Spin recovery strakes?

I have been spinning Tiger Moths for over 20 years, both Private and Transport Category CofA aircraft, with and without ‘Anti Spin’ strakes fitted.

Trouble is… the original spin recovery problem was in 1941…not in the 1980’s.
As i posted earlier out of the 3 tigers tested at Boscombe for spinning problems ,only one was fitted with bomb racks,if you are flying a tiger which is close to its original design weight then you should not have a spinning problem,the CAA are presumably covering their ar5es and these days who can blame them in our increasingly litigious society.

cheers baz

I have been spinning Tiger Moths for over 20 years, both Private and Transport Category CofA aircraft, with and without ‘Anti Spin’ strakes fitted. There’s absolutely no difference at all in the spin recovery with or without these strakes, period. This despite what the CAA Test Flying dept would have you think…….

They were originally fitted during WW2 when the Tiger Moth was modified to carry light bombs under the centre fuselage.

Bloody hell – i was just about to post exactly that Baz!

Been looking into this, currently the UK CAA allow aerobatic sequences to be flown in Private Category Tiger Moths without spin strakes fitted. If however the Tiger is on a Public Transport Category CofA spin strakes must be fitted to fly aerobatics. Unless of course like us, your in the process of proving that a particular aircraft can be recovered from a spin without them fitted, the CAA will gives you a dispensation!

Seems strange that in the CAA eyes it all depends on whether the passenger in the front seat is paying to be there or not that seems the critical factor in the spin recovery tendancies – must be something to do with CofG and wallet I imagine!!!

FB

There are several references pointing to the fitting of bomb carriers and bombs to Tiger Moths which when initially trialled was found to disrupt airflow over the tail surfaces affecting spin recovery,the strakes were as a result of this for RAF Tigers,with the bombs fitted there is also the question of changes in C of G affecting spin recovery.

I am sure a Tigger spurt will be along shortly…but the answer may not be as simple as this…according to Bramson and Birch in ‘The Tiger Moth Story’.
During 1941 some tiggers were showing a marked reluctance to recover from a spin.Boscombe Down carried out tests on 3 machines which had gained vicious reputations,R4760,R5129 and N6221 but only R5129 had bomb racks (on one occasion this tigger took 13 turns to recover).
With the help of Farnborough it was decided that the RAF mods like addition of aileron mass balance weights,aileron box and spar reinforcement and 3 added paint schemes had increased the mass of the wings to an extent that they had a flywheel like effect of perpetuating rotation during spinning.
The mass balance weights were removed(VNE reduced to 170mph) ,spin strakes fitted to tame even R5129.
So looks like a civil tigger at its original design weight did not have spinning problems but at a certain stage in RAF tigger service the strakes were required because of weight gained in service.

cheers baz

The purpose of the strakes is to enhance Damping in Yaw which will result in a reduction in the yawing velocity (or rate). It is the keel surfaces, aft of the CofG that produce an aerodynamic moment to oppose the yaw with the greatest contribution being from the rear fuselage and fin. In this respect the cross section of the rear fuselage is critical and has a significant effect on the damping moment. Below are some comparative figures showing the variation in effect with cross section:

Cross section Damping Effect (anti-spin)
Circular 1
Rectangular 2.5
Elliptical 3.5
Round top/Flat bottom 1.8
Round bottom/Flat top 4.2
Round bottom/Flat top with strakes 5.8

The effectiveness of strakes will vary depending on the cross section to which they are applied. However in most cases they will be beneficial. The exceptions might occur where they produced a shielding effect and reduced the effectiveness if the fin/rudder combination.

exmpa

There are several references pointing to the fitting of bomb carriers and bombs to Tiger Moths which when initially trialled was found to disrupt airflow over the tail surfaces affecting spin recovery,the strakes were as a result of this for RAF Tigers.

When an RAAF Tiger was trialled with bomb carriers in 1942 (for the same reasons as those re-configured in the UK in 1940) I don’t recall spin was an issue. Maybe they just didn’t check.

There are several references pointing to the fitting of bomb carriers and bombs to Tiger Moths which when initially trialled was found to disrupt airflow over the tail surfaces affecting spin recovery,the strakes were as a result of this for RAF Tigers,with the bombs fitted there is also the question of changes in C of G affecting spin recovery.

The RAF mandated anti-spin strakes (often called ‘spin strakes’, rather like shock absorbers get called ‘shocks’!) for British used Tiger Moths. However the RAAF didn’t feel they were necessary and so they weren’t fitted to Australian Tigers. Presumably, therefore, they were not a critical anti-spin device. (The Dutch mandated a long, triangular fin for their Tiger Moths, another ‘unacceptable’ handling quirk everyone else didn’t mind.)

Guesswork, but it may have had something to do with different training syllabuses for the RAF and RAAF regarding spin recovery?

HTH.