Effect of shockwaves on primary flight controls
 

I understand that once an aircraft exceeds its critical mach number, shockwaves form on the wings and stabilisers and further increases in speed cause the shockwaves to grow and move towards the trailing edges. For aircraft with conventional flight controls (elevator, ailerons, rudder) this can lead to loss of control, or even control reversal, as the shockwaves and/or separated flow behind the shockwaves can render the controls ineffective. Hence trimmable horizontal stabilisers or all moving tailplanes / tailerons on aircraft designed to approach or exceed Mach 1.

My question is, how come elevons on delta wings still work in these conditions, e.g. Concorde, Eurofighter, Mirage fighters, etc? Is it simply a question of having enough force to move the control surfaces, i.e. through use of PFCUs? What about when the shockwave is sitting just in front of the control surface hinge line? Surely there must be a speed range where the control surfaces become ineffective temporarily?

you ask a question full of innocence.

back in the days before shockwaves were understood and the geometries developed to ameliorate the control problems the development of a shockwave at speed would lock controls, cause controls to cease working or cause them to operate in the opposite sense.

much talented development and experimental work went into understanding the dynamics of shockwaves and developing geometries that made the control problems manageable.

nowadays an aircraft that is expected to operate in a speed range where the problem exists will be designed with all the smarts needed to handle the situation.

I will take that as a compliment! But visualising transonic and supersonic airflow is hard without the benefit of considerable experience of working with a supersonic wind tunnel, which I don't have. Also, there do not appear to be many people around these days who have a really detailed understanding of this regime of flight.

Just to clarify: you are saying that, with the correct aerofoil and control surface profile, conventional flight control surfaces can remain effective throughout the transonic speed region and into supersonic? That makes sense, although I would like to know what the key differences are between a control surface design that remains effective and one that does not.

Let the fun begin...

Mirage 3: When supersonic, the stick was back for level flight, so far that you could see the up-elevon in the rear-vision mirror. This of course limited the g you could pull, as you didn't have as much available/spare backstick. The elevons were pretty big, I assume to provide satisfactory supersonic control.

The centre-of-pressure movement when transitting Mach 1 was fairly benign if S and L (push it thru with a kick of the ASI and aft-moving stick), but severe if pulling G. If decelerating, the sudden forward movement when coming subsonic would create a nasty pitch-up of about 2g which if you weren't ready to counter would cause an embarrassing over-G.

To understand how and why shockwaves affect flying controls we need to start by looking at what exactly shock waves are. A shock wave is an instantaneous increase in static pressure. When air flows through a shockwave the static pressure, temperature and density, all increase abruptly, while the velocity decreases. If the shockwave is very intense the reduction in velocity is sufficient to cause the boundary layer to separate from aircraft surfaces, thereby causing the airflow to become turbulence.

Shockwaves affect flying controls in the following ways:

1. In subsonic flight the pressure changes caused by the deflection of a control surface are felt not just on the control itself, but ahead of the control hinge line. So, for example deflecting an aileron may affect the pressure distribution all the way to the wing leading edge. But pressure changes cannot move forward through shockwaves. So if for example a shockwave forms on the hinge line of an aileron, the pressure changes caused by aileron deflection will be confined to the surfaces of the aileron. This will obviously reduce the control forces that the aileron is able to generate.

2. If a shockwave located ahead of an aileron causes the boundary layer to separate, this will directly reduce the ability of the aileron to generate control forces.

3. If intense shockwaves form on the upper surfaces of the wings, the resulting boundary separation will reduce the lift being generated and also envelope the rear surface of the aircraft (including the tail plane) in turbulent airflow. This will directly reduce the ability of the elevators to generate control forces.

4. The shockwave-induced loss of lift from the wings will reduce the downwash passing over the tail plane and elevators. This will alter the angle of attack that is experienced by the tail plane and elevators.

5. As an aircraft accelerates through the transonic speed range the formation of shockwaves, and their rearward movement, will cause the C of P of the wings to move aft. This rearward movement of the C of P, coupled with the loss of downwash over the tail plane, will cause the aircraft to pitch nose down (Mach tuck under).

6. As airspeed increases, the increasing dynamic pressure will increase the stick forces that must be applied to move the control surfaces.


The following design features are typically employed to overcome/minimize the above problems:

1. Use powerful hydraulic systems to reduce the stick forces that are required to move the control surfaces when flying at high speeds.

2. Use sweepback and reduce the thickness and camber of aerofoils in order to delay the formation of shockwaves and to reduce their intensity.

3. Use stabilators/all-flying-tail-planes (instead of separate elevators) to increase the pitch control authority. Increase the size of the other control surfaces.

4. Place the tail plane on the top of the fin to position it outside of the downwash and turbulent airflow produced by the wing.

