Skyhawk rudder

[kiwi]

Why do Skyhawks appear to have rudders with the ribs on the outside ?

[beagle]

they originally didn't, but this was changed as the original rudder with internal stiffners caused rudder buffet.

[mumbles]

IIRC, putting the ribs on the outside was an interim solution, pending a redesign of the rudder, which never happened in the end.

[beagle]

yes mr mumbles, you are right. so it must have worked ok, or was there a small buffet left that they thought was ok for a pilot to manage. Any ex Skyhawk pilots on here that can comment.

[FlyNavy]

My memory is failing here ;D but I'm not old enough to have flown an original smooth rudder bird, only the ribbed rudder. No buffet, however the aircraft had "stability augmentation" or 'stabaug' which if disabled by a switch in the cockpit or if U/S (unserviceable) in flight could make flying in a straight line an experience. The nose would wobble and weave without 'stabaug', depending on IAS this nose wobble could be disconcerting.

I don't recall any buffet rudder problem. You may know the vortex generators on the wing were important. The leading edge slats were the most problematic device on the Skyhawk. I have read that a device to allow them to operate together would have (at the time) cost around $10 US with a small weight penalty of course; but over the life of the aircraft would have likely saved many aircraft and lives.

In flight the rudder was not used by anyone to my knowledge although later A4G pilots have told stories of its use that would raise one's hair. So my first point that rudder was unused still is a good one. It had its use for spin recovery if required but [intentional] spins were not allowed. Using the rudder trim was important. Other than that my feet were planted firmly on the floor inflight with the aircon at full freeze so I lost all feeling in my feet. ;D I'm no stick (and rudder) guy.

[beagle]

so turning the aircraft was made by using aleron only ??
if, so, then one big fin without a movable rudder would have been the correct design for this aircraft.

[FlyNavy]

Beags,
No, I have made some generalisations for the sake of brevity in my response. Sorry for any misunderstandings. However the main idea (no need for rudders in a jet in ordinary conditions of flight) is a common one that gets close to the reality. I'll attempt to expand further:

In out of control flight situations (such as a spin for example) the rudder is the main control to regain flying control (amongst other possibilities) but it will do as an illustration. Having a large rudder surface on a large tail fin gives plenty of control effectiveness at low speed such as in a spin at a steady low airspeed thus allowing more rudder authority to regain control. [However there are spins where the airspeed is high and increasing.]

When there are asymmetric leading edge slat extensions in an A4 the rudder can be used to quickly bring symmetry back to the slats (depending on other factors). Conversely the rudders can be used at low airspeeds to test/demonstrate the slat asymmetry (in a fashion but not recommended). I'll just add that apparently (in the USN at least) the slats were tested before hard maneuvering to see if there would be any problem. We did not think to do this in my time flying A4Gs in early 70s but it is an excellent idea. To recover was a bit easier because the asymmetry was being anticipated - and the quicker the recovery, the better. I have glossed over lots of detail here but I hope that you have an idea of how the rudder is useful in certain very dramatic situations.

An every day use would be during takeoff and landing in a crosswind to control the strong tendency for the A4 with the large tail fin to point its nose into the cross wind. This could be disastrous if not brought under control with the large rudder being very effective at slow airspeeds. And of course the spoilers dumping lift on landing so that aircraft weight went on to the main wheels for more effective braking and less tendency to weathercock.

In my day learning to fly with the RAAF - before rejoining the Navy (FAA) - we learnt to fly on Winjeels where all kinds of spins were allowed and practiced. In the Vampire NO. Spins were forbidden - this is where we learnt the 'feet on floor' technique. In the Sea Venom it was a similar situation but by this time we were also flying Vampires in the Navy (1969-70) the technique was ingrained to not use rudders in flight and now not in the Skyhawk.

We were encouraged to fly the Skyhawk "on the burble" but not beyond. Here the slats were just starting to drop. If they dropped more than an inch or so too much energy was being lost. If rudders were used in this situation then BINGO - asymmetric slats. This looked comical from behind but not comical for the aircraft concerned depending on altitude and other factors. Flying in this careful way (learnt in Vampires) helped keep us safe from bad stuff in an A4. Later (because I guess Macchis required rudder to get anything happening in ACM) perhaps these lessons were not so well known. Only my guess - I was not in the RAN if and when any rudder techniques (or lack thereof) were changed.

