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Frostie
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I know this topic has had recent attention but something doesn't add up for me.

 

USAF F-15 instructors dictate 230 knots and 30 units AoA at 5000' AGL.

 

attachment.php?attachmentid=167589&d=1502824010

 

Does this mean a fully loaded F-15 can be at 120 knots, 70 degrees nose up at 36+ AoA and go on to perform a perfectly controlled loop without stalling. I am confused because for me this doesn't add up.

I'd just like to understand how this is possible.


Edited by Frostie

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I don't know what the Level 2 standard is but this doesn't sound like something done at full weight. Seems like something that should be possible, though, with limited weight. Been a long time since I've had F-15 seat time, though.

 

From my reading, starting at 400 kcas at 5000 altitude, pull up into 60-70° climb. When speed slows to 230 kcas pull over the top at 25-30 units AoA and recover.

 

EDIT: Unfortunately I don't know what 30 AoA units equals. It's not an AoA of 30°. Presumably it's more than that. If so, the wing should be stalled.


Edited by Ironhand

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EDIT: Unfortunately I don't know what 30 AoA units equals. It's not an AoA of 30°. Presumably it's more than that. If so, the wing should be stalled.

 

It's 20 degrees.

 

The document does state mil power. Is the F-15 in the video in AB?

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It's 20 degrees...

Oh! Each unit equals less than 1? 20° should be do-able, I think.

 

Translating into numbers that are more meaningful to me, that's starting at 1500 m altitude with an airspeed of 740 km/hr. Pull into a climb. As your speed deteriorates to 430 km/hr pull back and hold 20° AoA over the top. Recover (this word does suggest a stall or incipient stall) with an airspeed of 280 km/hr or more. I'll have to try that, when I get back to my computer.

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Oh! Each unit equals less than 1?

 

Specifically, units are actually equal to degrees, but offset by 10 (units - 10 = degrees). The reason for this was to avoid negative values for AoA numbers. Or at least that's what I've heard said on this on the forum here.

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Specifically, units are actually equal to degrees, but offset by 10 (units - 10 = degrees). The reason for this was to avoid negative values for AoA numbers. Or at least that's what I've heard said on this on the forum here.

Somehow, that explanation doesn't seem right. I did a quick search for other significant F-15 AoA units. Optimum cruise is: 12. Maximum endurance is: 14.5. That would make these numbers 2° and 4.5° respectively using that -10 system. And those are completely out-of-whack.

 

What seems closer to the truth is that 1 unit AoA equals .66 of a degree. Or 1.5 units make a degree. Then the other F-15 recommendations make sense (8° & 10° respectively).

 

EDIT: This conversion might be true for our F-15. That doesn't make it a standard, though. It might be that each aircraft type using units uses them differently.


Edited by Ironhand

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I don't see anything wrong with what the Tacview recording is showing other than a clear mistake by the Su27 driver. Like Ironhand already said, the F15 driver pulled over the top at its lowest energy level. Any other maneuver and it would have stalled completely and fallen out of the sky, but pointing the nose over the top and then down as he did prevented that and allowed the plane to recover. Look at the 19 second mark. The flanker driver makes a mistake and pulls 75 degrees AoA and completely tosses his energy advantage out the window at that point of the engagement. For this reason he also has to dive to recover before the F15 does, which gives the F15 driver the clear energy and altitude advantage from that point forward.


Edited by OnlyforDCS

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From the other thread, regarding AOA conversion.

The conversion for AOA to units in the eagle is

AoA[units] = 0.7728*AoA[Degrees] + 12.22

 

https://forums.eagle.ru/showpost.php?p=3223400&postcount=35

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I don't see anything wrong with what the Tacview recording is showing other than a clear mistake by the Su27 driver. Like Ironhand already said, the F15 driver pulled over the top at its lowest energy level. Any other maneuver and it would have stalled completely and fallen out of the sky, but pointing the nose over the top and then down as he did prevented that and allowed the plane to recover. Look at the 19 second mark. The flanker driver makes a mistake and pulls 75 degrees AoA and completely tosses his energy advantage out the window at that point of the engagement. For this reason he also has to dive to recover before the F15 does, which gives the F15 driver the clear energy and altitude advantage from that point forward.

 

What has this thread got to do with the Flanker, if you ignore the Flanker your post amounts to the F15 pulled up and over the top. Good observation.

Now pay attention to the details, the F15 starts its climb at 150 knots at 34 AoA and continues this posture to complete a controlled loop. Try doing that in any other aircraft.

