Speed brake deploy cause a nose drop?

Operating an F-16 on the ground or in the air - from the engine start sequence, over replacing a wing, to aerial refueling procedures
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saberrider

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Unread post05 Nov 2016, 06:34

When I fly in simmulator F-16s and deploy speed brakes nose drop some degrees. Is this the case in reality?Why?
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jbgator

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Unread post05 Nov 2016, 20:01

Simple response-no. More detailed response would need to come from engineer like Johnwill, but I believe the main reasons are that the speed brakes are symmetrical and located in a way that produces little to no pitch input, and the flight control system would prevent any pitching motion resulting from speed brakes anyway as it dampens out any uncommanded rates. Perhaps JW will correct or add to this.

Other aircraft are not the same. I only have experience with the T-37, T-38, and F-4 and they all have significant pitch transients with speed brake deployment depending on airspeed. Where the speed brakes are located I am sure has an input as well. In the case of the above 3 the speed brakes open downward and produce a pitch up. I suspect the F-15 barn door that opens upwards may produce the opposite but I have no experience with that large multi-engine aircraft.
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35_aoa

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Unread post06 Nov 2016, 06:23

There is a mild nose-down pitch transient, even with the FLCS working to damp it out……..particularly if you are going fast, and fan the boards out all the way. Same thing is true in other fighter aircraft with digital FBW/CBW (or otherwise). I'd have to ask my test pilot friends for the actual scientific explanation for the phenomenon, but it happens in the real jet as well. As an aside, on jbgator's comment about the F-15, in the F/A-18A-D with a similar speed brake configuration, you still get nose down pitching……..actually a little more pronounced in the Hornet than it is in the Viper. My *guess* is that it has more to do with these FBW control systems attempting to auto trim to 1G unaccelerated flight than it does with traditional aerodynamics, but I could be completely wrong there…….I'll leave that part to the engineers to explain away :)
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saberrider

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Unread post06 Nov 2016, 07:55

35_aoa wrote:There is a mild nose-down pitch transient, even with the FLCS working to damp it out……..particularly if you are going fast, and fan the boards out all the way. Same thing is true in other fighter aircraft with digital FBW/CBW (or otherwise). I'd have to ask my test pilot friends for the actual scientific explanation for the phenomenon, but it happens in the real jet as well.
Thanks.My thought is because very aft locations of boards ,but been simetrical deployed must alleviate any tendencies to nose's down.Anyway guilty is positive-AoA- in moment of deployment . Don't you think so?
Last edited by saberrider on 06 Nov 2016, 16:17, edited 1 time in total.
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johnwill

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Unread post06 Nov 2016, 08:41

Not in a sim but in the real world, the abrupt decel may induce a small nose down moment from the pilot inadvertently pushing tbe stick. Even hands off the decel acting on the stck mass will generate a small nose down command.
With a brake of F-15 design, drag on the brake will produce nose up pitch. But pressure build up on the upper fuzslage forward of the brake will result in a nose down moment. Similarly the brake can result in a small pressure reduction on the upper fuselage aft of the brake, resulting in more nose down moment.
On the F-16, with zero AoA, pitch from brake extension is very small. But with + AoA, the lower panels have more drag than the upper, so there is nose down moment.
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saberrider

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Unread post06 Nov 2016, 09:30

Thanks for answers.From your answers ,is reduction in deferential pressures responsible for the phenomenon in F15,Su27,MiG29,Tornado .I believed induced drag is principal factor .But F 105with petals breaks and Vipers in special must not have this tendencies.Possible causes-lowering turbine's blade rotations- act as an wall? Just a thought.

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