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

[Tbolt]

35 minutes ago, Bertie Psmith said:

 

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. 😄

 

It's not holding the wheel to prevent it from turning as if you watch the video, the door does not contact the wheel the only time it does is on extension.

 

It's just nice to not have a wheel spinning at high speed into a gear bay where there are other components that could be damaged if you have a tyre tread damaged on take-off. Most airliners have nose wheel snubbers to stop the nose wheels in the bay but the main wheels are braked automatically.

[Paramedic]

3 hours 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

 

I have the book somehwere, translated, can have alook later.

 

Some excellent stuff -thanks guys!

[Blimpyboy]

1 hour ago, Tbolt said:

It's just nice to not have a wheel spinning at high speed into a gear bay where there are other components that could be damaged if you have a tyre tread damaged on take-off

 

Plus, there's all the crap and corruption being flung off a wet, filthy tyre, into a well that may also have some nice electrical, pneumatic and hydraulic conduits running through it!

I seem to recall part of a training lecture long, long ago, about centrifugal force temporarily expanding a tyre's diameter, so a tight fit in the wheel well would be exacerbated unless the spinning was slowed. From about the same time, I recall that on some aircraft it reduced the noise and vibration - clearly, they'd never flown a Tracker.

 

Edited October 26, 2021 by Blimpyboy

[Tbolt]

1 hour ago, Blimpyboy said:

 

Plus, there's all the crap and corruption being flung off a wet, filthy tyre, into a well that may also have some nice electrical, pneumatic and hydraulic conduits running through it!

I seem to recall part of a training lecture long, long ago, about centrifugal force temporarily expanding a tyre's diameter, so a tight fit in the wheel well would be exacerbated unless the spinning was slowed. From about the same time, I recall that on some aircraft it reduced the noise and vibration - clearly, they'd never flown a Tracker.

 

 

While the centrifugal force does cause the tyre diameter to increase I would hope that aircraft are designed with that clearance in mind other it could get messy!

[Jay Gee]

There is a video on youtube showing a flight inside a restored B-24. Just after take-off they tap the brakes before selecting the gear up. Also with the C-17 when the gear is selected up, the brakes are applied automatically to prevent the gyroscopic effect. Given the length of the B-24 MLG and the rotation of the C-17 gear it makes sense.

 

Back onto the original post regarding the colour according to my copy of Southern Cross Mustangs, from K-15NTs onwards the rub strip was painted yellow zinc chromate. But looking at most pictures it was left bare.  

Edited October 26, 2021 by Jay Gee
speeeling

[Paramedic]

Yes, Pierre Clostermann mentions not braking his wheels before retracting and they make the plane vibrate fiercly (my words, my book is in Swedish). And he hopes he did not geta  flat tire from that..

[TAG]

On 10/25/2021 at 11:56 AM, 72modeler said:

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

That's exactly the reason why they primered the rub strips (or scuff plates).

 

Unlike the aluminum clamshell doors (NAA called them "fairing doors") the rub strips were made of stainless steel and, as Mike pointed out, you can't have bare steel touching bare aluminum since dissimilar metals corrode when in contact with each other. Hence, the need for primer.

 

The reason they primered the rub strips and not the  doors is because they needed the aluminum to be totally clean in order to spot weld parts together. Of course you can prime the whole assembly after the spot welding is done, but that took time and during the war the emphasis was on speed, not corrosion control.

 

That's why you'll see period color photos of earlier Ponies with YZC rub strips, since, as discussed in the wheel well thread, the further you go into P-51 production the more primer was used on the airframe, with the entire well sprayed YZC from dash 20's onward.

 

Here's a period color pic with the rub strip in all its YZC glory. Also, this photo is a prime study of the different hues and tones of paint and primer used in Mustangs.

This one is just a few days after VE-Day (you can tell by the squadron codes applied under the wings), but Jolie Helene was an early P-51D-5-NA that made it to the end of hostilities so its wheel well was mostly NMF, meaning it had a YZC rub strip. It also has several field-applied mods, like the DFF filet and AN/APS-13 early warning radar on the tail.

In any case, it was rare for the clamshells to be fully down anyway, I went through an inordinate amount of my reference pix to find these two and the vast majority of times they were either completely up or slightly asymmetrically open. But that is a whole other discussion and a can of worms in its own right.

[Paramedic]

That is a pair of very nice photos, thanks! Makes sense on the different metals too, I should have thought and reacted on that earlier.. 😚

[RZP]

4 hours ago, TAG said:

 

In any case, it was rare for the clamshells to be fully down anyway, I went through an inordinate amount of my reference pix to find these two and the vast majority of times they were either completely up or slightly asymmetrically open. But that is a whole other discussion and a can of worms in its own right.

 

Interesting, just to open that can slightly, I find the opposite. Examining just a couple of hundred of my photos of D/K Mustangs of different air forces (mostly RCAF, Polish and Commonwealth, but also including the USAAF/USAF), I find as many or more photos with clamshell doors down, it's far from rare. Looking through photos in Mustang books I find the same. 

