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Chipmunk Is Beautiful

Old 25th Sep 2010, 13:38
  #101 (permalink)  
 
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The Canadians never fitted anti-spinning strakes.
Spinning and spin recovery was a subject to be taught during flight school. Why on earth would you want to do it in an airplane modified to prevent spinning?

Last edited by twochai; 25th Sep 2010 at 16:28. Reason: typo
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Old 25th Sep 2010, 14:25
  #102 (permalink)  
 
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Dora-9, Mike Sassin who designed the 'Spraymaster' told me that some owners did in fact remove the strakes for reasons I cannot remember. I can clearly remember the placard after my 'event' prohibiting spins in RSK but after all this time you've got me on when the strakes were fitted. I have an undated photo of me taxiying in at MB and yes it has strakes, they extend to a little hole that I think was used for lifting. As for the last poster the 'anti-spin' strakes are in a way, mis-named they were only there to stop you obtaining a mining permit, it was all about recovery, not spinning.
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Old 25th Sep 2010, 14:44
  #103 (permalink)  
 
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The following is an excerpt by RE Gillman, former Senior Training Captain BEA Elizabethan Fleet, circa 1957, Croydon Aerodrome: Spinning the Chipmunk

The spinning exercise was somewhat unusal, and having been forewarned by this, I was rather curious to try it out. On introducing the spin with full rudder at the stall in the normal manner, the aircraft begain to rotate fairly fast, but the speed increased although the stick was still held back, and at 100 knots, the whole manoeuvre was noisy and uncomfortable, but recovery was simple enough, though at rather high airspeed during the ensuing dive. Although the aircraft turns at such a speed as to give an impression of autorotation, it can still be nothing more than a spiral dive. A full spin was achieved by applying opposite aileron, after the stick had been brought back centrally, thus stalling the inside wing completely on a subsequent occassion, and the resulting spin was fast clean, and a positive forward movement of the stick was required to regain control.

Last edited by evansb; 25th Sep 2010 at 20:45.
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Old 26th Sep 2010, 07:32
  #104 (permalink)  
 
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By George is absolutely correct; they are NOT anti-spinning strakes! (PM sent, btw).

But there are some in the UK who claim that fitting the strakes was a pure “political fix” (the story involves a prominent MP’s son being killed in an RAF spinning accident) and further that the strakes are of limited effectiveness. Certainly the Canadians were unimpressed for the need to equip their Chipmunks with these. Is it significant that following their exhaustive investigation into Chipmunk spinning, the Australian DCA published a report in June 1960 (i.e. two years after the RAF fitted their strakes) which never mentions the effectiveness or otherwise of this item?

As fitted, the UK strakes were distinctly crude, not even matching up to the tailplane leading edge. In contrast, the tapering Sassin/Aerostructures strakes are a very elegant solution:





There have been volumes written about Chipmunk spinning. I’m not really qualified to further comment, other than to point out the Chipmunk has several incipient “gotchas” lurking around:
  • Any brake selection limits rudder travel.
  • Failure to use the published entry technique can result in nasty surprises.
  • The aircraft won’t simply recover by letting go (unlike some types); the correct recovery technique is essential.
  • One quirk of Chipmunk aerodynamics is that while control forces are normally quite light the elevators can really firm up in a spin, with the result that the pilot thinks because of the push force involved that he has full forward stick when he hasn’t. At least one Aero Club had witness marks painted on the control box and taught students to push the stick until the marks aligned.
  • Sometimes the first indication that the aircraft is recovering (i.e. the correct technique has been employed) is that the spin “tightens up” with an increasing spin rate initially – this one still gets me!
In 1967 at Bunbury WA I did something really stupid in a Chipmunk (see item 2); the ensuing manouvres and spin frightened me witless. It was 28 years before I got in a Chipmunk again!!!
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Old 26th Sep 2010, 08:40
  #105 (permalink)  
 
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I did lots of spins in the T10 and never remember anything particularly surprising, either solo or dual, including during a ten-turn spin. However, we always did everything by the book - CFS standardization at its best - and I certainly never felt like experimenting with different entries or recoveries.

bri, concerning Captain Gilman's Croydon experience, that sounds very odd. Maybe he had a very far-forward C of G. In my experience of teaching spins in a Blanik, it was quite common to enter a spiral dive at forward C of Gs, particularly of course, if the student wasn't holding the stick all the way back.

