Aim-120(and AIM-7) Drag

I've been doing some AA in the hornet lately and I've noticed that whenever I bring missiles on the wing pylons the plane becomes dog slow.

I understand that weapons will come with a significant amount of drag but I felt that it was too much.

Of course I'm no expert on this at all so for all I know this is completely realistic. However. I did some testing.

And one of these results MUST be wrong.

I made a mission with 2 hornets. Everything equal except one hornet was loaded with 8x-mk83 bombs.

The other was loaded with 4x aim-120c on the wings. Single missile per pylon.

Then I just did a speed test, same altitude. Full power until they ran out of fuel and recorded their top speed.

The one with bombs topped out at 799 knots Ground speed(mach 1,22)

The one with missiles topped out at 644 knots ground speed(mach 0,99). It wouldn't even go supersonic!

There has to be more drag from the bombs? In addition to the massive weight difference which will create more drag also.

I don't know about AA missiles, but if you try rocket launchers you will be amazed. It is like having speed brakes in the wings.

If you want to compare the drag, you can always test the top speed with pylons and missiles, then jettison the missiles, test again, then jettison the launchers.

That way you will find where is the drag coming from.

Why did you move it to this obscure place? It's clearly a hornet issue.

Why did you move it to this obscure place? It's clearly a hornet issue.

Probably because weapons have their own drag values, so they should effect all aircraft equally.

In any case I did a quick test and it seems you're right. Mk bombs provide a higher top speed and better acceleration than AIM-120's. Tracks attached for stable version of DCS.

Each track is 2 minutes of maximum acceleration.

AIM-120 ~ M1.2

Mk82 ~ M1.4

Mk84 ~ M 1.3

F18AIM120drag.trk

F18Mk82drag.trk

F18Mk84drag.trk

Why did you move it to this obscure place? It's clearly a hornet issue.

This part of the hornet is still WIP and there are other deviations (training pod, CAP-9).

Edit sniped ;)

AIM-9X is also draggy, between AIM-120 and Mk82. Didn't save the track for that one.

It could be the launchers have incorrect numbers. Since the f15 can carry these missiles without becoming super slow. Also having missiles on the fuselage mounts doesn't affect the speed nearly as much. Now I would expect that carrying them tucked under the fuselage is inherently more aerodynamic so that may explain some of the difference.

There may be a bug but you have to remember the bluff body shape of typical missiles, the rocket nozzle creates a large low pressure zone relative to the frontal CSA of the missile right behind the missile, the bomb on the other hand is tapered front and back, reducing any bluff body effect it will suffer to nearly zero.

The effect may be exaggerated, but it is still there, or at least it should be if ED have modelled the missile aerodynamics correctly.

There may be a bug but you have to remember the bluff body shape of typical missiles, the rocket nozzle creates a large low pressure zone relative to the frontal CSA of the missile right behind the missile, the bomb on the other hand is tapered front and back, reducing any bluff body effect it will suffer to nearly zero.

 

 

The effect may be exaggerated, but it is still there, or at least it should be if ED have modelled the missile aerodynamics correctly.

When you approach supersonic speed though, wave drag dominates. The missile nosecones and narrow diameters should be much less draggy at speed.

At low speed, there is also induced drag to deal with, which should be much larger with Mk84 bombs than lightweight AAM's.

You're underestimating the effects of a bluff body shape.

If the effect was that dramatic then surely they would have put a cone on the back of the missiles which would simply get blown of when the missile fired. I've seen some rocket pods which have similar cones.

You're underestimating the effects of a bluff body shape.

These effects are less important at transonic speeds and beyond. You can't have pressure recovery on the tail section when a everything separates after a shockfront.

A quick search is showing a much higher transonic drag rise for the Mk82 than a generic AAM:

http://www.dtic.mil/dtic/tr/fulltext/u2/a329921.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/a556680.pdf

You don't have to do anything....

Recovery may not happen very locally but it will happen, and not everything separates at a shockfront, at a tail section it must at some point recover, the question is how far aft of the tail it does that, if you have a bluff body on one shape and a boat tailed shape on another there will be a difference, and that will include how the tail shocks behave, and using a model that is more comparable to an AIM-9B with a rounded dome nose and pretending that that is comparable to the profile of an AIM-120 (which this topic is about) is not a valid comparison or argument, such a shape will always generate more drag than a nose profile like the AIM-120 in the circumstances of this thread, and that link about the missile provides no actual data on the missiles drag coefficient at all, it's not featured in it, so saying one is showing more drag than the other, and citing that source is not valid either, so no conclusions can be drawn in either direction from that.

Until actual relevant data is available for the actual missile in question, you can't write off the effects of a bluff body on this.

that link about the missile provides no actual data on the missiles drag coefficient at all

The axial force coefficient (C_A) is the CD at 0 degrees AoA.

Yes the missile is more representative of the Sidewinder because I didn't have time to look up AIM-7 data, which didn't appear in my initial search. The AIM-9 is going to be draggier than the AIM-120/AIM-7, so this supports what I'm saying further.

Until actual relevant data is available for the actual missile in question' date=' you can't write off the effects of a bluff body on this.[/quote']

This doesn't really have anything to do with the missile. The bomb doesn't appeared to be optimized for supersonic flight and you're trying to apply aerodynamic rules of thumb where they don't apply.

I did some further testing.

Same mission, everything is the same.

Altitudes are virtually the same, maybe 50 feet apart.

Clean plane:

Speed(indicated): 741 kts

Loadout 1: 4 single aim-120cs on the wings, nothing else.

Speed(indicated): 579 kts

Loadout 2: 8 Mk-83 on dual racks on the wings, again, nothing else.

Speed(indicated): 687 kts

And now the interesting result:

I fired of the aim-120s so now it's only the single mount launchers left from the amraams:

Speed(indicated): 602 kts.

The 4 racks cause considerably more drag than the combined drag from all the bombs AND their racks!

Launching the missiles only gained us 23 kts. The racks alone cost us 139 kts of IAS.

There is no point in discussing the drag from the missiles, which seems to be pretty low, we have to discuss whether the racks should be causing so much drag.

It seems wildly unrealistic to me. They are not that big and look pretty aerodynamic.

I'm getting the same result, the missiles only have a marginal impact while jettisoning racks provide a large speed increase.

The weapons' drag most like isn't the issue here. When putting them on the wingtip and fuselage pylons, their effect on the Hornet's performance is marginal. But don't dare to take those racks on the wing pylons, they're almost like extended drogue chutes. On the other hoof, keeping the racks after dropping off Mk. 83 or those training bombs in fact does boost the performace which is even more weird even though I have to admit it's lots of fun to supercruise this thing around