5. Locate vortex generators ahead of ailerons to prevent shockwave induced boundary layer separation.

6. With very long thin wings, aileron deflection can cause wing twisting to the extent that the effects of the controls are reversed. Raising an aileron for example, may twist the wing trailing edge down, thereby increasing the angle of attack of that part of the wing. So instead of producing a wing-down rolling moment, the overall result would be a wing-up rolling moment. This problem is prevented by fitting an addition set of ailerons further inboard where the wing is stiffer. The ailerons at the wingtips are used in low speed flight then locked in neutral as speed increases. In high speed flight only in inboard ailerons are used.

The above lists are by no means exhaustive.

Australian built single seat Mk 30 Vampires had this problem in the early 1950's. To accommodate the larger airflow requirements of the RR Nene engine, a pair of intakes were installed on the top of the fuselage directly aft of the cockpit. These intakes had a curved top surface something like a typical carburettor air intake seen on radial engines like the DC3, DC4, Beaufighter and so on. It was discovered (too late to save three crashes in which the pilots were killed because in those days the Vampires did not have ejection seats) that at around Mach 0.68 in a dive, shock waves formed on the top of these two intakes and caused loss of elevator effectiveness which could not be regained until lower altitudes. The intakes were known as "Elephant Ears"

For example, two Vampires experienced what was known then as compressibility while practicing formation aerobatics around 15,000 ft. Once over the top during a loop the pair would deploy airbrakes to keep speed under control in the dive recovery. In the accident case the leader did not deploy his air brakes in the dive and was unable to pull out from the dive due to loss of elevator effectiveness. The No 2 aircraft formating closely was similarly affected by compressibility and both went in.

It wasn't until the Australian test pilot for de Havillands at Bankstown, NSW, "Blackjack" Walker was tasked to investigate the behaviour of the Vampire at the Critical Mach Number, that compressibility was found to be the problem. If I recall, he took a Nene powered (no ejection seat) Vampire to 40,000 ft and rolled into a steep dive. He described losing elevator control and was only able to recover by 10,000 ft. It was a mighty close shave. The fix was to relocate the elephant ear intakes under the fuselage and the problem disappeared but not before another fatal crash due to compressibility. That aircraft was practicing a diving quarter attack at another Vampire around 20,000 ft when the pilot reported that he was in compressibility. The aircraft was seen to crash almost vertically.

When this writer first flew the single seat Vampire Mk 30 (no dual available in those early days and all of us on the fighter course only had about 250 hours total time in our log books) we would accompany a experienced Vampire pilot in formation for what was called Mach Runs. He would stay very close formation as No 2 and talk us into the dive. He knew from experience the symptoms of compressibility before we would be aware of it.

As soon as it was felt and the elevators were becoming ineffective he would call on the radio for throttle close and dive brakes out before things got out of hand in the dive. Eventually all the single seat Vampires in Australia were modified to have the intakes installed under the fuselage and as an added bonus, ejection seats were installed.

Complicated subject, but you will find a useful discussion on this (and much else) here:

DESIGN FOR AIR COMBAT - Главная

Go to Rolling Motion of an Aircraft

Don't be put off by the Russian bit in the title - the book is is good English

Thank you
 

All,

thank you for your very helpful replies, much appreciated. I already had most of the "pieces of the jigsaw", so to speak, but I am now able to put them together in a much more coherent fashion.

One thing I had been taught (erroneously) was that conventional flying controls became ineffective at high transonic speeds, which clearly is the case for some aircraft but is not always the case. It seems to me that the issue is essentially one of control authority, i.e: are the control surfaces big enough; can they deflect sufficiently; does the authority remain sufficient to cope with the main wing centre of pressure movement; and is the airflow over the horizontal stabiliser and ailerons (or elevons in the case of a delta) sufficiently clean and attached to allow control authority to be retained. Truly a complex topic!

Thanks again.

WF

You seem to be there now. One of the things that I think may have made you uncertain was your repeated use of the word ineffective. To me that word means that whatever one is talking about has 'no effect'. In actual fact what is really needed/meant in the circumstances of this topic is 'reduced effectiveness'.

Keith has made it quite clear why reduced effectiveness happens and what the various fixes are.

A final point is you mentioned 'control reversal'. This just does not happen through purely aerodynamic reasons. It happens if the torque needed to move the control surface (usually ailerons) is so great that it causes the wing to twist. So a down going aileron can twist the wing trailing edge up to such an extent that the overall net aerodynamic rolling moment is opposite to what the control surface (pilot) was asking for.

Wonderful book
 

Thank you so much OwainGlyndwr! Great pleasure to discover many things in many domains, very clear, nobody may resist to that vortices' lift for the planes and the thought !:DThank you again!