Only when the Macchi came along to replace the Vampire/Venom did we regain the ability to spin and use rudders in flight in an RAN jet anyway. The Macchi had benign flying characteristics mostly with or without rudder; but having straightish wings with small ailerons it really helped to be able to whack in full rudder (at suitable speeds) to get things moving. This brings me to another point.

The Skyhawk has an incredible roll rate. 720 degrees of roll per second. No one on this planet can roll a Skyhawk at full aileron deflection at the best roll speed and not roll beyond the mandatory NATOPS 'one rotation only'. It just was not possible. Before the aileron could reach full deflection the one - single - roll allowed was completed. An A4 can roll - ask any rear seat passenger. ;D You will find their helmet marks on the canopy; even when they know the roll is coming. So you can see for rolling, the rudder is unnecessary in an A4.

Back to the Macchi: Sadly (after a couple of years) the RAAF found out that the MB326H was unreliable in recovering from an inverted spin - but did not tell the Navy. We lost a Macchi (both crew ejected OK) after they could not recover from an inverted spin. Prior to this all kinds of spins were allowed. So even in some situations perhaps even the large rudder on the large tail fin of our Macchi was not effective (for whatever reason).

Hope this has cleared up some points. Yes rudders are necessary in jets for specific situations but for ordinary use not required (a big simplification - but we are talking about the A4 here). Please ask more questions. Probably I can refer to NATOPS (USN Flying Manual) to get some specific answers but I think for example the best max roll rate airspeed was 350 KIAS but it may have been 250? AND why the max revolution was only one (straight and level) was due to the tendency for the rear fuselage to roll not in sync with the nose (the proper term escapes me for the moment - if you can imagine the rear was barrel rolling while the nose is just rolling around a point) so quickly the rear fuselage would be really out of control if the roll continued beyond more than one and a bit. During a roll one would expect the entire fuselage to roll at the same time but the weight of an A4 was in the rear (engine, large tail etc.).

I'm long winded eh. To answer about the turn - yes only aileron was used in a turn in an A4. No rudder needed. However due to constantly carrying drop tanks or stores the rudder needed to be trimmed using the trim though after turn complete or adjusted for flight conditions but that was it. I'm not saying that any aircraft is being flown by using the trims for example - just that unlike a prop aircraft a jet does not usually need to have co-ordinated stick and rudder control movements in a turn. See now you know why jet jocks have it easy. ;D

[kiwi]

Thanks for that it was all very interesting , a ground crew told me that if an aircraft had a stabaug failure some pilots would not fly it as they knw it would make them sick .

[FlyNavy]

Yes that would be correct. I would not be worthwhile to fly knowing beforehand that the stabaug was U/S. If it failed inflight then getting back on ground ASAP was the go. And I have heard the same stories that stabaug failures could induce motion sickness, the severity of the motion varied with airspeed and any outside turbulence such as that found at low level could make the combination of motion very troublesome.

[cribble]

Again, three years on...

From Combat Aircraft Designer The Ed Heinman Story (Edward H Heinman and Rosario Eausa) isbn 0 7106 0040 2
p224

"we also had trouble wth rudder buffet and used a North American idea to correct it. Essentially, it involved reworking the rudder inside out. We made it a single surface item with the skin down the middle and the supporting ribs on the outside...."

[fyl]

That's great information, FlyNavy. Thanks for taking the time to put it all down..

[cracker8]

Re Skyhawk rudder......wasn't the final design a clever form of boundary layer control?

[baz62]

Thats a fascinating insight into flying the A4 FlyNavy. Thanks very much. Not in the same class but I flew Cherokees and often described the rudder pedals as foot rests! You certainly didn't need much input for turns and primarily used for steering (on the ground linked to the nose wheel), picking up a wing in a stall and cross wind landings.