Hawk Rudder

[RetiredBA/BY]

Can anyone explain the exact purpose of the trailing edge flat strip on the Hawk rudder. It extends from the top of the trim tab. Looks rather similar to the flat strip on the top section of the Canberra rudder which, iirc, was to increase rudder effectiveness. Does it have any anti-flutter effectiveness ?

Anyone help ?

[John Farley]

Such a strip is very effective at ruducing rudder buz. It was also added to the Hunter for the same reason.

[RetiredBA/BY]

Thanks John, thought it might be something like that but my CFS days are a long time ago. Thanks again.

[RetiredBA/BY]

May I ask some further Hawk questions:
Do Hawk rudders (both the manual rudder with trim tab and the Australian type Hawk , no trim tab, so presumably hyd. powered)have any mass balance and if so where is it mounted ?

Where is the balance point of the stab, in relation to the pivot axis ?

What is the real reason for the use of dome headed rivets on the rear fuselage skinning. (Aerodynamic, or structural - just curious.

The reason for asking those questions is that I am completing a 1/3 scale model of the Hawk T1 with a 50 pound thrust gas turbine and some other builders of this model have experienced rudder flutter and poor pitch control possibly due to insufficient servo power to the stab. And I am trying to determine the optimum balance point of the stab an adding the appropriate mass in the leading edge root.

I have done everything possible to eliminate rudder flutter (except mass balance) including the anti-buzz strip on the trailing edge, but any help on this and any other of the points I have raised would be gratefully received.

Finally, long after leaving the RAF, I cadged two trips in a Hawk at Valley, and as an ex JP QFI and having flown the Vampire T11 and the Gnat ( plus the Valiant, Victor and Canberra, I was incredibly impressed by this superb aeroplane.

[Chris Scott]

Hi RetiredBA/BY,

Quote:
"I am completing a 1/3 scale model of the Hawk T1 with a 50 pound thrust gas turbine..."

With an 11-foot wingspan, will it need a CofA? ;)

And roughly what is the AUW (empty)? (Am wondering about the thrust-to-weight ratio.)

[RetiredBA/BY]

It is actually a little less, precise scale is 1:3.8 so span not quite 11 feet. Finished weight is projected at 50 pounds, dry, plus 6 litres of fuel, so T/w ratio is about .9 on take off. It does not require a C of A as such, but has to go through an inspection process and proving flights.

I already have two other Hawks flying, scale 1:4.5 weight 44 pounds with 36 pounds thrust engines. They fly superbly and are all equipped with state of the art, professional, dual uplink plus data downlink ( including downlinked warnings of critical parameters) radios on 2.4 GHz.

Now retired from flying big jets at least they fill one of my aviation passions !

Oh yes, the electrical system is powered by twin lithium batteries, but unlike those in the 787, my battery packs have internal electronics which monitor individual cell voltage and temperature. So far, no battery fires !

[longer ron]

Yes the Manual rudder versions do have mass balance weights - they are bolted directly onto the leading edge of the rudder,they are wedge shaped (tapered) to give adequate clearance with internal Fin structure.
Cannot remember any exact figures for checking the balance but they hang very nose low on the check rig.
I would think that the later marks with powered rudder would also have balance weights as they have manual reversion to cater for hyd failure.

On the Hawk tailplane circuit there is a non linear gearbox mounted in the rear fuselage forward of the Tailplane PCU (power control unit) to give adequate tailplane range of movement,this NLG gives Fine control movements around T/P neutral but gradually gives coarser movements (IE increased angle change for given stick movement) as the tailplane angle increases.

Edit...just re read your posts...
the T/P (stab) is not mass balanced and its c of g is behind the pivot points,it will naturally and gradually 'droop' trailing edge down when hyd pressure dissipates.

[Brian Abraham]

JF, could another possible explanation be the trick of using a Gurney flap to aerodynamically increase the apparent size of the rudder, as on the MRA 4?

An example of aero trickery from http://www.dept.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf

Trailing edge T-strips have been used to improve the performance of aircraft vertical tails. An example is the use of T-strips on the vertical tail/rudder of the Sino Swearingen SJ30-2 business jet to increase dutch roll damping. Flight tests showed that a 1% chord T-strip on the lower third of the rudder increased dutch roll damping for the flaps up, 180 KIAS flight condition from essentially zero to 0.12, more than double the FAR 23 minimum.

Edited to add - Testing the Honda Jet http://www.icas.org/ICAS_ARCHIVE/ICAS2004/PAPERS/437.PDF

During the initial flight testing, the airplane exhibited a small amplitude rudder oscillation in the mid-speed range. To eliminate this, various sizes of T-strips were added to the trailing edge of the rudder. An example of the effect of the partial span T-strip on the rudder oscillation is shown in Figure 22. A T-strip eliminates the oscillation (Fig. 23).

What is the real reason for the use of dome headed rivets on the rear fuselage skinningPerhaps the boundary layer is so thick at that point that the effort to countersink is not worth the meagre return.

[FE Hoppy]

Brain is fading a little but I think we had T strips on the MR2 Ailerons.