Dora-9, I see your aircraft still has the UHF aerials. Did it come with the whole fit, or had the boxes been taken out? I remember when the UBAS Chipmunks arrived back after the summer break (1968 I think), freshly painted in the grey and dayglo scheme with white canopies, with the new UHF radios. It actually made our life at Shawbury more difficult, since the other resident aircraft were Marshall's VHF equipped Vampires and Piston Provosts, used by the ATC trainees for PAR practice. The UHF didn't help for cross-country very much either, since it was a limited channel set with only the RAF common UHF frequency (362.3?) and 243 MHz for communicating with other towers. I remember ATC at Syerston trying to get us to change to their Approach frequency and refusing to believe that a UHF equipped aircraft could not do so.

Does anyone know about the background of the UHF conversion. It must have been incredibly expensive. Were these sets designed and built for the Chipmunk or were they used in other aircraft?
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Old 26th Sep 2010, 09:22
  #106 (permalink)  
 
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I have spun the Chipmunk a few times with a very highly qualified chipmunk pilot in the rear. I have never dared to do it solo. Apart from anything else, I have wondered at the effect of the C of G being much further forward solo and whether there is a danger of going inverted when pushing the stick fully forward during recovery. I would be interested in comments!

Is the increased rate of turn not due to the nose dropping further and therefore the rotation increasing to preserve angular momentum? I believe the Slingsby has that characteristic.
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Old 26th Sep 2010, 17:33
  #107 (permalink)  
 
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Was was awarded a Flying Scholarship while a schoolboy in the early 60s - did the training at AST at Perth - we were only allowed to fly Chipmunks with the anti-spin strakes and broad chord rudder.I found the spin characteristics somewhat unpredictable - I believe the Egyptian Air Force lost many to spinning incidents.
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Old 27th Sep 2010, 03:19
  #108 (permalink)  
 
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With about a thousand hours instructing on them, I've done many hundreds of spins on about twenty different airframes. I never had any uncomfortable moments and although different airframes exhibited slightly different characteristics, the recommended entry and recovery technique leads to safe spin training.

However, there are different ways to enter a spin and this leads to different characteristics. The recommended entry is to apply full rudder at 50 knots and soon after full back stick with the ailerons neutral. It's unlikely that an inadvertant spin will have such an enrty - a more plausible entry would while in buffet, induce wing drop with rudder and attempt to correct with out spin aileron. This will very likely induce a spin and this spin is very different to the intentional. The former is a stable non-oscillatory manouvre which settles into the fully developed stage at about a full turn with about 50 degrees nose down, recovery taking about two turns. The latter is a flatter, slower rotating spin with oscillations and which enters the fully developed stage in about half a turn or less. Rudder is noticeably less effective and the recovery takes about three to four turns.

There are few records remaining as to the accidents which led to the fitting of the anti spin strakes, but it's clear they are result of a number of accidents occurring to the RAF aircraft soon after entry into service. De Havilland came up with the modification and it's not dissimilar to the one fitted to the DH82 Tiger Moth. About the same time, the wide chord rudder was fitted and this was intended to give better rudder authority during aerobatics, especilly slow rolls. But not for spin recovery, and in fact contemporary documents mention that the wide chord rudder does not aid spin recovery.

And the term anti spin strakes is a bit of a misnomer. We referred to them as such in the RAF, and the CAA's CAP562 leaflet 11-1 (Chipmunk Spinning and Aerobatics) also refers to them this way. But a 1960 DH document simply refers to them as 'Fuselage Strakes' and I suspect the wrong nomenclature has stuck with time. CAP562 describes them as increasing the aerodynamnic drag of the tail, damping rotation in yaw and steepening the spin. This doesn't really make sense if you think about it and differs from the RAF's explaination of how they work which is that at the high angle of attack of the spin, they act as a vortex generator and re-energise the airflow over the rudder making it more effective. This would help recovery in the latter type of spin I described, so I'm sure they were fitted in response to inadvertantly entered flatter spins which recovery from which would quite clearly benefit from an increase in rudder effectiveness.

