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P-51B inner gear doors - the diagonal-ish section, colour and function?


Paramedic

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I have looked a round a little but did not find the answer. Which surprised me, sicne I thought it would have been asked several times. Let us see if i need to be ambarrassed or not.. But here goes:

 

The inner gear doors on the P-51B/C - that diagonal plate that is sometimes painted Zinc Chromate (or Zinc Chromate GReen or even Interior Green!) and sometimes some metallic - what is it and what is correct colour (-combinations)? Speciafically I am building Margie Maru.

This borders wheel well colour questions a bit but I am quite satisfied with other threads on those overall on P-51s. I think.. :) (Bad choice of pics perhaps since this is a restored Warbird? But ignoe the green.. ;) )

 

Loaned this pic/link - it is froma  D model but looks the same (here?)

And since I brought ina  D model - are there differences here between the B/Cs and Ds?

 

9782319371_e7193a424c_h.jpg

 

 

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1 hour ago, gingerbob said:

Stainless steel, and is a scuff plate for where the tire might contact the inner door.

Yep, You are correct, sir. That being said, I have seen period color photos of P-51B's and D's that show the scuff plates painted in ZCY, which doesn't make a lot of sense, as I would think the tires would scuff the primer off after repeated landing gear cycles. Maybe in some instances, aluminum instead of stainless steel was used at the factory- thus the need for primer to prevent corrosion? You see this a lot on restored airworthy Mustangs.

Mike

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1 hour ago, Paramedic said:

are there differences here between the B/Cs and Ds?

 

If you are referring to the wheel bays, there are some differences. IIRC, due to the different leading edge extensions and a slight change in strut geometry when retracted, the hydraulic piston/strut for the inner fairing door was located at the front of the door on the B/C, and it was at the back of the door on the D. The inner fairing doors also had different external contours between the two variants. The D also had the landing light relocated from the leading edge of the wing to the LH wheel bay, where a small wooden roller (You can see it in the posted photo.) was contacted by the landing gear strut to retract it back into the wheel bay- the light was extended with gear extension.

Mike

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Thanks guys, as usual, fast and informative, :)

 

20 minutes ago, 72modeler said:

If you are referring to the wheel bays, there are some differences. IIRC, due to the different leading edge extensions and a slight change in strut geometry when retracted, the hydraulic piston/strut for the inner fairing door was located at the front of the door on the B/C, and it was at the back of the door on the D. The inner fairing doors also had different external contours between the two variants. The D also had the landing light relocated from the leading edge of the wing to the LH wheel bay, where a small wooden roller (You can see it in the posted photo.) was contacted by the landing gear strut to retract it back into the wheel bay- the light was extended with gear extension.

Mike

 

I was thinking about that door mostly. :) I had noted that plate on B/Cs more often. It was not really until I made the thread I remembered seeing it on Ds as well. Weird. Maybe because they are often (? not even sure about that but it feels like that) in different colours on those earlier ones?

But that was nice info none the less! I knew of some of it - but far from all.

 

Sounds Like I will be painting it Stainless Steel then. Many thanks!

 

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Pretty clever, so clever I could not even guess - even though I did not try really.. ;)

 

Is that a common thing? What happens if the tyres keep spinning - should not imballance? Fire hazard..? 😕

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Hmmm, I would first apply the toe brakes to make the wheels stop and then move the lever into "Up" position. Actually I do that every time (of course I'm not flying a P-51) I'm not an engineer either but everybody knows what happens if you are trying to turn a spinning wheel by 90 degrees (as Bertie PSmith says).

 

Cheers,

Antti

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10 minutes ago, Antti_K said:

Hmmm, I would first apply the toe brakes to make the wheels stop and then move the lever into "Up" position. Actually I do that every time (of course I'm not flying a P-51) I'm not an engineer either but everybody knows what happens if you are trying to turn a spinning wheel by 90 degrees (as Bertie PSmith says).

 

Cheers,

Antti

That sounds like suicide! What if you stopped in mid air from applying the brakes?! As a matter of fact I did get my education from Warner Bros. cartoons.........

