Extending air brake on approach
 

Hi all,

This is my first post in PPRuNE. Hopefully I've got this thread in the right forum. Please be kind.

I am an aero engineer undergrad and I've recently had a pilot friend (He's training on the venerable A-4 Skyhawk.) ask me:
Why do some military aircraft extend their air brakes all the way from approach to touchdown?

I ventured an answer along the lines of:
Maybe the A-4 is not able to slow down to landing speed while on a descent without the air brakes extended. Or even worse (with a hint of bad airmanship, I know), maybe the pilot wanted to land slightly below nominal touchdown speed to reduce landing roll distance; taking the risk of a stall while flying low and slow.

He just got back to me with an answer:
The extension of the air brake increases stall speed. It has got to do with the drag curve for delta-winged aircraft (like the A-4). When the air brake is extended, the drag curve bottoms out where normally a trough exists. As such, when approaching the low speed regime, the drag increase of the aircraft would not happen suddenly.

It all seems a little strange to me as an engineer as I would have thought that no one in hell wants their stall speed to increase, especially on approach. Also, with the air brakes out, won't the whole drag curve shift upwards instead of just having the trough bottoming out? To even have the trough bottom out seems weird to me as it would mean a reduction of total drag! Totally opposite to what an air brake was designed for!

Of course, I may be wrong in all my assumptions here.

So, will anyone enlighten my friend and I, so that I may gain additional knowledge and so my friend doesn't get skewered by his QFI or IP?

Thanks in advance!

I'm not sure if this is a particularly enlightening answer, but one reason you might put the airbrake out would be to allow you to maintain a desired approach speed with a higher power setting than otherwise, which could be a good thing if your engine (probably more applicable to older types) has a long spool-up time, i.e. takes an appreciable time to accelerate from idle to some useful power as might be required in a quick go-around.

Arm, that IS THE answer....

Immediate power is available.

Particularly prevalent for aircraft designed for carrier ops, perhaps?

I flew the A4 "back in the good old days" and the posts above are correct, it's to keep the engine spooled up for quicker response on an overshoot, when you simultaneously select mil power and speedbrake in (mind you, the J52 has a pretty quick spool up time from idle anyway).

Any A2 should be able to draw you a total drag curve and show how popping your airbrake increases ZLD drag without an increase in lift dependent drag. This moves the whole total drag curve left a bit (with airspeed as the X axis), so that the speed at which you end up "on the wrong side of the drag curve" is decreased.

If you're on the wrong side of the curve, a decrease in IAS equates to an increase in total drag, which in turn decreases the IAS further and exacerbates the problem. The increase ZLD curve means that the bottom of the curve equals a lower IAS, so you can fly your original approach IAS but be on the "good" side of the drag curve, where a drop in IAS equals a drop in drag and hence the best chance of recovering your speed.

5F6B
Good explanation 5F6B, but poor use of Colour :ok:

YS;)

G'day ex DougyDriver (from 885bender) :=

Spoolup time is key for old Naval Aircraft use of speedbrakes on approach at Optimum Angle of Attack (airspeed will vary with weight below max landing weight but at correct attitude for the arrested landing). More information than youse ever wanted to know about Naval Aviation especially the Skyhawk especially the A4G variety (RAN FAA) is/are at:

www.a4ghistory.com
OR
https://skydrive.live.com/?cid=cbcd63d6340707e6&sa=822839791
(smaller files)

The USN Hornet F/A-18 A/B/C/D did not use speedbrake on approach due turbulence interfering with tail and whatnot. The SuperHornet does not have a conventional speedbrake. [Which brings to mind is the S/B function used in Shornet on approach? I'd guess yes.]

Correct me if I'm wrong, but Vulcans and Buccs used airbrake on final approach for the above reasons; IIRC, B-47s may have popped the drag chute prior to landing for the same purpose.

Blackburn's entry in the World's Ugliest Aeroplane competition was soundly trounced by the Spams with the F-4.

