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Comparing La-5 family cowlings
by Massimo Tessitori
Updated on December 2, 2004
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The shape of the engine cowling of the La-5 family was interpretated in different manners by many scale drawings authors.
The main interpretations are:

The argument was recently examinated on , and their thesis, as you can read, is that only La-5 and La-5F had rastremated cowling, while La-5 FN had cylindrical cowling.
Voronin's drawings are a precious and highly detailed source on this type, but some defects are suspected. The argument is of high interest for a modeler, because they are the main reference on the subject, even for some kit manufacturers.
So, I started to check this theory, asking help to friends and looking carefully all the photos I have on the subject.

The first surprise came from this photo.
The sides of the engine cowling look parallel or, if we consider the perspective, nearly parallel. 
They are well differents from those of Voronin-Kolesnikov drawings, where the contours of the cowling are in line with those of the rear fuselage, that is obviously rastremated.
At a first look, I thought that this confirmed the cylindrical shape of La-5FN, but then I discovered that this was an early La-5.
It is clearly distinguishable for the shape of the fairing over the nose and on the outlet flaps.
So, was the early La-5 cowling cylindrical too as that of La-5FN, or was it different?
A clear confirmation of the circularity of section of La-5 and 5F comes from this image from the rear (on the right). 
The cooling air outlet flaps of these versions, when in fully closed position, create a rather quick passage from the circular section of the cowling to the flatter section of the fuselage sides.
On the small image below, one can see (as a change of shade) a curved nail-shaped bending line on the flap; it separes the front part (brown, adhering to the cowling and approximatively cylindrical) from the central and back part (green, adhering to the fuselage surface and approximatively plan).

The bended aside radio mast is noteworty; it is not clear if this is accidental or common to other aircrafts too.

Here is a La-5 side.
We see that outlet flaps are in partially open position, as necessary when the engine needs to be cooled by air.
On the side of the cowling, we see fast locks that are common to La-5F, but they are missing on FN.
The oval section of the exhaust stack is well visible.
Here is a La-5F nose; it is identical to that of La-5. 
Here we see that the flap can be fully closed and adhere to the fuselage surface.
Only the passage for fumes from exhaust collector is always pervious.
The small image below shows the logo "F" for the aircraft equipped with the ASh-82F engine. They usually were "bubbletop" aircrafts, but sometimes this engine and logo was observed on "razorback" aircrafts too.
Here is a La-5FN from a nearly identical perspective, that allows an easy comparison to a La-5F. 
We see that the cowling section  immediately in front of the outlet flap is identical.
The rectangular metal plate behind the exhausts seems a bit more bended than the one of La-5 and 5F; consequentially, the section behind the exhausts should be a bit more curved and wider than earlier versions.
This was allowed by replacing of the only large exhaust stack with many thinner ones.
Although the image shows the outlet flap in slightly open position, it is clear that its cross section is more curved than the one of the fuselage and that the passage can't be fully closed even in fully retracted position.
From a functional point, this is obvious, because there are the exhaust pipes under the flap, and the fumes need space to pass.
On La-5FN only there is a triangular metallic plate on both sides, behind the rectangular one common to earlier versions.
The cowling profile is straight for all the considered versions.
Small deformations could be due to the fact that the cowling is not perfectly rigid and could be bent while forced to close.

An interesting point about the cowling of La-5FN is that the side panels are hinged in two positions, on the upper side and in middle position, at the heigth of the propeller axis; this allowed the side panels to rotate each on the other one while opened (see image below). 
Well, to function, an hinge must have a rectilinear axis; so the cowling should be cylindrical.
But look at the image on the right: the upper hinge is clearly rectilinear, while we see that the one on the side is curved. How is it possible?
The cowling is not too rigid: while there is a middle ring between the cylinders rows (see below), the front and rear edges of the side cowling are fixed by metal coils and forced to be adherent (on the right), so they bend. 
Probably the cowling sides are moulded as cylindrical, but they bend on the middle internal ring when they are locked in close position, so they look slightly onion-shaped; perhaps there was something creating interference between the internal side of cowling and the central ring.
In correspondance of the upper hinge of FN the cowling is cylindrical, and the hinge linear. 
The profile can give the impression of double curvature only because the dorsal air intake fairing becomes narrower between the first and central ring.


