need evidence Unrealistic speedbrake behavior?

[Manolo]

Hi guys, I'm new here. I started to fly the F-18C  just a few months ago. I noticed a terrible lack of power when using the speedbrake. Compared to reality this speedbrake is almost non existing. In real F-18 fliying, touching the speedbrake at 400 kts is like braking a F1 race car. If you are not firmly strapped down you can smash your face against the instrument panel. That's the thing which impressed me more during my first fly in 1998 (a twin seater in the Spanish Air Force). Opening fully the speedbrake in DCS generates almost no negative g's (on the HUD). On top of that, deceleration is too slow compared to reality that make me thing about a problem in the mathematical model. Does anyone noticed this or is it just my lack of experience in DCS?

Cheers

Manolo

Edited September 26, 2022 by Manolo
tags

[razo+r]

Do you have any video or sources or whatnot other than feelings that could be useful to reference in order to adjust the flight model?

Edited September 26, 2022 by razo+r

[ricktoberfest]

I believe you wouldn’t get negative g reported on the hud as that’s measuring G in relation to the vertical component, not acceleration or deceleration. 

[Exorcet]

The airbrake seems fine. I'm assuming your demo flight took place at relatively low altitude with a lightly loaded plane. Are you comparing to a similar condition in DCS? Airbrakes don't work like the ones on cars. They lose effectiveness as you slow down and are also less effective with altitude.

[Manolo]

Hi again, I did an informal experiment to document my feelings. I took 2 videos from the HUD (same lenght, 23 seconds). One is using the speedbrake + idle thrust and the other is the normal deceleration going only to idle. Same altitud and conditions.

With speedbrake speed the aircraft goes from 505kts to 319kts in 23 seconds

Without speedbrake speed the aircraft goes from 486kts to 329kts in 23 seconds

The difference of using the speedbrake or just idle between both is only 29 kts.... in 23 seconds. This doesn't look realistic to me. 

 

 

 

[cofcorpse]

39 minutes ago, Manolo said:

Hi again, I did an informal experiment to document my feelings. I took 2 videos from the HUD (same lenght, 23 seconds). One is using the speedbrake + idle thrust and the other is the normal deceleration going only to idle. Same altitud and conditions.

With speedbrake speed the aircraft goes from 505kts to 319kts in 23 seconds

Without speedbrake speed the aircraft goes from 486kts to 329kts in 23 seconds

The difference of using the speedbrake or just idle between both is only 29 kts.... in 23 seconds. This doesn't look realistic to me. 

I tested and I have different results - 22 and 39 seconds. Could you make a track showing the problem?

[Bankler]

@ManoloI'm sorry if I'm stating the obvious here. But just want to make sure that you are aware that you need to hold the speed brake extend button for approx 3 seconds for it to fully extend. I don't have DCS available atm to verify, but the numbers you present indeed sound very weak. This in combination with cofcorpse numbers made me think maybe you're not using the speed brakes correctly (please verify in F2 view that it fully extends).

Edited September 28, 2022 by Bankler

[Manolo]

hace 6 horas, Bankler dijo:

@ManoloI'm sorry if I'm stating the obvious here. But just want to make sure that you are aware that you need to hold the speed brake extend button for approx 3 seconds for it to fully extend. I don't have DCS available atm to verify, but the numbers you present indeed sound very weak. This in combination with cofcorpse numbers made me think maybe you're not using the speed brakes correctly (please verify in F2 view that it fully extends).

 

Hi Bankler. Yes of course, I'm aware of how to operate the speedbrake. As a mater of fact I flew the F-18 in real life in the Spanish Air Force, and that's why I'm not so happy with the behavior in DCS.

Edited September 28, 2022 by Manolo
Correction

[Razor18]

Hmmm... Just wondering, why would the speed brake opening ever induce any negative G value on the HUD?...