 

The issue is how did the F15 get from level flight 34AoA 150knots on to do a complete 360 not why did the Flanker lose.

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The issue is how did the F15 get from level flight 34AoA 150knots on to do a complete 360 not why did the Flanker lose.

 

That....

 

How did the F-15 manage to pull all that with a so low speed and high angle of attack.

 

Nothing to do with the Su-27. Everyone now just forget the whole Su-27, and if you can't, then at least pretend that there is a F-18 instead Su-2So 7 and still focus what the F-15 did.

 

So the F-15 should go from level high speed to 60-70 degree high-nose vertical climb, wait that speed drops to 425km/h (230 knots) and then pull nose over and recover.

Now the F-15 started to pull up at 320km/h and is at 250km/h when started to go past 60-70 degree

 

So the fully loaded F-15 (what fully loaded means in this case?) manages to pull a maneuver about at 200km/h slower speed (50% of the mentioned in the document) and recover nicely from it.

 

You did something correctly ;)


Edited by Fri13

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Could you please add the track and the tacview file instead of the video? I want to check the whole telemetry data.

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From the other thread, regarding AOA conversion.

 

 

https://forums.eagle.ru/showpost.php?p=3223400&postcount=35

Thanks I didn't see that post in the other thread.

 

I now had a chance to watch the video. I don't have much of a feel for Kph but your Mach number did get pretty low. AoA was above 30 degrees consistently, which should start to show wing rock according to the DCS F-15 manual.

 

It wouldn't be unreasonable for the plane to fall away from its initial attitude given that info.

 

DCS does simulate the wing rock last I checked, but the intensity never felt really strong to me.

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Frostie, any idea what your weight was at the time? Starting the loop at 150Kts is really slow, but could be possible if your T:W was high enough at the time. Burner or mil thrust?

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Frostie, any idea what your weight was at the time? Starting the loop at 150Kts is really slow, but could be possible if your T:W was high enough at the time. Burner or mil thrust?

 

I'll have to dig around and see if I still have the track, I should imagine at least half a tank with 4 MRM and 2 aim9. I do remember that it was full rudder with burner all the way.

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FWIW, here are the tacviews and trks for 2 flights. One light (50% fuel, no suspensions) and one "heavy" (6 Aim-120 and 100% fuel). The profile was do-able in both. Actually a bit easier in the "heavy" version because there's less of a tendency to over pull.

Flight Heavy.trk

Flight Light.trk

F-15 Flight.txt.zip

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I'll have to dig around and see if I still have the track, I should imagine at least half a tank with 4 MRM and 2 aim9. I do remember that it was full rudder with burner all the way.

 

Alright- I was in the middle of doing the equations at full up given your original comments, and will present them here, but dropping two AMRAAM and JP down to half a tank substantially changes things; you'll see where I switch gears below:

 

Does this mean a fully loaded F-15 can be at 120 knots, 70 degrees nose up at 36+ AoA and go on to perform a perfectly controlled loop without stalling. I am confused because for me this doesn't add up.

I'd just like to understand how this is possible.

 

The first thing to understand is the noted exercise is a control lesson during F-15 conversion. This is a test of the student's competency in AoA control early on in their training; it should in no way be considered indicative of a maximum performance limitation, although proficiency at the 230 knot standard is important because it's a direct lead-in to their chandelle reversal technique.

 

Of most importance here, at least as far as I can surmise, is Vs, which is derived from weight. If you're topped off, with a 6x2 configuration, you're right around 44500 lbs if you've been running around in blower for a couple of minutes (I can't tell based on the track timing, so I'm giving credence to the "fully loaded" statement). Call it 44850 for good measure, and raw calculated Vs at that weight, using:

 

Vs = 17.2*sqrt[wt/(density ratio*608*Clmax)], or

Vs = 17.2*sqrt[44500/(0.867*608*1.33)], equals

137.6 knots

 

(note: density ratio used is ICAO's 70's standard for 5000', as it's one I know by memory)

 

The best way to proof this when dealing with an aircraft that isn't in straight line 1.0G flight is to note the G's used and confirm what the speed is where its being used. So let's say 2.0G, based on your initial pull to the vertical; the first speed a given G load becomes available is Vs times the square root of the desired load. So looking for 2.0G, we need the following:

 

x= [sqrt(2.0)]*137.6, or 1.414*137.6, so 2.0G is available as of 194.52 knots.