This image allows us to observe other details too.
The air flow inlet shutter allows to stop cooling air flow when the engine is cold and needs hotting. These shutters are two rows of 12 blades each, overposed and hinged in opposite sides.
When the air flow has to be stopped, the shutters rotate to fully close the section.
When air flow is needed, the shutters rotate and disappear behind the spinner.
In old drawings and kits, these shutters were often represented as a fan.
Note the starter tooth: its screw inclination is opposite to that of the prop blades, to disconnect from the ground starter shaft when the engine starts.

What about La-5 and 5F? 
The cowling opens in different way, with upper panel hinged upwards and lower panel hinged downwards; here the panels were dismounted.
There is a visible strut, parallel to the flight axis, on which the panels were adhering when closed; it is absent on the La-5FN.
The ring between the cylinders rows is present here too.
One of the coils used to lock and take adherent the side panels is visible on the photo.
We see the exhaust fumes collector that ends in a large exhaust stack on each side of the aircraft, and is different from the arrangement of La-5FN engine.
Was La-5 and La-5F cowling bended on its sides as that of  La-5FN?
Yes, here we see the junction line between upper and lower panels, and it is clearly curved.
Note the poor fit and the accidental bending of edges of some panels.

The outlet flap can show two possible shapes, fitted indifferently both on La-5 and on La-5F:
        . The first type of flap, that is the most common on photos, has a shorter and straighter exhaust fairing. 
The oval section of the exhaust stack is evident.
The second type of flap has an exhaust stack fairing longer and more rounded in section; it is more protruding too.
It's unclear if the exhaust stack section was the same; probably it was circular.

Let's resume our observations into some sketches, showing in red the modifies that should be done to the drawings by Voronin-Kolesnikov:
The upper view of the nose of La-5 and 5F should appear not rastremated, only a bit curved because of bending.
The rivets and fast locks should be moved shightly forward.
The exhaust pipes fairings should be cutten about orthogonally to the flaps; the illusion of a strongly oblique cutting is due to the fact that they often appear partially open on profile photos.
The flaps should show a slight bending line close to the exhaust pipes fairings(drawn in pink).
The reptangular plates behind the stacks seem to be narrower and more parallel to the aircraft axis than on original drawings.
The upper view of La-5FN should appear not rastremated, only a bit curved because of bending.
The cooling air outlet flaps don't adhere to the rear fuselage even in fully closed position.
The reptangular plates appears a bit wider than those of La-5 and 5F just behind the exhausts; in section, they should appear a bit more curved in this position.
The profile of the nose of  La-5 and 5F should be slightly modified:
the fast locks and rivets on the side of the cowling should be moved slightly forward.
The exhaust stack fairing is wrong in original drawings, it appears cutten obliquely on the rear, but it should be symmetrical. 
Two alternative shapes can be found in photos.
A bending line is drawn in pink; it divides the front part (about a cylindrical surface) from the rear part (nearly plan).
The lower profile of the cowling should be about straight; an hardly perceivable curvature could be seen on the lower profile, immediately behind the coil ring.
The profile of La-5FN should be slightly modified:
the fast locks and rivets on the side of the cowling should be moved slightly forward.
The lower profile of the cowling should be about straight; an hardly perceivable curvature could be seen on the lower profile, immediately behind the coil ring.

On the whole, these conclusions could justify partially both the description of the cowling as cylindrical, that seems to be the base geometry, both the description as onion-shaped, due to bending of the side panels when locked.

I would thank Aleksandr Ruchkowsky, Matthias Erben, Milos Vestsik and Aleksej Ilic for their help on this matter.
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