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Duel: PBY Catalina vs. H5Y Cherry


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#11 Rick65

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Posted 03 February 2018 - 01:45 PM

The interesting thing about this comparison is how it highlights that for non well known or unpopular topics much stuff on the internet  is straight cut and paste from a very limited number of sources. Most articles on the Cherry are almost word perfect repeats of the data from Francillon quoted by Corsning in post #2.

There was one reference to structural issues in one source but no more information.

 

I am not sure you can claim it has too small a wing, 108 sq m is a lot for a twin engined plane and the weight per sq m of wing area was less than for the Catalina which has a huge wing without flaps. A B-17 wing is only 2 sq m bigger than a Catalina one.

 

Maybe it just wasn't very good. Look up the Saro Lerwick for an example of a twin engined seaplane disaster.



#12 Rick65

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Posted 03 February 2018 - 01:46 PM

Double post so I will edit and ask a question.

How come Cherry got an Allied reporting name given the small number produced (2 +28) and the limited service.

The later Aichi H9A a smaller training seaplane was produced in similar numbers, served longer but never gained an Allied reporting name.

Was it because the Cherry was a pre war plane where obtaining information was easier?


Edited by Rick65, 03 February 2018 - 01:54 PM.


#13 bearoutwest

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Posted 03 February 2018 - 03:21 PM


"... I am not sure you can claim it has too small a wing, 108 sq m is a lot for a twin engined plane and the weight per sq m of wing area was less than for the Catalina which has a huge wing without flaps. A B-17 wing is only 2 sq m bigger than a Catalina one. ...."

 

Not saying it's a small wing, just smaller than the PBY, which is one comparable "guess" as to why it had such a slow rate of climb compared to the PBY.

 

The B-17 had more power (4x turbocharged 1,200 hp motors) and lot less drag, even with all those gun turrets, because it lacked the boat hull.  So not really an apples with apples comparison.

 

The PBY had no flaps, so a simpler structure with no need for hinge points, control lines and flap retraction runners.  The H5Y (in some internet photos, including linked here: http://i1.wp.com/www...?resize=346,173), had about 20% of central wing area (i.e. between the sets of ailerons) devoted to flaps.  That's a lot more wing structural weight devoted to the high lift device and supports.  Weight-wise, PBY had more "efficient" wing lift-weight ratio....(my guess only, don't have any numbers available).

 

...geoff (or if you prefer "other-jeff")



#14 Romantic Technofreak

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Posted 03 February 2018 - 06:34 PM

 

...geoff (or if you prefer "other-jeff")

 

Sorry for lacking concentration. In German, you both are named "Gottfried"! (please speak "freed" in the end...)

 

Regards, RT



#15 Rick65

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Posted 04 February 2018 - 01:48 AM

My understanding is that the hardest task for a sea plane was taking off.

The "stiction" (adhesion due to surface tension & friction) of the water makes this a harder task for a seaplane than for a land plane on low friction wheels rolling over hard tarmac, dirt or grass. \

I have read many stories of Catalinas at high weights either failing to take off, suffering structural damage in attempting to do so or only leaving the water after extended and stressful runs at high power.

I suspect the wings for seaplanes are sized for this difficult task which makes them over sized for most other tasks.

The smaller Cherry wing should create less drag and is fitted with flaps to improve its efficiency for the critical take off and landings at a cost of greater structural weight.

The fact that the Cherry is not clearly faster than the huge winged Catalina underlines the point made by BoW that the rest of the Catalina design was well streamlined.

It is also possible that the exceptionally low climb figures for the Cherry are not correct as they seem to only come from one source, Francillon, and with the last Cherry being produced in 1939 they could have been old news by the time he got to Japan.



#16 bearoutwest

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Posted 04 February 2018 - 03:22 AM

Now you are taking me back to the dark recesses of my past, when I was young and silly and more interested in drinking beer and chasing girls*, rather than listening to my aeronautics lecturers.  (Though as we speak of flying boats, perhaps it was more disinterest in doing extra work for optional credits.)

Surface friction on boat-hull and wing lift - two quite separate though obviously linked and related subjects.

Surface Friction.

Part of the take-off technique is to get the boat-hull up on the "step" (use this photo-link for reference https://history.nasa...P-468/p166a.jpg) as soon as possible, it reduces the "sticky" force on the hull-afterbody.  In the reference photo of the PBM Mariner, that's about a 40% reduction in "stick-down" force.  You can then build up to take-off speed, while planing on the forward hull (forebody).  For wing weight efficiency, I guess you would design between this take-off load or your maximum take-off weight climb out load, whichever is higher.

