Spinning Strakes

[CoffmanStarter]

A bit of help with a technical question ... Ex QFI's/CFS feel free to jump in :ok:

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 :ok:

Coff.

[smujsmith]

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

[longer ron]

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

[fantom]

Google 'strakes+chipmunk' and it's all there.

[NutLoose]

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.

[CoffmanStarter]

Fantom ... thanks but my question isn't Chipmunk specific :ok:

[Bigpants]

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.

[CoffmanStarter]

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.

[CoffmanStarter]

BP ... Most interesting re the Tutor :eek:

[NutLoose]

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.

[CoffmanStarter]

Nutty ... There's a surprise ...

Not a "slag off the Tutor" thread ... but I note the Firefly has stakes (which has similar empennage)

http://www.alexhamilton.net/media/training_generations_fs.jpg

[diginagain]

.... as did the Auster AOP9.

[smujsmith]

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 (http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/2855-anti-spining-strake.html)

Smudge

[Duplo]

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.... ?

[kluge]

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: 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 (?).

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.

[Dysonsphere]

As I recall strakes were fitted as and when needed, a fact normally detected during protype testing.

[pontifex]

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.

[CoffmanStarter]

Thanks Pontifex ... good to have a very experienced TP's view on the topic :ok:

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 :eek:

Best regards ...

Coff.

[longer ron]

Just to add a little to Kluges earlier post...
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.

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

[longer ron]

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&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&sqi=2&ved=0CD4QFjAC&url=http%3A%2F%2Faviation-safety.net%2Fwikibase%2Fwiki.php%3Fid%3D20364&ei=bDxfUf73Iqi70QWTjYGgBg&usg=AFQjCNH12Ub_XFLz7DI-CqiE5JH5d4tT_A

[Dan Winterland]

''I have no idea what advantages the strakes have although the suggestion that the vortices they produce might have some beneficial effect is seductive''.

This is the official answer according to CFS exam wing - and I was asked the question on my A2 check which was done on the Chipmunk. At higher angles of attack, the vortices generated by the strakes re-energise the airflow and make the rudder more effective while also having the benefit of exposing more of the rudder to the energised airflow.

They were fitted after several fatal spinning accidents. The Chippy has a tendancy to flatten out during the spin and due to the geometry of the tail design, the rudder will be progressively blanked by the tailplane as this happens. The re-energising of the airflow from strakes in the flatter spin will give a better chance of recovery - apparently. If a Chippy is on the UK register, spins are prohibited without the strakes and the wide chord rudder. Also, 8 turns is the limit.

Incidently, the wide chord rudder was fitted as response to comments that the aircraft ran out of rudder authority during slow rolls and not for spin recovery - although it should help.

A mate owns a Chippy in Aus (where there is no limitation as in the UK) with the small rudder and no strakes. He says he has no problems recovering from spins - although he hasn't explored the recovery from an extended flattened spin.

[Dan Winterland]

"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.... ?"


The RAF pilot is trained to recognise the incipient stage and before the full spin has developed. I don't think I have ever had to recover from the full stage and I doubt many pilots have.

And although the incipient recovery is standard for most types, there is no such thing as the ''standard spin recovery''. There was once, but all types have subtle differences in their recovery techniques. it was the introduction of the Chipmunk and it's tendancy to flatten out in a prolonged spin that prompted a change from the standard and since the 1950s, the doctrine has been to apply the specific technique for the type. I have flown all five types you mention (instruced on two) and they are all slightly different.

[longer ron]

Yes reference Chipmunk spinning and to add to what dan posted...istr that also during spinning if the airflow over the elevators became blanked - then during the recovery whilst going stick forward the blanked elevators would have felt 'light' until they 'bit' into the airflow at which point the stick load became higher and could be mistaken for hitting the stop.Hence insufficient forward stick to unstall the a/c !

[CoffmanStarter]

Many thanks all for your contributions ...

Back to the Chipmunk one final time ... did you know that when Spinning Trials were carried out from 11/57 to 07/58 at A&AEF, WP804 was equipped with a tail parachute and strakes as part of a series of tests to improve spin recovery. Not sure if the tail parachute was ever deployed ... or does anyone know different :confused:

Coff.

[SOSL]

The JP Mk 5 did have strakes on the nose. It was operated at BFT for FJ selection ISTR.