An Australian document written in 1960 after flight testing of Chipmunks in response to series of accidents mentions that;

"It was found that although the strakes had no effect on the entry, the spin itself or on the recovery of an aircraft with good recovery characteristics, they did tend to shorten the recovery time slightly on an aircraft normally slow to recover, but it was only a reduction in the order of three quarters of a turn in the worst case"

The UK CAA don't agree, and UK registered Chipmunks without fuselage strakes and the wide chord rudder are prohibited from aerobatics and spinning.
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Old 27th Sep 2010, 03:55
  #109 (permalink)  
 
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''But RAF Chipmunks DID eventually have heaters! Mod H335 introduced in 1979 involved fitting the Canadian style heater system, detected by the small projecting intake in the front of the cowl and the steeply raked exhaust stack - it's readily apparent in the photos of the EFTS formation for instance.''

Yes, and it did an excellent job of raisng the temperature of your left leg by two degrees!



''Does anyone know about the background of the UHF conversion. It must have been incredibly expensive. Were these sets designed and built for the Chipmunk or were they used in other aircraft?''


RAF Chippies has several radio fits. I have flown them with four fits. First, the crystal VHF sets which had only about 8 channels numbered from A to G (?) with a controller on the coming (as fitted to Spitfires in WW2). Later these were fitted with a tuneable Marconi radio which had 720 channels, controller again on the coaming. The original PTR170 UHF set had again a limited number of channels, 1 to 12. Contoller yet again on the coaming. Later, some were modified with the PTR1751 with digital controllers in each cockpit which were the same ones as fitted to many frontline arircraft and even had the ability to control frequency agile anti-jam Have Quick 2 radio tranceivers - a bit excessive in the Chippy!

The EFTS aircraft I flew had the PTR1751 UHF sets with a separate Dittel 720channel VHF which was under the intrument panel by your right knee in the front cockpit and very difficult to see and tune.

Army and Navy aircraft had different fits.
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Old 27th Sep 2010, 06:35
  #110 (permalink)  
 
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India Four Two,

"Dora-9, I see your aircraft still has the UHF aerials. Did it come with the whole fit, or had the boxes been taken out?"

No, all the boxes were removed although the Channel Selector Panel on the RH of the rear cockpit was still there.

bcgallacher:

"Was was awarded a Flying Scholarship while a schoolboy in the early 60s - did the training at AST at Perth"

Oddly enough, I also was an ATC Flying Scholarship recipient in the early sixties (1961) in Perth, only my Perth was in Western Australia! I'm not saying we were better (this scribe clearly wasn't), but our Chipmunks all had the narrow-chord rubdder and no strakes. In fact I doubt there was a Chipmunk with strakes anywhere in Australia then!

Dan Winterland - a very well written piece I thought. Nice to see you point out that the broad-chord rudder had NOTHING to do with spinning.

"The UK CAA don't agree, and UK registered Chipmunks without fuselage strakes and the wide chord rudder are prohibited from aerobatics and spinning."

Similar here now in Australia too.Our authorities accepted the T.10 on the register unlike the UK where it had to be a Mk.22. But with the advent of an Approved Flight Manual for the T.10 (there simply wasn't one until the late 1990's) this restriction now applies, although only to aircraft without the strakes (the larger rudder is not mentioned). also.

Because a large number of Chipmunks that came here in the late 50's were surplus RAF aircraft that were disposed of before the advent of the strakes (and never qualified to be re-fitted with the "big" rudder either) even today the majority of Australian Chipmunks don't have strakes.
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Old 27th Sep 2010, 08:08
  #111 (permalink)  
 