On calm days, when the sea is still, getting the flying boat "up on the step" during take off is a big problem.  This indicates to me, that for maximum wing weight-lift efficiency - most designers don't try to come up with a wing that can lift the max take-off weight of aircraft AND generate enough extra force to break the surface friction on a hull forebody and afterbody.  From my reading over the years, techniques to aid take off during calm seas included 1) using the supporting marine launches to generate wake disturbance across the take-off path; or 2) if no launches available, to do a high speed taxi in a circle so that the take-off run cuts across the flying boats own wake.

Taxiing across the wake disturbance was generally enough to break the surface friction force, and get the hull afterbody out of the water.  The rest of take off then proceeds normally.

Wing Lift.

Flaps are always a trade-off between high-lift and high-drag - can't avoid it.  Dropping the flaps changes the wing profile (and increases the C(L) in the Bernoulli equation previously discussed).  This has the effect , though, of increasing the C(d) - Coefficient of Drag - in a similarly related Bernoulli equation.  Increasing the drag, decreases available force in forward-motion, essentially decreasing the speed - results in a decrease in lift force (in proportion to the speed-squared).  There's a band of tolerance in which the use of flaps increases lift before the drag over-compensates and decreases lift.  Simply putting bigger flaps on the wing, doesn't always work to your benefit.

The PBY wing is sized correctly to fly efficiently without needing flaps, but even it can't get off the water on a calm sea without using the maritime take off techniques.  It's plus benefit, is that the PBY can carry a relatively large payload with minimal drag increase.

Aeronautical design is typically a number of compromises between competing tasks - aerodynamics, structure, power, weight-balance, operational cg shift, client requirements.  Sometimes you get it right - Spitfire, Mustang, PBY, etc.  Sometimes you get it wrong.

Additional option for extra credit.

Check out the PBY's bigger brother - PB2Y Coronado.  The wing is long and slender, as opposed to the short squat wing of the PBY.  Known as the Davis wing, the design was similar to the laminar wing profiles of the P-51 Mustang - deeper at the 40% chord rather than at the 25% chord popular in the day.  This meant more efficient lift/drag ratio and deeper wing for storage of fuel tanks, weapons, etc.  The Davis wing would be further refined on the B-24 Liberator, and the B-32 Dominator.  One of the key selling points of the Davis wing was improved lift at small angles-of-attack (angle to horizontal).  Ideal for the flying boat take-off run and transition "on to the step".


* (I'm older and wiser now, and have learnt to drink beer, talk aeroplanes and chase girls at the same time.  Though the only girl is my daughter, and I still can't catch her running the obstacle course through the house...despite her laughing herself breathless at the time.)


Edited by bearoutwest, 04 February 2018 - 03:25 AM.


#17 Ricky

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Posted 04 February 2018 - 08:58 AM

If you put the beer down, it is easier to negotiate the obstacle course ;)
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#18 CORSNING

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Posted 04 February 2018 - 01:30 PM

* (I'm older and wiser now, and have learnt to drink beer, talk aeroplanes and chase girls at the same time.  Though the only girl is my daughter, and I still can't catch her running the obstacle course through the house...despite her laughing herself breathless at the time.)

 

These are the good old days Got'Fried.

 

2_supgoose1346846074.jpg

 

By the way RT, my wife says I am not fried. She says I am half-baked. B)

 

Well, RT said to speak "freed", that's freely in actual real English. :)


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#19 bearoutwest

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Posted 04 February 2018 - 02:06 PM

If you put the beer down, it is easier to negotiate the obstacle course ;)

In my defence, I didn't have time to put down the beer....we were racing for the last packet of available snacks (Mum/wife being out shopping for more supplies). 

 

Jeff,

That Turbo-Duck (oh alright, I'll use the proper name Turbo-Goose or is it a Turbo-Mallard, can't tell which one is bigger) gives me the distinct impression that it's doing a inverted, low-level fly-by.  I'd never noticed the upside-down arrangement of PT6A turbo-prop and intake before.

 

RT,

It's amazing how many people speak military German correctly without thinking about it.  We (almost) all pronounce Heinkel as hine-kel, not heen-kel (as in English).  Junkers is Yunkers and not with a "Jay".

 

...gottfried/geoff/other-jeff/hey-you

 

(Call me almost anything, just don't call me late for dinner!)






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