But looks can be deceiving - the Bucc became a first rate strike jet once the Gyron Juniors were replaced by Speys. Now, a low level strike aircraft requires good gust response and long legs at high subsonic speed. Hence a small wing and coke bottle fuselage for excellent performance and bags of room for fuel. And a small fin so that the jet would fit into Victorious' hangar deck, or so I was once told. All of which made low speed operation quite tricky, if it was to land safely on HM's flat-topped war canoes of the day.

So the excellent high speed autostabs and ailerons were given a low speed mode - the aileron gear change being a pretty agricultural lever which was pulled up when below 300KIAS. Even so, the approach speeds needed to be reduced to below the normal stalling speed, so blown flaps, ailerons and a compensating tailplane flap were fitted. These needed 20psi (IIRC) of blow to work properly in the 45-25-25 configuration; this in turn needed 80%+ HP RPM. To avoid the beast accelerating into the middle distance, 17'6" of tail cone airbrake was needed to stabilise events. Activity from breaking into the circuit until established at datum speed on final was quite busy - involving changing hands on throttles, control column, landing gear button, arrestor hook, autostab switches and aileron gear change - and checking mainplane flap, aileron droop and tailplane flap indications plus blow gauges sprinkled liberally around the cockpit amongst switches plundered from a Victorian signal box, all whilst monitoring ADD indications and keeping an eye out on the rest of the formation. Which could well be at night. But when finally set up at 45-25-25 and datum speed with the AOA pegged, full airbrake and high thrust (I'm an A2, I don't say 'power'!), the banana jet was very responsive and precise. But get it wrong and throttle back too far...a horrid wheezing noise as all the blow disappeared, the ADD started 'blooping' horribly and the voice from the rear seat went up in both volume and pitch in inverse proportion to the rate of descent...:eek: Hardly surprising as the jet would be very close to stalling and departing! Only solution was (as Bruce described it) "FullPowerAirbrakeInGearUp....keep the world level and wait until control is regained!".

Yes, high drag airbrake was used in the Vulcan on final; after flying downwind at Pattern Speed and about 72% RPM, you selected mid-drag and 69% at the start of the final turn reducing to Approach Speed plus 10KIAS, then rolled out and reduced to 66% and Approach Speed, selecting high-drag at 300ft and reducing to Threshold Speed. 66% was a pretty low power setting; we also flew 'airbrakeless' approaches now and again which were pretty demanding as to get back to approach speed and ultimately to threshold speed without any airbrakes and with unresponsive engines could be quite a challenge. The Vulcan was massively overpowered at light weight, so for normal touch-and-go landings we weren't supposed to use more than 80% when climbing away.....:E

Nope, was wrong about Super Hornet Speed Brake. It 'retracts' automatically as shown here:

U.S. Navy F-18 NATOPS Flight Manuals | Public Intelligence
from NATOPS FnA18-E/F:
"There is no dedicated speedbrake surface. Instead, a ″speedbrake function″ is provided by partial deflection of several of the primary flight control surfaces."
&
"2.10.6 Speedbrake Function. The aircraft is not fitted with independent speedbrake surfaces. A ″speedbrake function″ is provided to increase drag by partial deflection of several of the aircraft’s primary flight control surfaces: ailerons, rudders, TEFs, and spoilers. The stabilators are commanded to counter pitch transients during speedbrake extension and retraction. The full speedbrake function can only be commanded in UA.

At subsonic speeds in UA, the speedbrake function flares the rudders and symmetrically raises the ailerons TEU to approximately 95% of the capability of each surface at the given flight conditions. This makes sure approximately 5% of surface authority is available for yaw and roll control. If needed, rudder and aileron priority is given to yaw and roll commands. TEFs are also symmetrically lowered to further increase drag and to counter the loss of lift caused by deflecting the ailerons TEU. The spoilers are raised to the full up 60° position only when the speedbrake command reaches 75%. At subsonic speeds, the stabilator is used to offset any pitch transients that occur due to the deflection of all speedbrake surfaces except the spoiler. Delaying spoiler deflection until 75% allows the pilot to use partial speedbrakes for speed modulation, while avoiding minor spoiler induced pitch transients.