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25 minutes ago, Antti_K said:

Hmmm, I would first apply the toe brakes to make the wheels stop and then move the lever into "Up" position. Actually I do that every time (of course I'm not flying a P-51) I'm not an engineer either but everybody knows what happens if you are trying to turn a spinning wheel by 90 degrees (as Bertie PSmith says).

 

Cheers,

Antti

 

I guess.. I have no clue. :) But is not the wheel already turned when the plate stops it? Or did i misunderstand?

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If those were meant to stop the tire turning once retracted, we'd see a lot of chafing and scuffing of that area in period photos.  I believe that they are  meant to protect the inside of the door from damage if it doesn't open as it should before the strut begins to swing out from its closed position.

 

EDIT: Aha! Note the tire on the aircraft's right (viewer's left) slightly dragging on the bottom of the inboard door during gear extension in the video linked by @72modeler below... 

Edited by Rolls-Royce
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On a P-51, I think the undercart is sequenced so that  the gear struts are engaged into the uplocks before the inner fairing doors close; the operation is reversed when  the undercart is extended. The video below shows it better than I can explain.

Mike

 

Note that the gear is fully up before the inner fairing doors close, and how the landing light is pushed up into the wheel bay by the strut. It is also a characteristic of the Mustang that the inner fairing doors close again when the undercart is down- they are only open during retraction, extension, or when the pressure in the hydraulic system bleeds off after shutdown or when the inner doors are released manually. Footage via YouTube.

 

 

 

Edited by 72modeler
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38 minutes ago, Rolls-Royce said:

If those were meant to stop the tire turning once retracted, we'd see a lot of chafing and scuffing of that area in period photos.  I believe that they are  meant to protect the inside of the door from damage if it doesn't open as it should before the strut begins to swing out from its closed position.

 

EDIT: Aha! Note the tire on the aircraft's right (viewer's left) slightly dragging on the bottom of the inboard door during gear extension in the video linked by @72modeler below... 

Yep- that's one of the things that can be adjusted, and one of the reasons an aircraft is put up on jack stands and connected to a hydraulic power source. Ideally, both struts extend and retract at the same rate, but on some aircraft, not sure about Mustangs, you will see first one strut retract and then the other- never been sure if that's by design or improper adjustment, as you see this a lot on Mustang warbirds. (Looking at the video, it does appear that the RH inner fairing door  doesn't fully open before the strut extends- good eye!)

Mike

 

A little off-topic, but see this video on retraction/extension of an FG-1 Corsair's landing gear; just another example of all the sequences and operations that have to be adjusted for proper operation of the undercart. I was not aware that the Corsair had a strut that compressed the oleos to fit the assembly into the wheel bay, just like the P-47. Footage via YouTube.

 

Edited by 72modeler
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15 minutes ago, 72modeler said:

Yep- that's one of the things that can be adjusted, and one of the reasons an aircraft is put up on jack stands and connected to a hydraulic power source. Ideally, both struts extend and retract at the same rate, but on some aircraft, not sure about Mustangs, you will see first one strut retract and then the other- never been sure if that's by design or improper adjustment, as you see this a lot on Mustang warbirds.

Mike

Probably because the single hydraulic pump wasn't up to providing the volume of fluid needed to move both struts at the same rate simultaneously. These were relatively low-pressure systems, so you had to move a lot of fluid instead. 

Did a few "jack-and-retract" gear swings in my USAF career!

Edited by Rolls-Royce
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11 hours ago, 72modeler said:

That being said, I have seen period color photos of P-51B's and D's that show the scuff plates painted in ZCY,

 

Yeah, frustratingly, it seems that every combination of priming/non-priming/painting, etc was tried throughout the P-51's life!

 

Just for yuks, here are some shots of the main wheel well of an ex-Guatemalan Mustang (44-77902).

You can see that, at some point, the well has been given a ZCY spray, followed by (presumably at some other, later point) an overall silver/aluminium coat.  I'm pretty sure I can see some scuffing of the plate, where the ZCY is showing through (at the bottom of the central embossing - which strangely, runs off the rub plate...).

 

246161993_10223288707980209_744128054167

 

246216072_10223288707620200_265204016645

 

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On 10/25/2021 at 1:45 PM, Bertie Psmith said:

 

Gyroscopic effects could be embarrassing perhaps?

Maybe not so much on a P-51, but definitely on larger aircraft with a nose wheel.  For example, on the C-135 family of aircraft there are two strips of rubber in the wheel well that stop the nose gear tires from turning on retraction because of the gyroscopic affect on the aircraft.  There is a name for them which escapes me right now (damned CRS!).

Later,

Dave

 

2220 PDT 26 Oct - I just remembered the name.  Spin Brakes!  I've never looked up on the inside of the nose gear well on a C-17, but on -135s, C-141s, and for sure C-133s, all had spin brakes.

Edited by e8n2
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8 hours ago, Bertie Psmith said:

 

Gyroscopic effects could be embarrassing perhaps?

 

7 hours ago, Antti_K said:

Hmmm, I would first apply the toe brakes to make the wheels stop and then move the lever into "Up" position. Actually I do that every time (of course I'm not flying a P-51) I'm not an engineer either but everybody knows what happens if you are trying to turn a spinning wheel by 90 degrees (as Bertie PSmith says).

 

Cheers,

Antti

Somewhere in the depths of my memory bank is a wartime memoir where the pilot mentions retracting the wheels the first time he flew a (I think) Hawker Typhoon, where the still rotating wheels set up a vicious shaking as they retracted, after which he made sure to hit the brake lever before retracting, so it definitely was a thing though what we're seeing on the P-51 is less sure in it's design effect as that I think.

Steve.

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4 hours ago, Rolls-Royce said:

Probably because the single hydraulic pump wasn't up to providing the volume of fluid needed to move both struts at the same rate simultaneously. These were relatively low-pressure systems, so you had to move a lot of fluid instead....

I believe that no pump would ever move two legs at the exact same rate. There will always be very small variations and the pressure will try to move whatever gives first.

I'm also convinced, that the designers and engineers at the time were painfully aware of this, but dismissed a perfect simultaneous retraction as irrelevant.

 

/Finn

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I've always assumed it severed the purpose that it has on other aircraft - when you alternate extend the gear you are unlocking the door and the leg (the doors have a separate release on the P-51) and the leg falls under it's own weight and contacts the door pushing it out of the way. This strip is just to prevent any damaged to the door in that situation, but the above video does indicate that the wheel contacts the door when extending even with hydraulics on, that's if they've got the sequencing timed correctly ;)

 

 

EDIT

 

Just having a read in the P-51A AMM and it states "As the pressure supply of the engine-driven pump has been purposely designed to be inadequate to perform all these functions simultaneously [all gear and doors operating when down is selected] , the ones requiring the largest fluid displacement—the operating struts and fairing door struts—will not actually be moved hydraulically during the initial part of their travel, but will fall of their own weight." Which I guess is why in the video above the tyres do actually contact the door in the initial part of their travel.

 

 

1 hour ago, stevehnz said:

 

Somewhere in the depths of my memory bank is a wartime memoir where the pilot mentions retracting the wheels the first time he flew a (I think) Hawker Typhoon, where the still rotating wheels set up a vicious shaking as they retracted, after which he made sure to hit the brake lever before retracting, so it definitely was a thing though what we're seeing on the P-51 is less sure in it's design effect as that I think.

Steve.

 

It's pretty standard to apply the brakes to stop the wheels spinning and a lot of more modern aircraft do this automatically but in the case of the P-51D it says in the Pilot's Manual (1945) "Caution: Don't brake the wheels after takeoff. Doing so may fuse the discs of the brakes that are hot from extended taxing".  So I'm guess this had happened on the Mustang so they decided it was safer not to do so. In the P-51B PM, there's no warning against this but it doesn't say to do it either.

