Acceleration & AoA - Some Observations.

[Deano87]

Hey All.

 

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I don't want this post to be taken as me slamming on EDs representation of the F-16, I understand that its WIP and subject to change. I truly love the work that ED have done so far on the Viper, Flying it in VR is basically a dream come true for me, so I'm only interested in improving the sim as best I can. The intention of this post is to try and incite mature conversation about the topic from people more knowledgeable than I, that is all.

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I was recently browsing Youtube and found this fun F-16 HUD tape of an F-16N which includes a Mach 2 run. 11min to 14mins

 

 

This piqued my interest and I thought I'd have a go at flying a similar profile in the DCS F-16 and see how it compares. Now I fully understand that this isn't an apples to apples comparison because the F-16N is a different beast to a Block 50 Jet, BUT still I figured that any weight or thrust difference between the two wouldn't produce that big of a difference, at the very least it would produce some interesting results that might be worth discussing. The Jet in DCS was setup clean with no pylons and full internal fuel.

 

 

Well... thats interesting.

 

My non-perfect flying aside, You can see that the DCS version keeps up pretty well until about Mach 1.35 at which point the IRL jet starts to pull away... and keeps pulling away.

 

By the time the simulated jet is at mach 1.6 the real jet is at 1.83.

By the time the real jet gets to mach 2, I've just passed 1.75.

But what is really telling is how long the real jet takes to go from mach 1.80 to 1.90, which only takes 16 seconds. The DCS jet does the same in 1 minute and 4 seconds!

 

So I think there is some high mach drag issue here, although the F-16N is lighter it should also have less thrust then the Block-50, And I wouldn't think that any of the airframe additions (IFF interrogators , big intake, bulged gear doors etc) to the Block-50 would increase drag this much? But perhaps I am wrong?

 

Also While I was editing the video together another thing I noticed was the difference in apparent AoA between the two aircraft. I calibrated the two hud images using the pitch ladder (actually pitching up in both cases as it was easier to get a clear image of the Pitch Ladder, but not shown in the image below) so I could attempt to get a readout of the AoA between the two videos. Here is what I found.

 

IMAGE UPDATED TO V2 - ERROR WITH G LOADING CORRECTED - ORIGINAL IMAGE STILL ATTACHED BELOW

 

I found the fact that in both cases the sim jet had an AoA 133% of whats shown in the F-16N video very interesting.

 

- EDIT -

Pointed out to me that my G loading in the top and bottom images were not 1G, Corrected that issue, now the AoA numbers are 133% and 126% of IRL respectively.

- EDIT -

Could that be accounted by the weight difference between the two aircraft? Or does it point to the DCS F-16 being down on lift in general? These are questions I don't know the answer to. But are interesting to discuss, for sure.

 

I have subsequently noticed that the CAS displayed in my sim footage is a fair chunk lower then in the F-16N video, This is no doubt due to me not adjusting the temperature to better represent the conditions of the real HUD tape. I intend to re-do the speed test with corrected conditions to see if it makes any difference. I don't imagine it will change the lack of acceleration at high mach, but at least it's another variable to disregard.

 

Anyway, apologies for the long and rambling post. Hopefully some of you have found this interesting. Anybody who has more intimate knowledge of how lift relates to Aoa in regards to the F-16, I'd be interested to hear your thoughts.

 

Cheers

D

Edited January 13, 2020 by Deano87

[RaisedByWolves]

I have nothing to add, other than I wish I had this much time on my hands.

[Deano87]

This is what happens when you get the flu. Lol.

[The Falcon]

Very interesting

Edited January 11, 2020 by The Falcon

[Gnadentod]

That's the spirit.

[jcdata]

I believe the naval variant has titanium wings. IIRC the FADEC in the later-model is a reason why there's a time difference

 

Sent from my SM-N950U using Tapatalk

[Deano87]

I believe the naval variant has titanium wings. IIRC the FADEC in the later-model is a reason why there's a time difference

 

Sent from my SM-N950U using Tapatalk

 

The F-16N has titainium wing mounting plates, not complete ti wings. Afaik.

[Bouli306]

In the last picture you are flying 4000ft lower than the F-16N. Also did you remove the pylons underneath the wing and centerline? Did you set 1013Mbar or 29.92Hg in the altimeter? If you want to compare you really need to have more info than you have...

[Deano87]

In the last picture you are flying 4000ft lower than the F-16N. Also did you remove the pylons underneath the wing and centerline? Did you set 1013Mbar or 29.92Hg in the altimeter? If you want to compare you really need to have more info than you have...

 

Yes I did remove all the pylons.

 

Altimeter was set to 29.92, yes.

 

I was 4000 ft lower because if you watch the video I was holding the same dive angle as the IRL pilot did, but because it took a lot longer to get to the same to the same speed I ended up much lower. I can try and recreate it more accurately in the future though.

Edited January 11, 2020 by Deano87

[Frederf]

Gross weight can make a significant difference in cruise AOA. I would be curious what are the AOAs for DCS in matching conditions at a wide range of GWs. The video might be a lightweight condition.

[Deano87]

Gross weight can make a significant difference in cruise AOA. I would be curious what are the AOAs for DCS in matching conditions at a wide range of GWs. The video might be a lightweight condition.

 

That is a good point. Is the connection between AOA and lift linear? If we presume that speed doesn't change then If say 4 degrees of AoA provides 35000lb of lift (level 1G flight for a 35000lb aircraft) then does it follow that 8 degrees would provide 70000lb of lift? or 6 degrees would be 52500? obviously at some point you reach a critical angle of attack where the wing and any other lift producing part of the airframe stalls but if we are talking about cruise angle of attack, does anybody know how that relationship works?

Edited January 12, 2020 by Deano87

[Frederf]

Near between zero and LDmax yes more or less linear. Nearer to stall it gets more curvy. I wouldn't trust in linearity beyond the single digits. I'm just curious if it's possible to replicate the video results at some reasonable GW in DCS.

[Deano87]

Replicate the video results, as in the AoA seen in the video?

 

I was in a clean jet with no pylons and just internal fuel. I can bring the internal fuel down a bit but I’d already burned a fair chunk by the end of the Mach 2 run. I’m not really sure how I can make it much lighter? Drop the gun ammo and Chaff/Flair I guess and just put enough fuel in to do the run without flaming out. I’ll try tomorrow.

Edited January 12, 2020 by Deano87

[bkthunder]

Acceleration is off by a huge amount, as reported here:

https://forums.eagle.ru/showthread.php?t=252003

 

I ran this test several times, generally at each update, and there have been no changes yet.

The engine also behaves weirdly at the moment.

 

Whether it is a matter of drag, or thrust, or both, only ED knows. I hope they give this stuff some attention and priority, because ti would literally suck to have a botched FLCS and FM modeling, it becomes especially noticeable when there's another sim that is pretty spot on...

[Robin885]

This should be a model for in-game observations. I hope your thread will be seen by ED.

 

 

I do have a feeling that the DCS F16 has a too big drag coefficient, and that may be where your observation comes from. I don't have any IRL experience or extensive knowledge to back this up, but from what I read and heard the Viper should be one hell of a monster in 2 circle fights, essentially having a massive sustained turn rate around 400ish knots. But it just looses so much energy as the AoA climbs, it feels like a massive aerobrake.

Edited January 12, 2020 by Robin885
Writing

[The Falcon]

At what external temperature were you when you reached 1.80 mach?

Edited January 12, 2020 by The Falcon

[Deano87]

At what external temperature were you when you reached 1.80 mach?

 

It was a standard caucuses day in the Mission Editor, I hadn't adjusted pressure or temp.

[mvsgas]

Curious, what on the video tells us the aircraft the HUD video was recorded was an F-16N? USN has used the F-16N but also started to receive F-16A back in around 2002. So how can we tell?

 

Also, are we aware of all the differences between this F-16's? Analog flight control vs digital, different engine and equipment (changing weight and balance, performance, etc.)