At supersonic speeds in UA, speedbrake surface deflections are changed. The rudders are not deflected above 1.05 M due to vertical tail loads. The ailerons and TEFs are not deflected above 1.1 M due to a lack of effectiveness. The spoilers are therefore deflected immediately upon speedbrake actuation, since they are the only effective surface at these conditions. At supersonic speeds, the stabilator is used to counter spoiler deployment. The speedbrake function is completely disabled above 1.5 IMN.

In UA, the speedbrake function is ramped out above 16° AOA or below -9° AOA to preserve lateral-directional stability and between -3.0 to -1.5g for airframe loads.

In PA, the speedbrake function is disabled with WoffW. With WonW and the FLAP switch in HALF or FULL, the speedbrake function only deploys the spoilers. While the spoilers can be deployed during landing rollout or aborted takeoff, the drag increase is minimal, and rollout distance is not appreciably decreased. With WonW and the FLAP switch in AUTO, full extension of the speedbrake function commands 20° of rudder flare, 23° of TEU aileron, 7° of TED TEF, 60° of spoiler, and a 2° TED stabilator change."
&
"7.7.2 VFR Landing Pattern Entry. See figure 7-3. Typically, the VFR landing pattern can be entered through several methods: the break, downwind entry, VFR straight-in, or low approach/touch-and-go from a GCA. Regardless of the entry method, enter the pattern at the altitudes and airspeeds prescribed by local course rules. A normal break is performed by executing a level turn to downwind with the throttles reduced to IDLE and the speedbrake function enabled (if required to reduce airspeed). The desired abeam distance is 1.0 to 1.3 nm. The g-level required to achieve the desired abeam distance will be a fallout of break airspeed.

As airspeed decelerates below 250 KCAS, lower the LDG GEAR handle and place the FLAP switch to FULL. If enabled, the speedbrake function will retract automatically when the FLAP switch is moved from the AUTO position. Continue to decelerate to on-speed AOA (8.1 deg). Longitudinal trim inputs are required with the flaps in HALF or FULL. The MI code for on-speed AOA is unit 14, address 15743, data 3300."

Supermarine Scimitar had blown flaps and so the extra drag due speedbrake means more power required and hence more blow. Landed on the wrong side of the drag curve. Great for carrier approach - stick for speed and power for rate of descent.

Ex Driver

A2 recurring nightmare!
 

Just thinking about the 'why does he land with his airbrake out' question brings me out in a rash!
The explanation about the drag curve is the one we were always expected to settle on (a great way to waste 30 minutes of the CFS examiner's time while you juggled with all those pens). However, I always felt it probably had more to do with the engine spool up time and throttle response. It is obviously jet and role dependant then I guess.
So were we all right in our own little way!
BV:confused:

4Greens,
..is there any other way? (speaking as a heli driver, of course) :ok:

"stick for speed and power for rate of descent". Surely not on final approach, where stick controls flight path and throttle controls speed.

Duncs:ok:

In the Victor we had infinitely variable airbrakes and one ideally flew the approach by setting the four engines at an average rpm for the weight and then finessing by moving the airbrake in and out as required to achieve the correct speeds. Thus one had the advantages of engine rpm in a more responsive range, only one variable for speed control and a more draggy cofiguration until we popped the 45 ft drag shute on the runway.

Buccaneer wing was also blown along the mainplane towards the leading edge.. Banned on all F1 cars next season.:E

Wrong Duncan,

Wrong side of the drag curve only, stick for speed and power for glide slope. Your formula only works on a 'normal aircraft'.

Bin there.

4Gs,

Sorry mate, I hadn't realised we were taliking about 'abnormal' aircraft ;)

Duncs:ok:

In gliding you approach with airbrakes out so that it steepens the approach, given that it's better to be at(for example) 100ft with 1/4 mile to touchdown, rather than 100ft with 1/2 mile to touchdown.

Wouild that be the case for carrier jets as well?






Stands by for disparaging comments. :cool: