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Author Topic: La-5 Cowling Shape  (Read 12121 times)
mholly
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« Reply #15 on: August 22, 2010, 12:05:12 AM »

Hi Massimo,
Quote
You are considering thin metal parts, as sheets and rods, as rigid pieces, and could be not exact. They could bend
Sorry for being so persistent but what you're saying defies some principles of aircraft construction. Those pieces are supposed to be rigid, they not only cover the engine but also bear forces and tensions imposed on the airframe by aerodynamic forces. If you look at this pic again you'll see that those panels are strengthened from inside!
http://www.postimage.org/image.php?v=Pq1W1gyi
Thickness of metal sheets covering the aircraft can vary from 0.6mm to 2mm. I just can not imagine how they can bend easily resulting, in addition, to material fatigue!
I didn't quite undestand what you meant "constrained by coils"?
Cheers,
Mario
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Massimo Tessitori
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« Reply #16 on: August 22, 2010, 09:33:13 AM »

Quote
Sorry for being so persistent but what you're saying defies some principles of aircraft construction. Those pieces are supposed to be rigid, they not only cover the engine but also bear forces and tensions imposed on the airframe by aerodynamic forces. If you look at this pic again you'll see that those panels are strengthened from inside!
http://www.postimage.org/image.php?v=Pq1W1gyi
Thickness of metal sheets covering the aircraft can vary from 0.6mm to 2mm. I just can not imagine how they can bend easily resulting, in addition, to material fatigue!
I didn't quite undestand what you meant "constrained by coils"?

Hi Mario, Smiley

For coils, I mean two thin strips of metal that seal the front and rear gaps of the cowling and are locked by some fast-lock tensioner; they are visible on photos, sometimes unpainted. They press the side panels from outside to inside. Should panels be undeformable, they would be unuseful.

This image shows the hinge of La-5FN seen from upper-front position, and the perspective shows the curvature. But I know that this could be the early type cowling without central hinges.


I have not yet found a definitive photo showing the shape of the cowling of late type. The most satisfying available till now is
,
that shows a more straight line of the early type. Have you found better images showing this line?

There is a photo of La-5FN in factory, with the coils not yet installed, at pag.52 of  Lavockins' piston-engined fighters of Yefim Gordon. If the plate stays in closed position, it means that it has already found some stop, probably the longitudinal strut aside the engine and/or something on the lower side.
The hinge line appears straight, but a gap of some millimeters is visible on the front.
When the coil will be installed, it will press the plate aligning it to the frontal ring, and bending it. However, it's question of few millimeters only.

Apart  from the discussion on La-5, I would make some considerations about fatigue: all aluminium alloy structures of the planes are projected for a limited fatigue life. The structure of DC-3/C-47 is the only exception  that I know.
The rod of the hinge is, by sure, made by steel, that is more resistant to fatigue than aluminium alloys. Besides, if the tension due to deformation remains within the limit of the Wohler diagram, the fatigue doesn't affect the piece.
The important in-flight rigidity of pieces is the rigidity of the ensemble in flight condition, not the rigidity of separed pieces. As an extreme example, one can think to fabric, that is not rigid in itself but only when constrained on a metal structure.

Massimo Smiley



 

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Graham Boak
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« Reply #17 on: August 22, 2010, 09:41:17 PM »

I agree with you that all aircraft parts will bend - making them totally rigid would make them too heavy.  You are also right that the strength of a structure is in the complete structure.  However, you are wrong that a panel edge, clear of a hinge, will move millimetres when tightened against a former.  It will move thous.  Thousands of an inch.  The metal isn't that flexible over short distances.

Even were you right, you are talking about only that part of the panel adjacent to the edge.  It will not result in a bowing effect over longer distances visible to the eye. Plus, as I said before, any bowing would result in distortion elsewhere and visible gaps and steps.

The metal strips around the cowling are simply fasteners, probably chosen to avoid the production problems of more normal fasteners and their resulting drag.  In order to produce the kind of compressive force your argument requires these cowl strips would have to be tightened with powerful jacks.  These don't seem to be visible in any photos I've seen, and indeed would rather run counter to the Soviet practice of minimal specialised support equipment.

If any were bulged, this was a design and manufacturing feature.
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John Thompson
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« Reply #18 on: October 20, 2010, 07:36:37 PM »

Here's a photo that was posted recently on Scalemodels.ru by mmoustaf. It shows a slight curvature of the La-5FN hinge line, which suggests that the cowling is, in fact, bulged slightly when closed:



Also of interest, I think, is the painted tips of the prop blades, not commonly seen during the GPW. The photo is captioned "Alexandr Matashov and Vladimir Morozov at Konigs", but is undated. As best I can determine, "Konigs" is the town of Konigs Wusterhausen, which is located a short distance to the southeast of Berlin. This was part of the area occupied by the Red Army during their advance on Berlin; the area in which Konigs Wusterhausen lies eventually became part of East Germany. This suggests that the photo was taken either very late in the war, or, more likely, early in the post-war period. If correct, it may provide some information regarding the implementation of painted prop tips on VVS aircraft.

John
« Last Edit: October 20, 2010, 07:47:41 PM by John Thompson » Logged
Massimo Tessitori
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« Reply #19 on: October 21, 2010, 11:34:21 AM »

Hi John,
good image; but we don't know for sure if this is an early-type cowling, or a late type hinged between the panels; so, it's difficult to take it as a proof of the theory that the hinge bends.
Regards
Massimo
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John Thompson
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« Reply #20 on: October 21, 2010, 04:06:14 PM »

You're right, although I don't see any sign of the latch which should be present if this was the early cowling.

Late La-5FN:



Early La-5FN:



I still think it's a late-type cowling, but I don't post these things to try to win arguments; I just like to present the "evidence" and let the jury decide for themselves. I'm easy... Wink

John
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Graham Boak
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« Reply #21 on: October 21, 2010, 06:11:42 PM »

This metal distortion suggestion can be shown to be wrong with a piece of paper and a curved surface.  Being in the UK, I'd suggest a milk bottle, but any sizable cylinder would do.  Take the paper and fix one end, wrapping the other around the cylinder - imagine this the Lavochkin's engine cowling and trim to an appropriate size.  Now fix the other end.  The paper conforms to the curve of the cylinder, with straight sides.  Now loosen the bottom edge, and try to create a bulge shape in the side of the paper.  You CANNOT do this without creating large distortions and gaps around the edges of the paper, particularly the bottom edge.

If there was a bulge in the cowling, then it was preformed in the part, and will be visible with the cowling open or closed.  I'm reluctant to take sides, but I will point out that such a piece would require a heavy press, and be much more expensive in time and effort than a simple 2-D curve which could be done on any metal former.  It would also require curved interior strengthening pieces.  This doesn't sound like Soviet WW2 production methods to me.
« Last Edit: October 21, 2010, 06:21:24 PM by Graham Boak » Logged
mholly
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« Reply #22 on: October 23, 2010, 08:51:00 AM »

Quote
This doesn't sound like Soviet WW2 production methods to me.
Cann't agree more. It will be a simple cylinder to me until someone (in Russia) finds a wreck and prove it otherwise.
Cheers,
Mario
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Walker
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« Reply #23 on: November 18, 2010, 03:28:03 PM »

About the bulbs, hood, studied a lot of photos and different views. There are photos where the bulb is very well seen, there are photos where the bulbs are not at all. Does not depend on the time of release, or the modification or the state of the aircraft. In the technical description of the plane of the hood is written very little. I do not think that the bulb hood was originally intended. But it is.
 The most probable cause of the bulbs have already voiced Massimo. The fact that the hood on the La-5 blew down the special steel strips. The anterior and posterior junction with the effort. In the middle of the hood is the support ring deflector cylinder engine. On this ring glued felt tape to seal the joint. Depending on the shrinkage of the ring, and the tension force of tightening steel rings, the hood can deform considerably.
In working on a master model, I tried to do the hood in the form of an absolutely correct cylinder. So, it looks a cylinder shaped hood quite unlike the picture. So I gave a barely noticeable 0.5-mm follicle and hood soon became like a photo.
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John Thompson
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« Reply #24 on: November 18, 2010, 07:33:00 PM »

Thanks for the information, Musa! I had decided that I wasn't going to dispute this any longer, but regardless of what logic may suggest regarding whether it's theoretically possible for the cowling panels to deflect or not, the fact remains that some photographs do show the hinge line to be bulged, just as you've stated.

John
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Graham Boak
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« Reply #25 on: December 04, 2010, 01:05:01 PM »

I understood we were talking about the sides of the panel, not the hinge line.  Is this not a piano hinge?  If it is, then it CANNOT bulge, for it HAS to be straight to work.  Not theoretical, but very real and practical.  This reminds me of the old (very old) plans and models of Bf109s, which had a curve from the cockpit to the nose instead of the real straight line, with the panel curving only outside the length of the hinge mechanism.

If it is not a piano hinge, ok, this posting is not relevant, but engineering realities are not "theoretical".
« Last Edit: December 04, 2010, 01:07:44 PM by Graham Boak » Logged
Walker
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« Reply #26 on: December 05, 2010, 01:25:32 PM »

Piano loop can not operate in a bent condition is true. But the conversation is not about that. It is a strain panels hood during the contraction of steel bands. Due to the elasticity of the metal in the free state loop is rectified and everything is working.
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Graham Boak
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« Reply #27 on: December 05, 2010, 02:00:38 PM »

I have doubts about that by itself.  However, for the hinge to bend, you are not only bending the (arguably flexible) panel but the rigid structure to which the other half of the hinge is attached.  This seems unreal to me. 

You also have to be opening gaps between structures - are these visible?  To say a little more about this, think of the circumferences of the sections.  It is one length where it is circular(ish) at the front and back of the cowling.  If it is to bulge in the middle, then it must grow to have a larger circumference, and this cannot be done without opening gaps.  Big, aerodynamically important, gaps.

If there is an "onion" shape to the cowling, then it was built that way not created by Samsonovitch bending metal structures.
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Walker
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« Reply #28 on: December 05, 2010, 09:37:54 PM »

Here's a picture from the factory documentation:
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John Thompson
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« Reply #29 on: December 06, 2010, 01:13:57 AM »

Here's a picture from the factory documentation:


Thank you, Musa - that's exactly what I expected. Slightly bulged in plan view, and not onion-shaped! How it gets that way is not important - both the drawing and photos show exactly what I've been saying.

John
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