 

DCS F-16C is still WIP. Why not wait until DCS F-16 is completed before comparing and speculating?

[Deano87]

Curious, what on the video tells us the aircraft the HUD video was recorded was an F-16N? USN has used the F-16N but also started to receive F-16A back in around 2002. So how can we tell?

 

Also, are we aware of all the differences between this F-16's? Analog flight control vs digital, different engine and equipment (changing weight and balance, performance, etc.)

 

DCS F-16C is still WIP. Why not wait until DCS F-16 is completed before comparing and speculating?

 

Nothing in the HUD says its an F-16N, but the video uploader is retired Navy Pilot Frank Furbish who was the pilot in the video. He has confirmed to me that the aircraft was an F-16N and clean with no pylons.

 

I'm fully aware of the differences, I mentioned that it wasn't an apples to apples comparison in the post. But at the end of the day, we are not comparing pitch rate, or turn radius or how the aircraft behaves at the edge of the envelope. We are looking at a simple acceleration test. Thrust vs Drag. I would expect the Block 50 aircraft to be slower then the F-16N just due to weight difference and general airframe cleanliness. But currently the Block 50 is not even in the same ballpark.

 

As far as why I'm making the comparison in the first place? Well I would have thought that was obvious... Its because its unfinished. Its much better to make problems obvious to the devs as early as possible in the development process. I haven't seen any other F-16 HUD tapes with Mach 2 runs in them, If they are out there, then I'd love to see them. But this is the only one that I could personally find. And its a data point we can work from. It also gives us a measuring stick to compare against as development continues, I can test it again in the future and see if there is any improvement/change.

 

Thanks for your input though.

[mvsgas]

Nothing in the HUD says its an F-16N...

Fair enough

 

I did not match the F-16 HUD video exactly, on the video from the moment the pilot said he will plug in the AB, mach .71 to mach 2.00 was 3:10.

 

But using Open Beta version 2.5.5.41371 (NTTR map, dynamic weather) it took me 1:49 from mach .88 to 2.00 reaching max of 2.20 mach on the first run when doing a cold start.

 

and 1:29 from mach 1.29 to 2.0 on the second run

 

The track is to big when starting on the ground, but here is the missions

F-16 mach 2 run.miz

 

If I start on the air, it took 5:22 sec from mach .77 to 2.0 here is the track

Speed run to mach 2.00 starting on the air .trk

 

There is something else causing the acceleration problem.

 

Edit

The weather is the limiting factor. Re-flew starting from the air since I did not realize the pylons where added. The time to mach 2.00 was around 3:42 to 4:14. Took, dynamic weather off and added tail wind and head wind at 25 knots with same results. Tried starting from ramp, with same times to mach 2 from mach .71. With dynamic weather, occasionally you get in a wind that will allow you to accelerate faster. All this was done in -1C temp in NTTR, 12 Januray 2020 in game date.

Edited January 13, 2020 by mvsgas
aditional info

[HILOK]

hi deano, i appreciate your efforts. as much as you, i just want the viper flight model to be as accurate as possible : )

when comparing both HUD pictures on page 1, i believe that the pitch ladders should be graphically calibrated as well, cause they don't match (see magenta lines in pic below). and that means the AOAs can't be compared graphically. but the AOAs could be deduced in each pic separately. that's what i did (see green arrows). either method isn't super accurate anyway, but i think it serves for the purpose.

so, although the corrected difference in AOA is not as great as before, it is still significant in my opinion: 13%

 

thanks,

HiLok

[Deano87]

hi deano, i appreciate your efforts. as much as you, i just want the viper flight model to be as accurate as possible : )

when comparing both HUD pictures on page 1, i believe that the pitch ladders should be graphically calibrated as well, cause they don't match (see magenta lines in pic below). and that means the AOAs can't be compared graphically. but the AOAs could be deduced in each pic separately. that's what i did (see green arrows). either method isn't super accurate anyway, but i think it serves for the purpose.

so, although the corrected difference in AOA is not as great as before, it is still significant in my opinion: 13%

 

thanks,

HiLok

 

Good point Hilok, but I actually calibrated the pitch ladders using a 3rd screenshot taken while both aircraft were climbing, the pitch ladders in the climb are much clearer and gave me excellent accurate reference for scale, from the horizon to 20 degrees. However due to both aircraft pitching up and down a lot during the flight I couldn’t actually get a climbing shot where we were both at a stable 1G, at the same Mach during the climb so I omitted it from the comparison and only used it as a calibration tool.