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52 minutes ago, stevehnz said:

 

Somewhere in the depths of my memory bank is a wartime memoir where the pilot mentions retracting the wheels the first time he flew a (I think) Hawker Typhoon, where the still rotating wheels set up a vicious shaking as they retracted, after which he made sure to hit the brake lever before retracting, so it definitely was a thing though what we're seeing on the P-51 is less sure in it's design effect as that I think.

Steve.

 

stevehnz,

 

Now that you mentioned it, I think it was Pierre Clostermann who told about this in his book. It was his first flight in a Typhoon. I remember trying that myself on my very first flight in a Piper Arrow (after reading the book) and it was shaking rather alarmingly. Brake first then choose the gear up has been my SOP ever since.

 

Cheers,

Antti

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21 minutes ago, Antti_K said:

 

stevehnz,

 

Now that you mentioned it, I think it was Pierre Clostermann who told about this in his book. It was his first flight in a Typhoon. I remember trying that myself on my very first flight in a Piper Arrow (after reading the book) and it was shaking rather alarmingly. Brake first then choose the gear up has been my SOP ever since.

 

Cheers,

Antti

Reckon you're right there Antii, thanks for the memory jog. :)

Steve.

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2 hours ago, stevehnz said:

Somewhere in the depths of my memory bank is a wartime memoir where the pilot mentions retracting the wheels the first time he flew a (I think) Hawker Typhoon, where the still rotating wheels set up a vicious shaking as they retracted, after which he made sure to hit the brake lever before retracting, so it definitely was a thing though what we're seeing on the P-51 is less sure in it's design effect as that I think.

 

Somewhere in the depths of mine is the gyro theory that I once taught at RAF Halton. After some digging, I've remembered the precession and rigidity rules and concluded that the turning force applied to a rotating wheel by inwards retraction would be translated 90 degrees in the direction of rotation causing a force which woulf tend to twise the u/c leg towards a toe-out position. This would cause some binding at the retraction axle resulting in the juddering vibration that your pilot mentioned.

 

Contrary to my thoughtless suggestion last night, the wheel in the well wouldn't be a gyroscopic problem because it wouldn't be rotating fast for more than a few seconds even if the pilot didn't tap the brakes before retraction, as I now remember is the common practice. So why would the P-51 and other aircraft have a little brake to prevent rotation of the retracted wheel? (I remember seeing these on other aircraft now, I think the Vulcan had little paxolin 'snubbers' in the u/c bays)

 

I think it's to stop them being driven around endlessly by the drafts which get in through the door seals, causing unnecessary rumbling vibrations and eventually bearing wear. That would explain why the P-51 strip doesn't show signs of wear - it doesn't stop the wheel but merely holds it in the stopped position.

 

I also think the theory about the wheel pushing the inner door open is valid. I would expect there to be some contact even if the door was opened and then closed hydraulically during the extend sequence. 

 

Thanks for exercising the old noggin through the insomniac night. It gave me something to do. 😄

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Similar bars are on current aircraft. Their purpose has nothing to do with stopping the wheels spinning. Rather, when the gear is selected down, the inner doors start to open, the uplocks open and the gear starts down. If the inner door isn't open far enough, the tire can catch on structure on the face of the door and damage the tire, or worse, hang up and jam the gear extension. The rub bars allow the tire to push the door open safely. 

So, you ask, how would the inner door not open enough? In a normal extension, the inner door uplocks open and hydraulic pressure is routed to both of the door actuators, if one is stiffer than the other, then the 'easier' actuator takes the majority of the hydraulic fluid and moves first and farther. The 'stiffer' door moves slower and can be in the way when the gear drops.

In a manual extension (emergency extension) the inner door uplocks are manually opened first, and the door starts to slowly bleed down (open), then the gear uplocks manually open , and the gear drops (under its own weight, with spring assist) and pushes the doors out of the way.

Watch any video of a gear extension, the inner (body) doors almost never open together, one always lags.

 

Colin

 

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