 

On the surface, that seems like a concern when reviewing the video, where I'm seeing a point where Tacview registers 2.2G at 192 knots, but there are a couple of things I can't tell from the video- the first of which is the actual weight. Lower your weight around 2300 lbs to 42500 lbs, and that shaves four knots off your Vs- now you're looking at 133.8 knots vs 137.6, and 2.0G is available at 189 knots- five and a half knots slower. That's about halfway to where you should see 2.2G (198, with the square root of 2.2 being 1.483).

 

The next proofing point for this becomes a factor is when you're nose high at 89 degrees pitch, and it's reading 1 G at 126 knots.

 

I'm going to stop the original example numbers, based on the suggestion there was substantially reduced weight versus my original calculation. However, I'm going to leave it there as an example of how to track what's happening, and from this point use that 1.0G point as a proofing value:

 

60% gas, four AMRAAM and two AIM-9s are 40,056 lbs.

 

Vs = 17.2*sqrt[40056/(0.867*608*1.33)]

Vs = 17.2*sqrt[57.1338]

Vs = 17.2*7.5586

Vs = 130.009

 

Again, looks like an issue. However, remember my note about using the 5000' density? ICAO's current 3000' density:

 

Vs = 17.2*sqrt[40056/0.9151*608*1.33)]

Vs = 17.2*sqrt[54.1307]

Vs = 126.54

 

And note- that's without compensating for the difference in temperature against the standard 15 degrees C at sea level(!), which would lower Vs to a point that we're not talking about a half knot variance.

 

Feeling better? As someone who has to track for simulation down to two knots on a slide rule, I myself am very confident with what is exhibited from a derived track on Tacview as illustrated.

 

Now, how did you do it?

 

You followed the limiter warning and got exceedingly lucky on your timing. It's also important to note that once you get your velocity above roughly sixty degrees, a similar function to what happens with angle of bank and required speed to maintain a level turn occurs, only it's trading the wing for your pure thrust component. So long as your net acceleration against gravity remains a positive number, you climb. And so long as you don't invoke more G than the wing has available, which you didn't, you don't stall- you're just no longer stalling at 1 G like you would in level flight because you're not in level flight.

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Fancy-pants calcs - I like performance graphs :thumbup:

Stall.thumb.jpg.afbdf082eb39fff539e049286694bd3b.jpg

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.

 

LF thanks for that awesome response, I made this thread hoping for your input, I understand we are borderline stall but not there with the speed but what about the inclusion of such high AoA, shouldn't there be wing rock at least. I get the feeling the FM hits a limit with full thrust and rudder which makes this maneuver so easy to go through.

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Regarding wing rock, first look here, PDF page 62, document page 51:

 

http://www.dtic.mil/dtic/tr/fulltext/u2/a256613.pdf

 

The gist of the full text is that wing rock begins in the F-15 around thirty units AoA, and is a function of speed, and gravity causes imbalance which increases the force.

 

When modelling, and when actually flying the aircraft, because a pitch up removes the gravity imbalance from the equation, its affect is essentially negated, so the rock is lessened (lowered amplitude). Plus you're slower, so it's already less pronounced.

 

It's been a while since I read the full document, but I would surmise that because you were so close to the limit you were at the point where rock, while present, should be minimal. How DCS calculates for such is unknown. At the same time, depending on the amplitude it may have only required minimal input to counteract, which may be smaller than the resolution of Tacview can show (consider how crazy roll maneuvers can look in some circumstances).

 

Understand- I don't doubt when you say you didn't feel a need to compensate for the rock; I just don't have the information available to get a better feel here- it may be a point that Belsimtek couldn't compensate for in their model, it may be modeled correctly and natural fingertip input may have been enough under the circumstances to take care of it. A track would help, if only to see the control input display.

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Now, how did you do it?

 

You followed the limiter warning and got exceedingly lucky on your timing. It's also important to note that once you get your velocity above roughly sixty degrees, a similar function to what happens with angle of bank and required speed to maintain a level turn occurs, only it's trading the wing for your pure thrust component. So long as your net acceleration against gravity remains a positive number, you climb. And so long as you don't invoke more G than the wing has available, which you didn't, you don't stall- you're just no longer stalling at 1 G like you would in level flight because you're not in level flight.

 

Outstanding post! Thanks for your efforts calculating & writing that! As an aside, this sort of thing is precisely why I love being a member of the DCS community :thumbup:

 

On subject: your calculations more or less confirmed what I thought might be happening: that the loop was possible due to good timing (deliberate or accidental) and T:W.

 

Again, great post. Wish we had more posts of this quality and fewer 'I think it's wrong because... reasons' posts.

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