[Manolo]

En 28/9/2022 a las 21:38, Razor18 dijo:

Hmmm... Just wondering, why would the speed brake opening ever induce any negative G value on the HUD?...

You are right, not in military aviation. I was thinking this HUD was like the Rockwell Collins HUD we have installed in our A220. There you can see the deceleration when landing.

[Razor18]

Yeah, but that one is refers to speed, not G. And deceleration is displayed on most of the modern airliners on the electronic ADI, just beside the speed scale.

How was your time in the Spanish Air Force?

[OLD CROW]

On 9/28/2022 at 9:38 PM, Razor18 said:

Hmmm... Just wondering, why would the speed brake opening ever induce any negative G value on the HUD?...

IMHO this effect is plausible. I'm only theorizing.... but due the upper and center position of the airbrake system in the F-18 (I guess near the physical A/C CG), once you actuate the airbrake this generates a high preassure  zone over the airbrake zone but also a turbulent air zone over the rudder and flaperons upper zone. So the lift generated by the lower flaperons surface tends to push the tail up... so it means pulling the nose down... ergo it should generate an induced negative G. Using the simplified (don't take it too seriously this ex. it's just only for visualizing the effect purpouses) car example: when brakes are actuated car nose tends to go down. This effect can't be seen in other A/C's like F-16 or F-14 due their airbrake position (tail end: off any surface control position) and upper and lower deployment. But as I said I'm not an expert so I will be pleased if someone can give us a clear explanation of this effect.

[Manolo]

hace 14 horas, Razor18 dijo:

Yeah, but that one is refers to speed, not G. And deceleration is displayed on most of the modern airliners on the electronic ADI, just beside the speed scale.

How was your time in the Spanish Air Force?

My time in F-18 was fantastic. 6 years working with the beast. Highest altitud reached was 65000 ft in a parabolic flight. Highest g's 7.3 Man, you have to be in good shape to fly that that was in the mid 90's, I'm 61 now and about to be retired from Lufthansa.

Edited September 30, 2022 by Manolo

[razo+r]

1 hour ago, Manolo said:

My time in F-18 was fantastic. 6 years working with the beast. Highest altitud reached was 65000 ft in a parabolic flight. Highest g's 7.3 Man, you have to be in good shape to fly that that was in the mid 90's, I'm 61 now and about to be retired from Lufthansa.

 

Have you tried reproducing your airbrake experiment in DCS yet? Because you should really do that and attach the tracks to see if your results agree with the one from cofcorpse. 

[Exorcet]

14 hours ago, OLD CROW said:

IMHO this effect is plausible. I'm only theorizing.... but due the upper and center position of the airbrake system in the F-18 (I guess near the physical A/C CG), once you actuate the airbrake this generates a high preassure  zone over the airbrake zone but also a turbulent air zone over the rudder and flaperons upper zone. So the lift generated by the lower flaperons surface tends to push the tail up... so it means pulling the nose down... ergo it should generate an induced negative G. Using the simplified (don't take it too seriously this ex. it's just only for visualizing the effect purpouses) car example: when brakes are actuated car nose tends to go down. This effect can't be seen in other A/C's like F-16 or F-14 due their airbrake position (tail end: off any surface control position) and upper and lower deployment. But as I said I'm not an expert so I will be pleased if someone can give us a clear explanation of this effect.

The airbrake is well behind the CG and the high pressure created by the brake will also push down on the fuselage ahead and the vertical stabs. The brake being so tall will also contribute a nose up moment. All together I think pitch up makes more sense than down. At least until you factor in the FCS which would just keep it at 1 g.

Cars aren't a good comparison. The car dives nose first because the wheel contact patch is below the CG. It's the opposite case of the Hornet.

[Manolo]

hace 17 horas, razo+r dijo:

Have you tried reproducing your airbrake experiment in DCS yet? Because you should really do that and attach the tracks to see if your results agree with the one from cofcorpse. 

