Flat noses on Po- fighters

[warhawk]

Hello,

I am interested in the reason why I-15, -153 and -16 had a flat frontal surface of the cowling, perpendicular to direction of flight?
I suppose the reason is probably better cooling (through the shutters), but doesn't it add a significant amount of drag when these shutters are closed?

[Massimo Tessitori]

Hi,
I suppose that it was to make shorter the shaft of the propeller. Any more aerodinamic frontal shape would increase the length and weight of the shaft and of the cowling.
Regards
Massimo

[KL]

I am interested in the reason why I-15, -153 and -16 had a flat frontal surface of the cowling, perpendicular to direction of flight?
I suppose the reason is probably better cooling (through the shutters), but doesn't it add a significant amount of drag when these shutters are closed?

Hi W
You have already indicated answer within the question;  cowlings designed by Polikarpov were a compromise between the need to reduce drag created by a relatively wide radial engine and the requirement to cool the engine.

On I-15 the stress was on drag reduction only;  this was achieved by installing a shroud around the engine crankcase and by installing a simple Townend ring around the engine cylinders.

on I-16, I-15bis and I-153 it was about drag reduction and the requirement to regulate amount of the cooling air

-  to deal with the drag created by the engine cylinders engine was shrouded in so-called "NACA cowling"
-  in order to regulate airflow which cooled the engine a frontal plate with adjustable openings was installed in front of the engine.  


check:
http://en.wikipedia.org/wiki/Townend_ring
http://en.wikipedia.org/wiki/NACA_cowling

I suppose that it was to make shorter the shaft of the propeller.

length of the engine shaft was given by the engine designer/producer and it was kept as short as possible because of its weight. Shaft had to be made of steel and it had to be massive in order to transfer energy from engine to the propeller.  Extending the shaft would be a challenging engineering task and the gain in terms of frontal drag reduction would be insignificant...

HTH,
KL

[John Thompson]

Or (I think) more accurately, heating rather than cooling, but maybe I'm splitting hairs - fully or partially closing the shutters in bitterly-cold Russian winter operating conditions would have helped to speed warmup and to maintain optimum operating temperature while preventing overcooling.

John

[KL]

Or (I think) more accurately, heating rather than cooling, but maybe I'm splitting hairs - fully or partially closing the shutters in bitterly-cold Russian winter operating conditions would have helped to speed warmup and to maintain optimum operating temperature while preventing overcooling.

Yes, those adjustable shutters did help in winter conditions- it was a bonus.  Their primary role was to regulate amount of the cooling air.

All high power radial engines in late 1930es required some kind of cooling air regulation.  Planes could climb to altitudes where air was as cold as it is during the "Russian Winter"...

Zero which operated in tropical climate also had cooling air regulation, called "adjustable cowl flaps":



In mid 1930es, instead of the "cowl flaps" Polikarpov designed that frontal plate with adjustable openings.  Eventually, in late 1930 he did start experimenting with "cowl flaps" as I-166 prototype shows:



All later radial-engined fighters designed by Polikarpov (I-180, I-185, I-190) had "cowl flaps" instead of the frontal plate.





HTH,
KL

[Massimo Tessitori]

Hi,

All later radial-engined fighters designed by Polikarpov (I-180, I-185, I-190) had "cowl flaps" instead of the frontal plate.

Similarly to Lavochkins. It's interesting that Russian chosed to stop the access of airflow instead of the exit (or, aside the exit on some types). Probably this had the additional advantage to prevent snow to enter the engine.

Regards
Massimo

[warhawk]

Thank you gentlemen for very concise and detailed answers!

fully or partially closing the shutters in bitterly-cold Russian winter operating conditions would have helped to speed warmup and to maintain optimum operating temperature while preventing overcooling.

Looks like I made the same mistake as the French and the Germans did - forgot about the Russian winter! Won't happen again i promise  

Regards,
Aleksandar

[Graham Boak]

The fitting on the Polikarpov design don't appear to match the true NACA cowl.  The extension of the outer ring aft is there, faired into the fuselage, but we don't see the forward extension with reduced area intake and smooth ducting.  The slotted openings will provide a restricted flow, which is then free to expand (and hence cool) aft of entry, as with the NACA cowl, but this does not occur in as smooth a manner and thus additional losses are inevitable.  What the various versions of "aft control" show is that controlling the exit conditions can be important as the entry, with the additional complication of excess oil being spread over the airframe.

[KL]

fully or partially closing the shutters in bitterly-cold Russian winter operating conditions would have helped to speed warmup and to maintain optimum operating temperature while preventing overcooling.

Looks like I made the same mistake as the French and the Germans did - forgot about the Russian winter!

Hi Aleksandar,
frontal plate with shutters had a role during winter operating conditions; that's true (thanks John   ).
But, for what I know, the plates weren't removed from planes during the summer.  Frontal plates remained on planes in Spain under the scorching sun conditions too! - so, the plate with shutters  must have had some other functions too...

The fitting on the Polikarpov design don't appear to match the true NACA cowl.

In reality, Polikarpov's design had some elements of the NACA cowl, combined with the elements of other designs and some original solutions.  In I-16 case, NACA cowl was the starting point:
TsKB-12 prototype had "true" NACA cowl



Another design that influenced Polikarpov was the so-called "Watters Tunnel cowl" developped by a Russian emigree Michael Watter and utilized on Luscombe Phantom 1 and Monocoupe D-145 (at that time Watter's identity wasn't known to the Soviets and his design was called "kapot Uottera")





"Watters Tunnel cowl" was tested at Zhukovskii Academy lab, results of these tests were used by Polikarpov's team to design cowling for TsKB-12bis in 1934



This prototype had a cowling with 9 openings on its front (an opening for each cylinder), but no shutters.  Back of the cowling fit tightly the fuselage so that most of the cooling air actually had to exit the cowling through the ducts which accomodated exhaust stubs.

Finaly, 1935  I-16 M-25 № 123954, a prototype for Zavod 21 series (nowdays known as I-16 Type 5) had shutters plate installed behind the plate with the openings



HTH,
KL

[Graham Boak]

Thanks for the fascinating additional information.  The view of the cowl does show the sharp edges of the flow ducts, which would result in losses at the engine side as it expanded.  Early days of fluid dynamics, of course, and an illustration of how progress is made by small steps at a time.