Over priming and cold weather starts

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Rocket_Bird
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Over priming and cold weather starts

Post by Rocket_Bird »

Starting the A2A P-51 in cold weather is a difficult task for me, but I found a way to do it. By cold weather, I mean subzero temperatures the range of -20 degrees Celsius and below. I could never get the engine to remotely catch with the recommended 4 seconds of primer. Going off of what I know of the A2A Spitfire, where it says to use 12 strokes of primer when temperatures dip to 0 degrees Celsius, and going by the similarities between the "Merlin" engines, trying to prime the engine more seems to be the key.

Of course, when it comes to temperatures like -20 and below, it seems that even 12 seconds of primer on the P-51 isn't enough. It catches just briefly, and even further priming while cranking does not seem to make it run. For something as cold as -20 and below, what really worked for me was to give it 4 seconds more prime for every 5 degrees below +15 degrees when the engine is not cranking. This would mean about 24 seconds of prime at -20! Works like a charm! But here my OCD gets to me. Is this real? How much is too much prime for the Packard Merlin?

I've overprimed aircraft engines before to the point where I can see avgas dripping below the engine and the extreme difficulty (not to mention danger) of starting afterwards. From what I have been reading about the electric primer on the P-51, it's extremely sensitive and easy to overprime. Is 24 seconds way too much? How else can one start this engine in the temperatures stated without killing the battery or burning out the starter?
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Scott - A2A
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Re: Over priming and cold weather starts

Post by Scott - A2A »

1 second of the P-51 electric primer is equal to one stroke of a manual primer, so you can use the Spitfire guidelines converting strokes to seconds.

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ft
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Re: Over priming and cold weather starts

Post by ft »

Considering that you'd never attempt starting a Mustang under those conditions today, I go the route of loading the aircraft before setting the weather to freezing, thus simulating having just rolled the aircraft out of a warm hangar.

The real thing saw extensive use in the subarctics. The lessons learned are briefly (but well) covered in an excellent book on the P-51 by Leif Hellström. Probably a bit hard to acquire these days, and even less accessible to most participants here.

The F-51D dash one also contains useful advice on cold weather operation.

Couple of key points:
Preheating - do it. In the simulator, we can't. We can, however, simulate it as per the first paragraph. Coal burning stoves were used in my neck of the woods. According to the above book, it took one hour ten in -25°C.
Oil dilution. Use it. While I'm not familiar with the oil grades offered in the A2A civ Mustang, they should be straight grades. At -20°C, those'll be almost entirely different from a fluid. Besides the mechanical resistance to getting the engine started, it'll take a good while for the oil to get to cams, rockers, drive shaft bearings and all the other places where you really want an efficient (and fluid) oil film when operating your precious Merlin.
In real life, the oil was drained while still warm, kept warm overnight and poured back in just prior to cranking. Again, we have to simulate this manually.
You get one shot at starting, without an external source of electrical power. I'm actually rather impressed that the batteries gave you even one go.
Dash one, cold starting procedure: "prime only when the engine is turning over". Overpriming and having the engine catch fire is one method of preheating for the next start attempt, I guess, but I don't think it can be considered a good one. Priming prior to cranking apparently went out of fashion about at the time the war ended.

Interestingly, landings in 30 cm of snow was apparently considered pretty much a non-issue. I've also seen pics of Mustangs on skis, but I don't think that ever became normal practice.
Be warned: Aero engineer, real life pilot, sim programmer. Nothing good can come out of that.

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Fedex B17Pilot
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Re: Over priming and cold weather starts

Post by Fedex B17Pilot »

ft wrote:
Interestingly, landings in 30 cm of snow was apparently considered pretty much a non-issue. I've also seen pics of Mustangs on skis, but I don't think that ever became normal practice.

doesnt sound any different than a Cub on skis, only with a little more umph under the hood! :D
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Rocket_Bird
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Re: Over priming and cold weather starts

Post by Rocket_Bird »

Ya I figured the P-51 isn't exactly designed to start in this kind of weather.

I live in Canada and work in a field where there is actually a P-51 here (never do see it in the winter time though). It can be quite cold during the winter time, and pretending that I myself am a P-51 owner, just knowing how to rightfully start the thing would be a good first step. Though I suppose it's probably right to say that if I did own a P-51, it'd probably be sitting in a warm hanger until I go flying.