The JP Mk 5a didn't have strakes but did have tip tanks. It was operated at Finningley in the Nav training role.

I guess the tip tanks and the strakes had a similar influence on the aircraft's aerodynamics. But I'm not sure it was to do with spinning.

Rgds SOS

[Dan Winterland]

The 'a' designation in JP5a denoted the improved avionics which were the civil standard transponder and VOR/DME. At some time in it's life - maybe the same time, it also aquired the nose stakes and the roughened leading edge to improve stability in the spin, as he larger canopy of the Mk5 made the spin oscillatory compared with the earlier marks. The JP5s used at Finningley for navigation training didn't do much spinning and weren't modified as the strakes and leading edges added 5lbs a minute fuel burn at low level. At 45lbs a minute burn the pilot trainers could only acheive about 38 minutes at low level on a good day. The Finningley aircraft also had tip tanks installed to increase their range and useful training time. There was a restriction on spinning JPs with tip tanks with fuel in due to the B/A ratios.

Later, when the JPs were replaced by the Tucano some lower fatigue 5as were sent to Finningley and tip tanks installed. These were known as 5bs.

[Dan Winterland]

Incidently, the nose strakes on the JP were to stabilise the spin and not aid recovery - as in the tail strakes. I span a JP5 without the strakes and leading edges at Boscome but didn't notice any difference from the modified aircraft. Apparently, the instability comes if the control column isn't held exactly central.

[Arm out the window]

a reduction of VNE to 170mph to avoid aileron flutter

Crikey, you'd think it'd be a difficult job to get one up to 170 in the first place ...

[Basil]

I have a recollection of someone telling me they'd been sent off to carry out an air test on an RAF Chipmunk which included a fully developed spin.
He climbed as high as reasonably possible before entering the spin. In-spin control was held until the spin went flat. It took a great many turns to recover and he was very glad that he'd commenced from a comfortably great height.

I've never permitted a spin to go flat and, at the levels we did them in training and AEF, if I had, I do not believe I'd be sitting here typing this now.

[Pontius]

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.... ?

Yep, me in Bulldog during an AEF SCT solo flight. Decided to explore the envelope a bit too much and found that a vertical roll, followed by over-exuberant use of back stick and full rudder (definitely NOT a flick-type manoeuvre, oh no) had an undesired result. Entirely predictable, I grant you and totally the fault of my gross mishandling :(. Standard spin recovery and straight into the next part of the (modified) sequence. Lucky it always happens to the other bloke :ooh:

[CoffmanStarter]

Do we have anyone that has spun both the RAF Firefly and the Tutor who might share their experience ... especially as the Firefly has strakes and the Tutor doesn't. :ok:

Coff.

[Courtney Mil]

Someone mentioned the "standard"spin recovery drill. How standard is this one?

Centralize
Speedbrake in
Check Altitude
Throttles idle
Stick central
Check ASI
<180 kts,
STICK FULLY BACK
Stick with turn direction

I span an Eagle at night. Supposed to be near impossible.

[CoffmanStarter]

I span an Eagle at night.

Bl**dy hell Courtney ... I assume you "intentionally" spun it old chap :eek:

I think I would have grabbed the Yellow and Black PDQ ... but there again 6 left and 6 right in Chipmunk was enough for me ! :uhoh:


Coff.

[Courtney Mil]

No. There was an odd corner of the flight envelope that Mac Air had only just discovered where it wasn't quite so naturally spin reistant. They hadn't mentioned it just then. Anyway, I found my way into it and she snapped straight into a high-rotation spin. Applied the appropriate controls and waited and after about three hours she recovered and flew on.

I mentioned it to the Mac Air guys and the cheif test pilot was delighted. "I knew it!", he said. Great.

[CoffmanStarter]

Just proves we train the best ... I bet the old adrenaline kept you buzzing that night :ok:

[MrBernoulli]

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.... ?

At least twice that I recall, whilst a QFI on the Bulldog!

[CoffmanStarter]

Mr B ... I seem to recall that in the early days the Bulldog Lycoming had a tendency to stop in the spin to make thinks a little more interesting :uhoh:

[longer ron]

Ahh yes our sooties used to spend hours running the feckin things whilst doing their RPM graph LOL :)
Great fun watching them freeze in the winter :)

Green Spot anybody ??