[RetiredBA/BY]

My Thanks to the further respondents, very grateful to you for your time and knowledge. Wherever its relevant and applicable I incorporate fullsize features into my jet models. My 2 Hawks already flying have all the wing vortex generators, set at the correct angle, wing fences and the gap in the flap vane. The result is a fine flying machine with excellent handling, no sudden departures at or near the stall, both at low speed, 1 G, in the approach config (flaps 40) and clean when pulling max G and what is probably past the buffet (when I can see the pitch rate reduce with increasing stab. angle.

One final question. I understand the gap in the flap vane on early Hawks (pre SMURF) was incorporated to counter the severe nose down pitch as flaps were extended which is reported in some media, (eg World Air Power Journal,) as due to strong downwash from the powerful flaps stalling the stab.

The flap vane gap is outboard of the stab. so how exactly was the flap vane gap effective in reducing downwash over the stab ? Again, just very curious, anyone able to put me out of my misery ??

[Brian Abraham]

RetiredBA/BY, put your questions to a group of Hawk pilots at an airshow on the weekend. Answer, "no idea, we just fly the thing". Sorry, not much help.

[Dan Winterland]

Hmmm. Wasn't covered in groundschool, but it was explained to me my an instructor early on. It was common knowlegde then. The Tucano has a 'blunt' trailing egde on the rudder for the same reason, as does the A320 I fly now

[longer ron]

Not 100% sure about the cut down flap vanes but I was always under the impression that it was cut away to preclude the 'possibility' of the flaps affecting tailplane effectiveness at slow speed on 'some' jets ie engineering/production tolerances may have come into play.The RAF accepted the resulting (5 knot ?) increase in approach speed.
The 'SMURF' came about (I believe) during the T45 design phase because obviously HS wanted to achieve the slowest approach speed but also with good tailplane effectiveness at low speed.
If you look at pics of later mark Hawks landing - the L/E of the tailplane is usually approx lined up with the smurf so I assume the smurf directs air over the t/p in some way or other.
Whilst working at a certain airfield in surrey :) I did actually assist one of our design team to attach a cardboard 'SMURF' to the side of a Hawk fuselage with masking tape as they worked towards the T45 design :).

[John Farley]

The full span flap vane gave more lift BUT it needed so much back stick to trim that you could under some circumstances stall the tailplane.

With the T45 when more lift was needed to meet the required carrier approach speed the full span vane was restored and the rear fuselage had strakes added to stop the tailpne stalling. You can just see one in this pic (above the airbrake and behind the Y and in front of the tailplane) the shadow rather confuses things.

http://img.photobucket.com/albums/v145/johnfarley/Tailplanestrakes_zpsf6d2497a.jpg

[longer ron]

Sorry late reply to this thread but not been home much recently,as referred to in my earlier post about production tolerances - I seemed to remember that during the Hawk flight testing there had been a fairly large difference with stalling characteristics between particular Hawks but I did not want to say too much without having a reference to use...

From Arthur Reed's book 'BAe Hawk' (Ian Allan,Modern Combat Aircraft 20)

Phase 4 of Hawk development began with the first flight of XX157 in april 1975.Although XX154 had achieved a satisfactory standard in handling at the stall,it soon became apparent that subsequent a/c were not consistent in their behaviour ,particularly in the approach configuration with u/c and full flap.
On one occasion this a/c pitched nose down over the vertical with full back stick,before recovering when flaps were retracted.
The problem was effectively cured by removing the outboard section of the flap vane.
As a further precaution the tailplane travel was reduced from minus 17 deg to minus 15 deg.

Arthur Reed was a highly respected aviation correspondent/journalist btw

[d2180s]

If it sticks out perpendicularly either side of the rudder, then it is probably a Gurney flap, which would increase rudder effectiveness as in this case.

[RetiredBA/BY]

Thanks, guys, for the further reply but we we still haven't got to the bottom of why the cutaway in the flap vane prevents the severe pitch down discovered in testing (by Duncan Simpson and Jim Hawkins, I believe ) and give the hawk such benign and pleasant handling.

I can see that a reduction in stab. travel (and that SMURFs directs airflow onto the stab. root, I guess they work best at large negative stab. deflection.) could prevent stalling of the stab. (even though MacD increased travel and power of the F4 stab by adding an inverted slat to the stab. LE,) but for the life of me I just can't understand why removal of just 15 inches of flap vane, outboard of the stab. span, produced such a dramatic cure for the pitch down.

Its not a smart A--- question, perhaps just the curiosity of the QFI in me, but I just can't see how it works. Anyone put me out of my misery !?

If this fails perhaps I should call exam wing at CFS at Valley !!

[John Farley]

More flap vane, more lift, more downwash, more general induced flow down onto tailplane, more negative tailplane AOA easier to stall when stick hard back. Jim Hawkins was the driver.

[longer ron]

I would certainly go along with JF's explanantion of the vane cut out etc...he is highly qualified to speak on the subject :)

I am sure JF will recognise the location of this pic :)
The very first Hawk - XX154 is still earning a living with ETPS,this pic was taken when we were invited back to dunsfold in 2005 and clearly shows the missing section of flap vane !

http://i695.photobucket.com/albums/vv316/volvosmoker/DSCF0682-1.jpg

The 2nd pic shows 'Fred Suttons rear end' (extended fairing below/behind rudder) and also an early version of the extended tail cone - both these mods were to improve directional stability !

http://i695.photobucket.com/albums/vv316/volvosmoker/DSCF0657-1.jpg