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twochai
Way to go, Bri! Get those poms thinking.
Actually, we Brits were operating a Lycoming Chipmunk back in the early 70s as a glider tug at Bicester. Very nice to fly, obviously better ptw ratio and much quieter cockpit. The electric start was handy too - we always hand swung our Gypsy chippies.
We also had 2 ex-crop duster Gypsy chippies with fixed leading edge slots, both single seat. One had a squat little box canopy and was a real drag-master but nevertheless had good climb rate and superb slow speed handling. The other had a blown canopy and was a dream to fly. I'm sure the bubble produced extra lift as the climb performance seemed as good as the Lycoming aircraft - handled better too. On good thermic days, when a glider pilot didn't pull off in lift and I knew their competence I would crank us into a thermal (left turn) and often achieved 1500fpm+, waving off with a roll to the right at the club's statutory 2000ft. Again the low speed handling was a dream and it was possible to sideslip at speeds standard Chipmunks would have been stalled at. We often had to increase power after landing to taxy up to the glider line, having cleared the boundary hedge with our tow rope. Standard RAF chipmunks never achieved half the launch rate these aircraft did - most of their time was spent taxying.
The Chipmunk is a beautiful aircraft. After I'd been flying a Pitts S2A every day for a year I was asked to show a Chipmunk owner some aeros in his aircraft. It was so nice to get back in an aircraft with such beautiful control harmonisation. We did a few basic aeros - loops, avalanches, barrel rolls, slow rolls and stall turns and, some 30 years later, I still remember landing with an ear-to-ear grin.
Sadly I no longer have any photos and I can't remember the registrations. My log book was 'borrowed' by the Lagos police and not returned and one of my ex-wives burned all my piccies - lovely girl, but she's a good mum.

Last edited by rodthesod; 27th Sep 2010 at 14:49. Reason: My memory of a/c registrations not what it used to be.
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Old 27th Sep 2010, 13:19
  #112 (permalink)  
 
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G-AOTF was one of the ag-chippys. Shown here in about 1985 with a 160 hp Lycoming in a fetching air defence grey colour scheme - which when you think about it has to be the worst colour to paint a tug. IIRC, she was a Mk23 which used a raised rear cockpit as the flying position, had a space for a hopper where the front seat was, and I think she had leading edge slots as well. A bit lighter than normal Chippys, she didn't suffer much performance loss with her 'uglification'. Except that with a Lycoming, she wasn't allowed to spin or aerobat under CAA rules, as she was modified as a glider tug and such certification wasn't deemed necessary.

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Old 27th Sep 2010, 13:37
  #113 (permalink)  
 
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Spins

Talking about spins, I always hated and feared them. In OUAS in the early 1960s we were regularly checked by our instructors and then expected to go off and do them solo. The entry was always of the classic variety, that Dan describes above. The first or second turn would be inverted, as I remember, and after a couple more the nose was firmly down, almost vertically it seemed. Recovery took a while when the spin was fully developed, but you just had to hang on. Thing was, to keep the wings level.
Would we have recognised an incipient spin from a messed up approach or bank at too low speed, I am not sure!
Happy days.

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Old 27th Sep 2010, 21:43
  #114 (permalink)  
 
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Spins and Strakes

Spinning
From my imperfect memory of Chipmunk flying with the UAS in the mid sixties, the strakes were described as spin recovery aids. They may have improved airflow over the elevator. I do not remember any of the aircraft going inverted. Going inverted could have been caused by poor rigging.
A general handling sortie would end with a spin followed by a PFL (not below 250 feet agl).
Stalling
Each stall was never the same. Perfect aircraft balance was essential to achieve a textbook recovery. I was told that one aircraft was taken out of service when it would go inverted in the stall recovery. The aircraft was scarapped even though the rigging tested OK.
I was always more careful with stalling than spinning.
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Old 28th Sep 2010, 00:36
  #115 (permalink)  
 
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OK, Dan Winterland gets the gong for the most coherent explanation of the evolution of the (not) anti-spin strakes on the
DHC-1.

But now, what about them canopies?? Why did the Brits and the Portuguese not use the bubble canopy in their production??

TC
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Old 28th Sep 2010, 09:59
  #116 (permalink)  
 
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Chipmunk is beautiful

Spinning in Chipmunks, Thoroughly enjoyable!
The ones I flew at BFTS at Burnaston in 1952 behaved impeccably in fact I once spun down from 10,000 feet to about 5000 just for fun.
It took ages to reach 10,000, and spinning was a quick way down without exceeding permitted airspeed.
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Old 28th Sep 2010, 13:21
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During my time on ULAS as a student, solo spinning was still in the syllabus. So off I went one fine day to do the deed....