 

The pitch ladder has no relationship to AoA. AoA is the distance between the gun cross and the Velocity vector. I only used the pitch ladders to scale to the two images correctly so I could make the measurement. Beyond that the position of the pitch ladder doesn’t matter.

 

Looking at you pic, I’m not quite sure why you’ve measured between the gun cross and the negative 5 degree line on the pitch ladders? Notice on the top left image the horizon line is out of view, but you can also see from the two purple lines on your image that my pitch ladders are quite well matched in size, but are in different places, which doesn’t actually matter when calculating AoA as that’s independent of pitch angle. I used the pitch ladders to calibrate the size of the images but then ignored it after that, their exact relationship to the gun cross in regards to AoA is not really relevant as far as I can see. Can you explain what you mean?

Edited January 13, 2020 by Deano87

[Deano87]

Fair enough

 

I did not match the F-16 HUD video exactly, on the video from the moment the pilot said he will plug in the AB, mach .71 to mach 2.00 was 3:10.

 

But using Open Beta version 2.5.5.41371 (NTTR map, dynamic weather) it took me 1:49 from mach .88 to 2.00 reaching max of 2.20 mach on the first run when doing a cold start.

[ATTACH]224933[/ATTACH]

 

and 1:29 from mach 1.29 to 2.0 on the second run

[ATTACH]224935[/ATTACH]

 

The track is to big when starting on the ground, but here is the missions

[ATTACH]224934[/ATTACH]

 

If I start on the air, it took 5:22 sec from mach .77 to 2.0 here is the track

[ATTACH]224936[/ATTACH]

 

There is something else causing the acceleration problem.

 

Edit

The weather is the limiting factor. Re-flew starting from the air since I did not realize the pylons where added. The time to mach 2.00 was around 3:42 to 4:14. Took, dynamic weather off and added tail wind and head wind at 25 knots with same results. Tried starting from ramp, with same times to mach 2 from mach .71. With dynamic weather, occasionally you get in a wind that will allow you to accelerate faster. All this was done in -1C temp in NTTR, 12 Januray 2020 in game date.

 

Very interesting post!

 

I’m not sure how wind would help and aircraft accelerate, the aircraft doesn’t know or care it’s moving through wind, it should effect its ground speed and track but nothing else.

 

In cold weather I’ve experienced a strange bug with the F-16 a handful of times where the airspeed indication will go haywire, displaying that I’m accelerating to mach 2.20 when I’m clearly not going that fast. I haven’t been able to replicate it reliably but I think you might be running into it. During the runs when you accelerate significantly faster than normal can you confirm you’re actually reaching the speed displayed in the HUD? Either F2 speed readout or create a track and read the Mach figure from Tacview?

 

Thanks for your time investigating.

Edited January 13, 2020 by Deano87

[The Falcon]

I think the outside air temperature changes the mach number, so if you can try to lower it and then see what happens

[bkthunder]

Edit

The weather is the limiting factor. Re-flew starting from the air since I did not realize the pylons where added. The time to mach 2.00 was around 3:42 to 4:14. Took, dynamic weather off and added tail wind and head wind at 25 knots with same results. Tried starting from ramp, with same times to mach 2 from mach .71. With dynamic weather, occasionally you get in a wind that will allow you to accelerate faster. All this was done in -1C temp in NTTR, 12 Januray 2020 in game date.

 

Guys, PLEASE, the wind doesn't affect an aircraft in flight. IF IT DOES SO IN DCS THIS IS A MASSIVE ERROR! Please report it if it is so!!