 

Lowering the inner doors was part of the shutdown and parking checklist for the D/K. Once parked (from the USAAF manual):

"Fairing door emergency release handle pulled out and down. ( when fairing doors are open, landing gear handle is mechanically locked in the DN position.) " 

 

The RCAF Pilot's Operating Instructions are similar under End of Flight Procedure:

"Pull undercarriage fairing doors emergency release to open doors and relieve pressure on hydraulic system."  

 

When the doors were released and fully down, a mechanical link was engaged with the landing gear handle which prevented the handle from accidentally being pulled into the UP position. 

 

An explanation from one of the leading Mustang restoration experts, Glenn Wegman (from P-51 Special Interest Group):
"When on the ground, gear down, with the engine running, the only thing holding the inner doors closed is the hydraulic pressure. When the engine is shut off the pressure should remain in the system for a while and hold the doors closed. Standard procedure though, is to pull the tee handle when the engine is shut down to dump the hydraulic pressure allowing the doors to fall open. 

There is a safety latch operated by the left inner door that locks out the gear handle from being raised if the doors are open when the airplane is sitting on the ground. Once the hydraulic pressure builds up after engine start and the doors close, the gear handle can be raised.

Dumping the pressure after shut down opens the doors and subsequently lessens the chance of inadvertently raising the gear handle." when the plane is on the ground which would cause the tail gear to collapse. That's why it is standard practice to dump the pressure upon engine shut down."
 

A modeller would be fine setting the inner doors in any position between fully closed and dropped, but personally I would drop them. According to the shut down checklist, the flaps should also be lowered fully "to prevent people walking on them", but that didn't always happen. 

 

Just some thoughts.

 

Richard

Edited November 10, 2021 by RZP

[TAG]

Thanks @RZP,

 

I've read so many contradicting accounts over the years about the proper procedure re: the fairing doors after shutting down the engine, a lot of them from apparent experts and even current pilots of the Mustang, it just hurts my brain. My comment is based merely on empiric observations of my own reference material, so I have no official corroboration to back up what I said, possibly I'm partial to photos of P-51's with raised fairing doors and seem to collect those more often than not?

 

In any case, it certainly wasn't uncommon to see the fairing doors fully or partially retracted once the plane was parked, so as you pointed out, no one can say your model is "wrong" no matter how you choose to depict the clamshells.

 

- Thomaz

[RZP]

Thanks Tomaz, it’s all still confusing to everyone…doors up/down, flaps up/down…and all kinds of combinations. All we can go by is the photos, and you’re right, so many of them show the doors firmly up. We all have our preferences, some like the clean sleek look, others like everything hanging. 
 

Like you said, no one can say you’re wrong however you build them...well, except one flap up, one flap down. 🙂
 

Richard

Edited November 10, 2021 by RZP

[Paramedic]

Or both ailerons down, as I saw on one build - not a P-51D though..

[Kari Lumppio]

Hi!

 

Great info in this thread. All I can add is that if you paint corrosion inhibitor only on one surface it should be the more noble one. That way a scratch in the inhibitor does not mean fast corrosion rate.

If a scratch reveals small area of less noble material to large area of noble surface the result will be very fast corrosion on the scratched area.

I think that is why the stainless steel scuff plate was painted with zinc chromate primer. Spot welding was perhaps factor, too. Though the primer would be in between either way to disturb the welding.

 

Cheers,

Kari

Edited November 10, 2021 by Kari Lumppio
rephrase

[72modeler]

All of the above-mentioned are good ones. My thinking as to why there are so few photos that show the inner fairing  doors partially open, is that this would occur shortly after shutdown, if the release handle wasn't pulled. I am guessing that most photos of Mustangs on the ground were taken well after shutdown, when those doors were both down, or that the 'posed' portrait was more attractive with both doors open; sort of like why there a lot more photos of the LH side than the RH side, as the LH side was traditionally  the side on which nose art/lettering was most frequently applied.

 

Most of the Mustangs I see at airshows have the flaps and inner doors down after being parked, and they both come up after startup when the hydraulic pressure is restored, so I guess the pilots pull the release as a matter of procedure. See the startup videos below and note the variations in the finish on  the inner side of the fairing doors.

Mike

 

https://youtu.be/wdy17GmkpWc

https://youtu.be/O2d5GbL_a30

https://youtu.be/_rQ48-vtkz0

https://youtu.be/eFBcBAsSrpQ

 

 

[Paramedic]

4 hours ago, Kari Lumppio said:

Hi!

 

Great info in this thread. All I can add is that if you paint corrosion inhibitor only on one surface it should be the more noble one. That way a scratch in the inhibitor does not mean fast corrosion rate.

If a scratch reveals small area of less noble material to large area of noble surface the result will be very fast corrosion on the scratched area.

I think that is why the stainless steel scuff plate was painted with zinc chromate primer. Spot welding was perhaps factor, too. Though the primer would be in between either way to disturb the welding.

 

Cheers,

Kari

 

..and I felt a bit stupid posting it since I was sure it had been beaten to detah, I could just not find it.. Why I love Britmodeller and the members so much, a wealth of knowledge and most of the time, very helpfully put forward!