Nevertheless, the priming thing made me rethink my winter operations SOPs, as I recently flooded the engine of a local plane here :oops: (I understand it, and know how to clear it, so it wasn't a big issue). My question with regards to overpriming is more of an intellectual one, since again, a P-51 would likely be sitting in a warm hanger in this day and age. Now, I know the Rolls Royce on the spitfire can take up to 12 primes (I think), I'd still be wondering if the Packard can handle 24 without catching on fire or otherwise not fire at all. Some good posts so far though.
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pilottj
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Re: Over priming and cold weather starts

Post by pilottj »

This was my message to another poster awhile back on cold starts. You can start the P-51 fine in cold weather if you give it more prime initially.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Just for ducks, I went up to Gulkana, Alaska to see if I could get the Civ P-51 started. Daytime temp was 28F(-2C), alt 29.29. Using winter grade oil. First attemps were using 4 seconds of primer initally, then after 6 blades, mags, prime on 4 sec, prime off 4 sec, prime on 4 sec..etc. I went through a few batteries and a few burned out starters with no luck. Even tried with the Military P-51 which has a GPU cart. no luck. Then I tried the P-40, normally you can get it started on 3-4 primer shots on a cool day. Reading the primer notes, it says use 12shots of primer for 0C. I gave it 14 shots. After a little bit of coughing, the P-40 fired up fine.

I went back to the Civil P-51. This time I had success. Here is what I did. Battery on, fuel pump on, 10 seconds of primer, starter - 6 blades, mags on then prime on 4 sec, prime off 4 sec, prime on 4 sec, prime off 4 sec...etc. After a few 4 second shots of primer, it coughed a little bit, and eventually started. Mixture - on, then I jumped right on the oil dilute for a good minute., then just let it warm up like normal.

The difference was just using a bit more primer initially. Maybe Dudley can chime in on extreme cold starts without GPU/engine heater.

Try that and see if it works for you.

Cheers
TJ

edit...just for fun, went up to Barrow, AK...temp -4F/-20C...10 sec initial primer not quite enough. 20 sec initial primer, it fired up fine using the method above.
"The knack of flying is learning how to throw yourself at the ground and miss." - Douglas Adams
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stephan.cote.1
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Re: Over priming and cold weather starts

Post by stephan.cote.1 »

To go in line with what Fedex said before...
First, sorry to revive this old topic, but as a guy from up north Canada, cold weather starts do talk to me...
Second, I'm relating second hand info here, my dad flew here up north for roughly 5000 hours... (sometime in temps as low as -50C)
Third, his experience was on radial engine, rather then liquide cooled engine such as the merlin...

So, what old bush pilots had to do around here was indeed to warmup the engine prior to takeoff, exactly like mentioned by fedex, they would normally drain the engine oil after the last flight while it was still warm, to bring it in the cabin or heated shed(when they had one) to bring it back to the airplane the next morning.
On the cold morning, they would heat the engine under an engine tent with a blewpot (semi open flame heating device I guess since they had no electricity) and would then poor the warm oil back in the engine once it had warmed up a bit. Then they would proceed to startup with only one extra stroke of primer(three instead of the normal cold engine 2 strokes(the DHC-3 Otter had a big priming piston)) and would then crank it, go full rich as soon as the engine started to catch. Priming more was too risky and could results in engine fire an using your engine extinguisher meant being grounded until a recharge could be brought to you, sometimes days later...

The big difference at that point (compare to normal cold engine start) was the fact that the engine would die soon after it had started, even at fullrich mixture, so they had to keep it alive by slowly feeding it with some long, gentle primer strokes... somethime for almost a minute before his big P&W R-1340 would run on its own...
As we know, they never used throtle control to try to prevent it from diying because of the risk of carburator fire... again, only one shot of engine cowling extinguisher....

As he sais, sometime they worked on the plane for two hours before they got the engine running, then had enought day time left for a single flight before it was dark... getting back to the camp, their winter coat damp with engine oil because of the time spent under the engine tent with the heater, tawning and condensing oil dripping on them...walking to the cabin with their two 5 gals of oil, and having earned 40$ for their 4-5 hours of flying...

When I think of it, you had to love flying to go through this type of flying during 3-4 months of very hash winter

Please had your experiences and ideas
Stephan
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