F14B High Altitude Payload Drag

[Apollonaut]

It seems that the payload of a Tomcat produces more drag at high altitude than at low altitude. Surely that has to be a bug right? I understand there is a difference between IAS, CAS, EAS, TAS, GS and I don't have any formal training in aerodynamics but since IAS is less than TAS at high altitudes, wouldn't you expect the drag added by payload to be lower? Even in TAS terms?

To be absolutely certain I performed the following test.

I left the F14 on altitude hold at exactly MIL power, allowing the 14 to reach steady-state speed over 200 miles up the Persian Gulf at 1000 ASL and then again at 30 000 ASL.

I repeated the test with different combinations of weapons and fuel tanks.
I didn't test every possible combination, or the TPOD or any bombs/rockets, but the results were similar.
Here are the results for two sidewinders, no fuel tanks. Fuel quantity shouldn't matter but it was an air start with a full tank. I also repeated the test by setting the initial speed above top speed and letting the speed decay to steady state speed and got the same numbers. I used the numbers from the F2 screen in all cases.
 

	CLEAN	2xAIM9Ms	Δ
KIAS @1000 ft	664	647	-17
KIAS @ 30 000 ft	408	386	-22

At low altitude, 2 AIM9s take away 17 KIAS, but at high altitude, they take away 22 KIAS (which means even more TAS). Why do the stores cause more drag at high altitude where the air is thinner? Am I missing something here?

[Skysurfer]

Known issue.

[BreaKKer]

4 hours ago, Apollonaut said:

It seems that the payload of a Tomcat produces more drag at high altitude than at low altitude. Surely that has to be a bug right? I understand there is a difference between IAS, CAS, EAS, TAS, GS and I don't have any formal training in aerodynamics but since IAS is less than TAS at high altitudes, wouldn't you expect the drag added by payload to be lower? Even in TAS terms?

To be absolutely certain I performed the following test.

I left the F14 on altitude hold at exactly MIL power, allowing the 14 to reach steady-state speed over 200 miles up the Persian Gulf at 1000 ASL and then again at 30 000 ASL.

I repeated the test with different combinations of weapons and fuel tanks.
I didn't test every possible combination, or the TPOD or any bombs/rockets, but the results were similar.
Here are the results for two sidewinders, no fuel tanks. Fuel quantity shouldn't matter but it was an air start with a full tank. I also repeated the test by setting the initial speed above top speed and letting the speed decay to steady state speed and got the same numbers. I used the numbers from the F2 screen in all cases.
 

	CLEAN	2xAIM9Ms	Δ
KIAS @1000 ft	664	647	-17
KIAS @ 30 000 ft	408	386	-22

At low altitude, 2 AIM9s take away 17 KIAS, but at high altitude, they take away 22 KIAS (which means even more TAS). Why do the stores cause more drag at high altitude where the air is thinner? Am I missing something here?

Stop going off of KIAS is going to go down at higher altitude anyway. Go off of TAS or GS

[Apollonaut]

 

On 3/11/2021 at 6:34 PM, Skysurfer said:

Known issue.

Really? Have Heatblur stated their intention to fix it?

[Skysurfer]

30 minutes ago, Apollonaut said:

 

Really? Have Heatblur stated their intention to fix it?

 

Yes. But it'll take some time.

[G.J.S]

On 3/11/2021 at 10:22 PM, Apollonaut said:

It seems that the payload of a Tomcat produces more drag at high altitude than at low altitude. Surely that has to be a bug right? I understand there is a difference between IAS, CAS, EAS, TAS, GS and I don't have any formal training in aerodynamics but since IAS is less than TAS at high altitudes, wouldn't you expect the drag added by payload to be lower? Even in TAS terms?

To be absolutely certain I performed the following test.

I left the F14 on altitude hold at exactly MIL power, allowing the 14 to reach steady-state speed over 200 miles up the Persian Gulf at 1000 ASL and then again at 30 000 ASL.