HASELLs diligently completed, close throttle, keep ball in middle, trim as instructed, then at the moment critique apply full pro-spin controls, up and over she went and into the spin.

After the first few seconds there was a loud thumping and banging from behind me, so I rapidly recovered - all was fine. A few gentle aeros - no problem.

I mentioned this when I came back - and the general consensus was that the knotted ends of the rear seat harness had perhaps been banging against the aluminium seat structure. So I went out to the aircraft (still 'soloed up') and experimented - yup, that was definitely the cause. Then a coffee, off for another trip including solo spinning (after first making sure that the rear seat straps were really tight and the ends tightly knotted together) - all was fine.

A few weeks later some edict came down from on high banning students from solo spinning. Rather a pity as it gave you lots of confidence!
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Old 29th Sep 2010, 00:15
  #118 (permalink)  
 
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Prior to Dan's comprehensive posting, I tried finding an online copy of the T10 Pilot's Notes to refresh my memory on the standard spin entry. Dan confirmed what I remembered - 50 kts, full rudder, stick back and neutral ailerons.

I had no luck with the T10 notes (well, not a free copy!) but I did find a copy of the DH Pilot's Manual for the civil Chippies (Mks. 20, 21, 22 and 22A) (http://is.gd/fyBII ). The recommended spin-entry includes the use of ailerons:

The aeroplane is difficult to spin properly at almost all centre of gravity positions. Therefore it is usually necessary to apply ailerons against the intended direction of spin, in addition to the normal pro-spin control movements. Entry with central aileron will probably cause the aeroplane to describe a semi-stalled spiral dive. This is often confused with a true spin
I find this very surprising. I did spins in two civil Chippies prior to my time with UBAS and in those we did the aileron-neutral entry, just like in the T10, with no problem entering a true spin. Is there something different about the civil versions or were DH just being very cautious? I don't ever remember getting into a spiral dive instead of a spin.

Beagle, I hadn't heard that solo-spinning was banned. Did you find out what happened to trigger that?
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Old 29th Sep 2010, 03:10
  #119 (permalink)  
 
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From AP 101B-5510-15 (Chipmunk T10 Pilot's notes) last amended 1993.





Similar, in that it mentions the opposite aileron to give a cleaner entry, but in the RAF flying training environment, this was never done. It was always an aileron neutral entry, to the point we taught the students to use both hands were on the control column to ensure it came back neutral. From the Student's Study Guide......



I gather the technique to enter the spin in the RAF changed over time. It used to be a lower speed entry which would be done close to the stall, this would probably have led to flatter more oscillatory spins which were harder to recover. The 50 knot entry is more of a 'flick' entry which is cleaner and leads to a more stable spin. The DH manual may be referring to the earler technique. This is from the Canadian flight manual published in 1957.


Last edited by Dan Winterland; 29th Sep 2010 at 03:48.
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Old 29th Sep 2010, 07:23
  #120 (permalink)  
 
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More really great stuff, Dan. I've never seen the 1993 notes, I have the 1953 and 1966 ones, plus a 1969 Chipmunk T.10 Student Study Guide.

Just in case someone decides to try this at home, could I point out in reference to item (iii) "...more positive spin entry...by applying aileron opposite the direction..." that CAP 562 states "If aileron is so used it should be centralized when entry is achieved. Under Spin Recovery: "b) check ailerons CENTRAL;"

Lastly, in my posting on 26th Sept I made the comment: "..the Australian DCA published a report in June 1960 (i.e. two years after the RAF fitted their strakes) which never mentions the effectiveness or otherwise of this item?" Having finally located my references to this, I can categorically state that I was wrong!

Dan's earlier posting stated:
"An Australian document written in 1960 after flight testing of Chipmunks in response to series of accidents mentions that;
It was found that although the strakes had no effect on the entry, the spin itself or on the recovery of an aircraft with good recovery characteristics, they did tend to shorten the recovery time slightly on an aircraft normally slow to recover, but it was only a reduction in the order of three quarters of a turn in the worst case"

He is correct, this is an exact quote from the DCA document.
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