I'm still doing informal experiments (I'm new and don't know how to track things in DCS other than this pictures). See the landing distance in RWY 07 Kutaisi. Same Vref speed (AoA) for landing (same weight) and aiming to same touch point with the velocity vector in the HUD (same full pedal braking in both). Stoping in the first exit with and without speedbrake. Yes, of course there is an effect in using the airbrake, but to me this is to small. 

 

[razo+r]

44 minutes ago, Manolo said:

I'm still doing informal experiments (I'm new and don't know how to track things in DCS other than this pictures). See the landing distance in RWY 07 Kutaisi. Same Vref speed (AoA) for landing (same weight) and aiming to same touch point with the velocity vector in the HUD (same full pedal braking in both). Stoping in the first exit with and without speedbrake. Yes, of course there is an effect in using the airbrake, but to me this is to small. 

 

When you flew a mission in single player and you get to the debriefing, you have the option to save the replay/track. After you decided where to save the track, you should upload it here.

 

Also, I don't think testing the effect of the speed brake on the ground is saying a lot to how little effect it has mainly because the slower you get the less effective the airbrake becomes. So just try again for example fly at 500kts, throttle idle and airbrake out, see how long it takes to decelerate to lets say 250kts or whatever and save the track. Then do the same thing again without airbrake and then upload both tracks here.

[Manolo]

hace 46 minutos, razo+r dijo:

When you flew a mission in single player and you get to the debriefing, you have the option to save the replay/track. After you decided where to save the track, you should upload it here.

 

Also, I don't think testing the effect of the speed brake on the ground is saying a lot to how little effect it has mainly because the slower you get the less effective the airbrake becomes. So just try again for example fly at 500kts, throttle idle and airbrake out, see how long it takes to decelerate to lets say 250kts or whatever and save the track. Then do the same thing again without airbrake and then upload both tracks here.

Great, thanks a lot Razo+r. I did it and now is clear. No need to discuss it further. With airbrake takes 26 seconds to go from 550 kts to 300 kts, whereas without airbrake it takes 52 seconds. Also thanks a lot to everybody participating in this subject

 

With speedbrake.trk Without speedbrake.trk

[Bankler]

On 9/28/2022 at 8:12 PM, Manolo said:

Hi Bankler. Yes of course, I'm aware of how to operate the speedbrake. As a mater of fact I flew the F-18 in real life in the Spanish Air Force, and that's why I'm not so happy with the behavior in DCS.

 

Yeah, I figured. And I of course I didn't mean to sound rude/patronizing/ignorant. Just noted that since you were new, there could potentially have been a binding problem. I remember there are some confusing details with the speedbrake bindings (something with an "OFF" binding and another "RETRACT" and so on), so it's not always obvious how to set it up even if you know how it should work. Anyway, I read your last message with the test you did and that everything seemed fine in the end, so happy to hear that!

[Foka]

On 10/1/2022 at 6:02 PM, Manolo said:

I'm still doing informal experiments (I'm new and don't know how to track things in DCS other than this pictures).

Instal free version of TacVIew https://www.tacview.net/download/latest/en/
It will record telemetry of all objects in the mission, then you can analize flight and all its telemetry.
 

Edited October 3, 2022 by Foka

[maxTRX]

On 9/26/2022 at 10:53 AM, Manolo said:

...Compared to reality this speedbrake is almost non existing. In real F-18 fliying, touching the speedbrake at 400 kts is like braking a F1 race car. If you are not firmly strapped down you can smash your face against the instrument panel...

Legacy Hornet is a giant speedbrake.  At 500 ~ 400 kts, if you pull the throttles back to idle, you will kiss the HUD if you're not firmly strapped.   I remember one of the Hornet guys talking about this long time ago.  I can live with DCS's version of speedbrake.  I would have to get some more feedback on RW Hornet's decelaration rates since I hear some 'chatter' about it here and there. (brake or no brake)