I repeated the test with different combinations of weapons and fuel tanks.
I didn't test every possible combination, or the TPOD or any bombs/rockets, but the results were similar.
Here are the results for two sidewinders, no fuel tanks. Fuel quantity shouldn't matter but it was an air start with a full tank. I also repeated the test by setting the initial speed above top speed and letting the speed decay to steady state speed and got the same numbers. I used the numbers from the F2 screen in all cases.
 

	CLEAN	2xAIM9Ms	Δ
KIAS @1000 ft	664	647	-17
KIAS @ 30 000 ft	408	386	-22

At low altitude, 2 AIM9s take away 17 KIAS, but at high altitude, they take away 22 KIAS (which means even more TAS). Why do the stores cause more drag at high altitude where the air is thinner? Am I missing something here?

What was the aircrafts AoA in each case?
The clue will be there, as the way the store is presented to the airflow will be slightly different.

A little more AoA - a little more drag for the same store.

Think about it.

It’s the same as a whole aircraft at 0 AoA, versus the same aircraft at 10 AoA, the higher the AoA, the higher the drag. Same applies to stores.

Edited March 16, 2021 by garyscott

[GGTharos]

Also there is a significant loss of thrust with altitude.

[IronMike]

Payload drag is ED side, there is little to nothing we can do about it. That said, I dont think it is a bug either, because as mentioned above, the higher AOA at higher altitudes will produce more drag. This is why you need in some cases afterburners to fly at 200kts in the danger zone, while down low you will breeze along with 75% RPM or less even... On top of that the thinner air adds to it and so on and so forth.

We believe that on some stores drag is generally too high, so this problem definitely permeates to high altitudes or is even more visible, but the fact alone that drag is higher at higher AOAs, is not really out of the ordinary.

[Machalot]

On 3/11/2021 at 2:22 PM, Apollonaut said:

Fuel quantity shouldn't matter but it was an air start with a full tank.

Fuel qty does matter because the weight affects steady state AoA.  Although the way you tested it looks like it removes the difference. 

Edited March 24, 2021 by Machalot

[Skysurfer]

On 3/20/2021 at 8:38 PM, IronMike said:

Payload drag is ED side, there is little to nothing we can do about it. That said, I dont think it is a bug either, because as mentioned above, the higher AOA at higher altitudes will produce more drag. This is why you need in some cases afterburners to fly at 200kts in the danger zone, while down low you will breeze along with 75% RPM or less even... On top of that the thinner air adds to it and so on and so forth.

We believe that on some stores drag is generally too high, so this problem definitely permeates to high altitudes or is even more visible, but the fact alone that drag is higher at higher AOAs, is not really out of the ordinary.

 

Nontheless currently there is too little thrust or too much drag from like 30k feet and up. You can barely get a clean jet past M1.2 in some instances. 

[Golo]

You are doing something wrong if you cant get past M1.2. I tried max speed dash today and few days before, I got slightly above M2.2 with 1*AIM-7F (st.5). Mil climb to 25k unload then burners accelerate past M1.4 then climb to 43k then shallow descent to around 35k. It took me 10.9k lbs of fuel from unloading at 25k. 

Note: F-14A

Edited March 24, 2021 by Golo

[Machalot]

12 minutes ago, Golo said:

You are doing something wrong if you cant get past M1.2. I tried max speed dash today and few days before, I got slightly above M2.2 with 1*AIM-7F (st.5). Mil climb to 25k unload then burners accelerate past M1.4 then climb to 43k then shallow descent to around 35k. It took me 10.9k lbs of fuel from unloading at 25k. 

Note: F-14A

 

When you unload at 25kft, do you maintain 0g until M1.4?

[Skysurfer]

18 minutes ago, Golo said:

You are doing something wrong if you cant get past M1.2. I tried max speed dash today and few days before, I got slightly above M2.2 with 1*AIM-7F (st.5). Mil climb to 25k unload then burners accelerate past M1.4 then climb to 43k then shallow descent to around 35k. It took me 10.9k lbs of fuel from unloading at 25k. 

Note: F-14A

 

 

Below 30k it's a non issue - above it's almost as if you hit a wall. Mach is a function of temperature and you should be achieving higher mach numbers higher up. It definitely is a bug that got introduced with the 14A.

