Spinning Strakes
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Spinning Strakes
A bit of help with a technical question ... Ex QFI's/CFS feel free to jump in 
I recall being told/taught (?) that RAF Basic Training Aircraft were fitted with Spinning Strakes on the grounds of "training policy" rather than necessarily for specific aerodynamic reasons (?).
Hence the Tiger Moth, Magister, Prentice, Chipmunk and Bulldog were all fitted with Spinning Strakes. But, for example, the RAAF didn't fit strakes to their Tiger Moths and I don't believe the Canadian Chipmunks had strakes either.
As far as the RAF Chipmunk is concerned ... I recall that the anti-spin strakes were extensions forward of the tailplane roots, some 3 feet in length, faired into the fuselage sides. Might be a bit rusty on this bit though ... the purpose of these strakes was to increase the aerodynamic drag of the tail hence damping rotation in yaw and thereby steepening the spin. So this sounds like aiding identification of the spin v spiral dive by the student ?
The strakes then improve spin recovery, by reducing the number of turns taken before rotation ceases after correct corrective control inputs have been applied.
So my question is ... was the policy to (1) damp/steepen and (2) reduce turns post recovery, common to the aircraft mentioned above ... or say the Bulldog at least.
If this is the case ... why aren't strakes fitted to the Grob/Tutor ?
Also interested to understand why the Tucano has strakes but the Provost stable (both piston and jet) didn't.
Ya never stop learning
Coff.
I recall being told/taught (?) that RAF Basic Training Aircraft were fitted with Spinning Strakes on the grounds of "training policy" rather than necessarily for specific aerodynamic reasons (?).
Hence the Tiger Moth, Magister, Prentice, Chipmunk and Bulldog were all fitted with Spinning Strakes. But, for example, the RAAF didn't fit strakes to their Tiger Moths and I don't believe the Canadian Chipmunks had strakes either.
As far as the RAF Chipmunk is concerned ... I recall that the anti-spin strakes were extensions forward of the tailplane roots, some 3 feet in length, faired into the fuselage sides. Might be a bit rusty on this bit though ... the purpose of these strakes was to increase the aerodynamic drag of the tail hence damping rotation in yaw and thereby steepening the spin. So this sounds like aiding identification of the spin v spiral dive by the student ?
The strakes then improve spin recovery, by reducing the number of turns taken before rotation ceases after correct corrective control inputs have been applied.
So my question is ... was the policy to (1) damp/steepen and (2) reduce turns post recovery, common to the aircraft mentioned above ... or say the Bulldog at least.
If this is the case ... why aren't strakes fitted to the Grob/Tutor ?
Also interested to understand why the Tucano has strakes but the Provost stable (both piston and jet) didn't.
Ya never stop learning
Coff.
Last edited by CoffmanStarter; 5th Apr 2013 at 13:44.
Coff,
I'm not an aviator to that extent but ISTR that the JP5 did have strakes, on the lower nose? I'm not sure what they were for though, possibly to do with airflow control for the engine air intakes.
Chipmunk and Bulldog exactly as you describe.
Smudge
I'm not an aviator to that extent but ISTR that the JP5 did have strakes, on the lower nose? I'm not sure what they were for though, possibly to do with airflow control for the engine air intakes.
Chipmunk and Bulldog exactly as you describe.
Smudge
Last edited by smujsmith; 4th Apr 2013 at 17:55.
There is quite a good explanation of the history/reasoning behind the RAF Tigger strakes in 'The Tiger Moth Story' by Bramson/Birch - I can recall some of the causes but prefer not to work from memory...if nobody else posts details then I will check when I get home tomorrow.
rgds LR
rgds LR
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Coff,
Civi wise details aeros with or without the strakes installed on the Tiger Moth, are online, see page 3 on
The AAN reference it all is here
http://www.caa.co.uk/AANDocs/22556/22556020000.pdf
I thought but cannot locate it Civi chippies require them fitted.
Civi wise details aeros with or without the strakes installed on the Tiger Moth, are online, see page 3 on
The AAN reference it all is here
http://www.caa.co.uk/AANDocs/22556/22556020000.pdf
I thought but cannot locate it Civi chippies require them fitted.
Last edited by NutLoose; 4th Apr 2013 at 18:54.
