems to describe an aircraft with only about 41000 lbf bench engine thrust.

-SK

Maybe the F-15 in lomac simulates the fact that most military engines in peacetime are underpowered to lengthen their expected lifetime. Anyway I'm also quite sure that in wartime these restrictions would be removed...

Anyway, for all those who want a bit of F-15C airshow action (included a brutal takeoff roll), here's a movie I downloaded from emule. It's not mine, but I didn't stole it, just downloaded from emule, so I suppose it's something shot by a person who wanted to share it with the Internet community.

It's a splitted rar (because my hosting service doesn't accept files larger than 10 MB), make sure to download both parts before trying to extract the movie.

here we go:

http://digilander.libero.it/airsuperiority/f15%20eagle.part1.rar

http://digilander.libero.it/airsuperiority/f15%20eagle.part2.rar

enjoy ;)

A note on this: CL and CD values are only one of the very few aspects of dynamic calculations. Falcon 4 does this bu ignored Inertia and moments of inertia and it ended up with a cardboard plane flight model.

I don't doubt your word, but a flight sim as accurate as Lock On needs more precise numerical data.

For example - how does Kula66 know the fuel load? The F-15C airshow routine and Viking takeoffs are reportedly done with full fuel.

How long was the aircraft going vertical, to what altitude, at what speed, and how do we know it was accelerating?

Note that the climb profiles shown above are for 350 KCAS. Whatever the precise details, vertical acceleration seems to be a very transient condition, that only happens near the ground imediately after take-off.

 

-SK

SK, Thanks for posting the graphs.

I have never said it would climb vertically ....

I have no idea of the fuel load, it was early-80s, sunny day, mid-summer at Mildenhall AB. The F-15 had performed a dull display because low cloud, the cloud cleared and a Mig-29 had just done a fantastic display! Then the bumped the display and the Eagle did a full AB, brakes on t/o and was off the runway and climbed near vertical in a few secs and just kept going until it was out of sight.

I know this is not good enough info for a sim ... but you are not able to re-create this in LO regardless of the fuel load. I get to 175ish mph and then start slowing, i.e. it isn't even close.

To be honest SK, I can't tell what that chart is telling me.

Interesting discussion you've got going here, however you are operating under several misconceptions::doh:

1) There's no such thing as a 38,000 Lb F-15 with full internal fuel, unless it's stripped down. The basic empty weight (Aircraft + Unusable fuel + Oil + Pilot) is around 28-29,000 (varies with each aircraft, but we flightplan a generic aircraft). Add to that full internal JP-8 which is roughly 14,000 Lbs & that gives an aircraft weight of 42-43,000lbs. BTW, can you even remove the wing pylons in LOMAC and do they account for any drag or weight to the aircraft (this will affect the climb - at least IRL). Weight of the aircraft with BEW+wing pylons+4LAU-114racks+Full internal fuel is roughly 44,000lbs (my memory is a little stale here but I think my numbers are correct).

2) The above climb charts are based on flying the aircraft in a certain manner - the dash-1 specific climb schedule. Hold brakes, run engines up to 80% RPM, release brakes, throttles into burner, rotate at 120KCAS to 10 degrees pitch, airborne at computed takeoff speed (or basically whenever the aircraft decides to fly - we compute this speed), gear and flaps up after airborne, accelerating to 350KCAS, hold 350KCAS to M0.95, hold this or 40degrees nose up, whichever comes first.

3) if you're using that chart to find a specific time to climb, you must hold your pitch THROUGH that altitude - i.e. don't try to level off, but maintain M0.95 through the desired altitude, just stop timing when that altitude is achieved.

4) the chart is based on a Standard Aeronautical Day = 59oF/15oC, 29.92inHg/1013.25mb, with standard lapse rate of 3.5oF/2oC up to 36,000ft. Is this even modeled in LOMAC?:huh: If you're flying in the Crimea in the summer, you must use the bottom left temperature corrections above standard day temperature.

5) Did you remember to add 30 seconds to whatever the chart gives you as a time to climb IF you're looking for time to climb from BRAKE RELEASE. This is a critical point, as the number for the chart only gives you time from attaining 350KCAS to level off.

6) Yes the F-15 can take off and climb straight up - however, to do this we do the above procedure, but hold the aircraft close to the ground (anywhere from 5ft to 50ft pilot pref.) and accelerate to 350KCAS (or more for a more spectacular climb), then pull the nose smoothly into a 90opitch attitude. The engine thrust will fall off as you climb, parasite drag will begin to decrease with decreasing airspeed & induced drag is negligible (pilots are aware of these things - in a big-picture sense, but don't need to know the numbers, just the shape of the drag curve.:music_whistling: )

That's about all i can think of right now.

Rhen, have you looked at the track I posted? The fuel load is 50% and a gross weight of ~35,000. And no, we can't remove the pylons in LOMAC so the drag is as low as the game will permit.

Can you give any indication as to loss of thrust at altitude? Maybe a % loss at 10, 20 and 30 K?

Interesting discussion you've got going here, however you are operating under several misconceptions::doh:

Interesting info Rhen ... now can you do the same in LO?

Here it comes... barrage of questions for Rhen.

Here`s mine: Has anybody ever used that little "Max power" switch in an

Eagle? (IRL)