[Golo]

9 minutes ago, Machalot said:

When you unload at 25kft, do you maintain 0g until M1.4?

No just set pitch like 5° nose down into descend, your goal at this point is to accelerate not hold altitude. Convert altitude into speed M1,4-1,5 and then climb around 10° up. You will still accelerate and you will climb like hell.

Edited March 24, 2021 by Golo

[Machalot]

13 minutes ago, Golo said:

No just set pitch like 5° nose down into descend, your goal at this point is to accelerate not hold altitude. Convert altitude into speed M1,4-1,5 and then climb around 10° up. You will still accelerate and you will climb like hell.

 

Ok. Is that what is normally meant by unload? I always thought it meant go to zero lift (by flying 0g) to minimize induced drag and get better acceleration. 

[Golo]

Well yes if you unload, or set 0g, you get no lift so your nose is going down, but only for moment to set descending attitude for acceleration. Well that is what I do, also you accelerate best at 0g, but only for a moment.

[r4y30n]

I know it’s in the docs somewhere but can’t find it, the engines will not last long at 0g, it’s like a handful of seconds in full AB, much shorter than negative g.

[bies]

On 3/20/2021 at 8:38 PM, IronMike said:

Payload drag is ED side, there is little to nothing we can do about it.

What about different pylons: i.e. AIM-7 at full drag pylon under the wing vs. low drag semi-recess fuselage pylon?

 

IRL and in real manual they have very different drag index.

 

IIRC they have different drag implemented in your F-14.

[Skysurfer]

6 hours ago, bies said:

What about different pylons: i.e. AIM-7 at full drag pylon under the wing vs. low drag semi-recess fuselage pylon?

 

IRL and in real manual they have very different drag index.

 

IIRC they have different drag implemented in your F-14.

 

Nope, in DCS those have the same drag per missile as it's a limitation of the sim. So it'll probably be too high for the tunnel stations and too low for the individual wing pylons. Just gotta live with it.

[bies]

1 hour ago, Skysurfer said:

 

Nope, in DCS those have the same drag per missile as it's a limitation of the sim. So it'll probably be too high for the tunnel stations and too low for the individual wing pylons. Just gotta live with it.

I did measure F-14 speed with 2xAIM-7 at wing pylons than at semi-recess fuselage pylons some year ago - there was a significant difference.

 

Similar with Hornet - AIM-7 or AIM-120 at semi-recess fuselage pylons slowing it way less than when mounted on wing pylons, but this require additional racks.

Edited March 26, 2021 by bies

[Quid]

On 3/24/2021 at 2:06 PM, r4y30n said:

I know it’s in the docs somewhere but can’t find it, the engines will not last long at 0g, it’s like a handful of seconds in full AB, much shorter than negative g.

Limits for zero or negative "g" maneuvers is 10 seconds with AB operating or 20 seconds at mil or less for the F-14A.  For the F-14B/D it's "Sustained 0 to -0.5g flight" while in afterburner, or 10 seconds in afterburner between -0.5 and -2.4g.  Mil limit is the same 20 seconds.

[Skysurfer]

1 hour ago, bies said:

I did measure F-14 speed with 2xAIM-7 at wing pylons than at semi-recess fuselage pylons some year ago - there was a significant difference.

 

Similar with Hornet - AIM-7 or AIM-120 at semi-recess fuselage pylons slowing it way less than when mounted on wing pylons, but this require additional racks.

 

 

The differemce is the rack afaik.

[bies]

I can't check it unfortunately but someone can test the Tomcat with 2xAIM-7 at wing pylons vs 2xAIM-7 at semi-recess fuselage pylons.

[Skysurfer]

7 hours ago, bies said:

I can't check it unfortunately but someone can test the Tomcat with 2xAIM-7 at wing pylons vs 2xAIM-7 at semi-recess fuselage pylons.

 

No I think the developers said that themselves. The only diference will be the pylons on the wing stations.

[bies]

7 hours ago, Skysurfer said:

 

No I think the developers said that themselves. The only diference will be the pylons on the wing stations.

I said just that: wing pylons vs semi-recess fuselage pylons.