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Tutor
The tutor did not get strakes because it never did the full Boscombe Down service introduction thing mores' the pity!
Mind you it turns out we were not applying the factory recommended spin recovery technique either which was somewhat disappointing for those of us blythly using the RAF drill.
These days we do not even fly the Tutor because it keeps shedding propellor blades... I love PFI deals.
Perhaps we could have some Chipmunks back?
JP had nose strikes as it had a habit of nodding in the spin I think but this subject needs a TP who can explain b over a ratios and so on.
Mind you it turns out we were not applying the factory recommended spin recovery technique either which was somewhat disappointing for those of us blythly using the RAF drill.
These days we do not even fly the Tutor because it keeps shedding propellor blades... I love PFI deals.
Perhaps we could have some Chipmunks back?
JP had nose strikes as it had a habit of nodding in the spin I think but this subject needs a TP who can explain b over a ratios and so on.
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Thanks Nutty ... still looks to me that the primary purpose of strakes are to create a defined set of recognisable conditions during a spin to help the (inexperienced ?) pilot to take correct recovery actions ... with the secondary effect of reducing the number of rotations post recovery actions being applied.
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The fly off's against I think a Robin, and Slingsby were carried out by Hunting at EMA, if i remember correctly the people doing it said the Tutor wasn't their first choice.
Last edited by NutLoose; 4th Apr 2013 at 19:28.
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Nutty ... There's a surprise ...
Not a "slag off the Tutor" thread ... but I note the Firefly has stakes (which has similar empennage)
Not a "slag off the Tutor" thread ... but I note the Firefly has stakes (which has similar empennage)
Last edited by CoffmanStarter; 5th Apr 2013 at 05:58.
I just found this on an American home built site, it's an explanation that pretty well covers the OP:
"There are several mechanisms available for improving controllability in a spin. One is as described above, where a triangular surface is placed (in a horizontal attitude) at the root of the horizontal stab. Practical examples of this can be seen on BD-5s and the Glastar. As was surmised above, for local angles of attack higher than normal, the strake creates a vortex which provides the airplane with additional stab and elevator control, helping the airplane maintain a nose low attitude for recovery.
A similar and more common addition is the dorsal fin - a triangular strake placed in front of the vertical stab. This was an effective fix (although not a cure) for the spin characteristics of the Grumman line of airplanes, which also tended to flatten out. As the plane spins, this strake also generates a vortex flow that is imposed onto the stab and rudder, thus providing a force counter to the spin direction.
The last option is rarely used but it is the addition of ventral fins at the aft end of the body. They serve two functions - they provide more aft body area thus resulting in slightly better yaw damping and stability and also, when in a spin they create countering lift, similar to that of the dorsal.
None of these are fixes - they are aids for better control and for providing the pilot with more control authority for getting out of an established spin."
anti spining strake
Smudge
"There are several mechanisms available for improving controllability in a spin. One is as described above, where a triangular surface is placed (in a horizontal attitude) at the root of the horizontal stab. Practical examples of this can be seen on BD-5s and the Glastar. As was surmised above, for local angles of attack higher than normal, the strake creates a vortex which provides the airplane with additional stab and elevator control, helping the airplane maintain a nose low attitude for recovery.
A similar and more common addition is the dorsal fin - a triangular strake placed in front of the vertical stab. This was an effective fix (although not a cure) for the spin characteristics of the Grumman line of airplanes, which also tended to flatten out. As the plane spins, this strake also generates a vortex flow that is imposed onto the stab and rudder, thus providing a force counter to the spin direction.
The last option is rarely used but it is the addition of ventral fins at the aft end of the body. They serve two functions - they provide more aft body area thus resulting in slightly better yaw damping and stability and also, when in a spin they create countering lift, similar to that of the dorsal.
None of these are fixes - they are aids for better control and for providing the pilot with more control authority for getting out of an established spin."
anti spining strake
Smudge
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out of interest, how many people say in the last 30 years have found themselves in a fully developed spin (clearly not training/practice induced) and had to use the RAF standard spin recovery on a JP/bulldog/tucano/firefly/chipmuonk.... ?
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Re NutLoose' AAN reference pertaining to fitting of strakes on the DH82A Tiger Moth, their effectiveness and necessity is debated still.
In addition to longer_ron post #3 a chapter dedicated to the history of the issue is also in Stuart McKay’s book dH Tiger Moth Legendary Biplane Trainer – Chapter 7 Getting into a spin - and how the fitting of strakes came to pass. Very interesting reading in the context of CoffmanStarters question:
Only the RAF Tiger Moths had strakes fitted to 'aid spin recovery'. The fitting of bomb racks, blind flying hood and nav lights and various weight gains all added to 'increased rotary moments of inertia'.
The RNZAF, RAAF, RCAF Tiger Moths were not fitted with strakes. The RCAF DH82C is quite a different beast to the DH82A though.
In addition to longer_ron post #3 a chapter dedicated to the history of the issue is also in Stuart McKay’s book dH Tiger Moth Legendary Biplane Trainer – Chapter 7 Getting into a spin - and how the fitting of strakes came to pass. Very interesting reading in the context of CoffmanStarters question:
I recall being told/taught (?) that RAF Basic Training Aircraft were fitted with Spinning Strakes on the grounds of "training policy" rather than necessarily for specific aerodynamic reasons (?).
The RNZAF, RAAF, RCAF Tiger Moths were not fitted with strakes. The RCAF DH82C is quite a different beast to the DH82A though.
Last edited by kluge; 5th Apr 2013 at 10:42. Reason: not enough coffee
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I have been lucky enough to fly and teach on many training aircraft and have spun them all many, many times. I have never had any problems in recovering when using the advertised technique. Some had strakes, some didn't. Never noticed any difference. That also goes for the Chipmunk of which I have flown both versions. Only slight problem I have had was with a Fitecracker which fell into an Inverted spin when doing a normal vertical recovery a la CFS. Normal inverted spin recovery technique worked as advertised. I have no idea what advantages the strakes have although the suggestion that the vortices they produce might have some beneficial effect is seductive.
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Thanks Pontifex ... good to have a very experienced TP's view on the topic
I was going to ask you about this when we met earlier this week ... but the couple of hours we had together over coffee just flashed by so quickly !
The inverted Firecracker Spin has got to be worth a few more lines
Best regards ...
Coff.
I was going to ask you about this when we met earlier this week ... but the couple of hours we had together over coffee just flashed by so quickly !
The inverted Firecracker Spin has got to be worth a few more lines
Best regards ...
Coff.
Re Spinning Steaks
Just to add a little to Kluges earlier post...
An extract from 'The Tiger Moth Story'...
As a result...
Mod 103 was introduced (removal of Aileron mass balance weights)
+ Mod 112 (anti spin strakes) and a reduction of VNE to 170mph to avoid aileron flutter
An extract from 'The Tiger Moth Story'...
A number of alarming reports had come in about some Tigers being reluctant to recover from spins.
Intensive spinning trials were carried out at Boscombe Down with 3 tigers which had developed a reputation...somewhat complicated by the fact that only R5129 had Bomb Racks fitted
on one occasion this particular a/c took 13 turns to recover.
Later testing at Farnborough revealed the fact that the addition of Aileron Mass Balance weights,the Aileron Box and Spar reinforcement and 3 separate colour schemes had increased the flywheel effect during spinning.
Intensive spinning trials were carried out at Boscombe Down with 3 tigers which had developed a reputation...somewhat complicated by the fact that only R5129 had Bomb Racks fitted
on one occasion this particular a/c took 13 turns to recover.Later testing at Farnborough revealed the fact that the addition of Aileron Mass Balance weights,the Aileron Box and Spar reinforcement and 3 separate colour schemes had increased the flywheel effect during spinning.
Mod 103 was introduced (removal of Aileron mass balance weights)
+ Mod 112 (anti spin strakes) and a reduction of VNE to 170mph to avoid aileron flutter
Last edited by longer ron; 5th Apr 2013 at 18:20.
This sad accident happened about a month before I graduated from Halton in 1972 ! I always remember it 
A Chipmunk spin accident ...
https://www.google.co.uk/url?sa=t&rc...CqiE5JH5d4tT_A
A Chipmunk spin accident ...
https://www.google.co.uk/url?sa=t&rc...CqiE5JH5d4tT_A