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Nico081
PostPosted: Thu May 24, 2012 1:05 pm 
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0 MPH !

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ssouchek
PostPosted: Thu May 24, 2012 1:09 pm 
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Joined: Sun Jul 04, 2010 11:13 am
Posts: 31
Location: Parker, Colorado
57.8mph increase at 17,000 ft.
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Puma257
PostPosted: Thu May 24, 2012 1:12 pm 
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Joined: Mon Sep 13, 2010 1:24 pm
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Location: Netherlands
I guess minus 30 kIAS because the AOA will increase above max that means increase of induced drag + increase of gravity component > parasitic drag > decreased lift > nose drop> increasing speed etc. > nose up etc. > did I say etcetera? > overall result: los of speed (30K?).

Trim that nose! ( :?: )

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Last edited by Puma257 on Thu May 24, 2012 1:21 pm, edited 2 times in total.

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Skycat
PostPosted: Thu May 24, 2012 1:19 pm 
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Location: Great Falls Army Air Base, Montana
Unladen? What do you mean, an American or European Merlin?

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Mickey
PostPosted: Thu May 24, 2012 1:36 pm 
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Airman Basic

Joined: Sat Feb 25, 2012 2:31 pm
Posts: 5
Scot,

I'm sure you have forgotten 'terrabytes' more on this subject than I will ever learn. But, won't the answer to your question depend on 'which version of Merlin was in use'?

Scott - A2A wrote:
The Merlin-powered P-51D Mustang comes with a dual-stage, dual-speed supercharger. At 17,000 feet and with the supercharger in LOW, the aircraft produces a little over 1,400 horsepower.


If I'm permitted to quote from Technical Order No. 1F-51D-I "For a -3 Merlin, the supercharger would cut into high blower at 19,000 feet, but for the -7 engine it would cut in from 14,500 to 19,500 feet, depending on the amount of ram air. The supercharger increases the blower -to-engine ratio from a low of about 6 to 1 to a high of about 8 to 1".

Scott - A2A wrote:
At 17,000 feet and with the supercharger in LOW, the aircraft produces a little over 1,400 horsepower.


You are the specialist here, but my understanding was that the aircraft produces a little over 1,400 horsepower at sea-level. As it would climb, the power produced would reduce till the super charger cuts in.

Scott - A2A wrote:
(HINT: The manifold pressure jumps from 54" in LOW to 67" in HIGH)


Your "Hint" has really thrown me off, and I still continue to grope with studying the charts. When I study the "Power Climb Charts" and "Flight Operation Instruction Chart" for the -7 version, I see that 67" of manifold pressure is only obtainable at 'war emergency' power settings (3,000 RPM). Max Mil power (for 15 min only) would only provide 61" of MP (which would progressively reduce as one climbs".

Scott - A2A wrote:
Question:
What is the approximate speed increase when the Merlin switches from LOW to HIGH


In my opinion, there cannot be one absolute value as an answer to your question. The charts provide for climbs at different RPMs (from 2100 till 2700). The charts also provide approximate TAS values for each of these RPMs at altitudes in steps of 5,000 feet. The 'High Blower' values are marked between 15,000 and 20,000 feet.

Illustratively, for a climb at 2700 RPM, at 10,000 feet the MP would be 42.5 providing a TAS of 346 MPH (300 knots) at 83 GPH fuel flow. At 15,000 feet, Full Throttle (indicated as F.T. in the charts, instead of a MP) will provide 373 MPH (324 Knots) at 90 GPH fuel flow (note the increased not decreased fuel flow, due to further opening of throttle). Somewhere between, 15,000 to 20,000 feet the 'High' setting of Supercharger would cut in. The corresponding values for 20,000 feet are 45" MP providing 389 MPH (338 Knots) at 93 GPH fuel flow.

The above illustration was for a RPM setting of 2700 RPM. Corresponding values would change if the RPM setting was reduced.

Scott - A2A wrote:
The first one who answers this correctly, will get a $50 gift certificate good for 12 months at the A2A online store.

I was so hoping to win this. Was planning to send my daughter to college on this lottery :)
I guess she will have to be satisfied being a 'high school' graduate now. (:
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Metalman64AV
PostPosted: Thu May 24, 2012 1:38 pm 
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Joined: Thu May 24, 2012 1:24 pm
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The change will be accompianed by a monentary power surge and increase in manifold pressure until the regulator catches up.
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crashaz
PostPosted: Thu May 24, 2012 1:44 pm 
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Joined: Thu Dec 29, 2011 2:52 pm
Posts: 7
394 MPH to 407 MPH

http://www.wwiiaircraftperformance.org/ ... titude.jpg
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mastercylinder
PostPosted: Thu May 24, 2012 1:57 pm 
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Joined: Thu May 24, 2012 1:51 pm
Posts: 1
A collective of minds supposes this will be a 92Knot increase. Doesn't sound right, but its as good a guess as any given we don't know any of the important variables...
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Mickey
PostPosted: Thu May 24, 2012 1:59 pm 
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Joined: Sat Feb 25, 2012 2:31 pm
Posts: 5
crashaz wrote:
394 MPH to 407 MPH

That is a very interesting chart crashaz. It even shows a 75" MP plot. Wonder how they managed to squeeze that power out of the engine?

I looked at this one quite closely, but how did you arrive at the 'jump' of 394 to 407 MPH. If I trace the the 360 MPH TAS line all the way up, at approx 17,500 feet, it branches out into a separate 61" MP trace. But, there is no increase in speed AT 17,500 feet, right?

I must be missing something very obvious here!
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thunderstreak
PostPosted: Thu May 24, 2012 2:05 pm 
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Joined: Sun Jul 09, 2006 5:18 pm
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Location: CDU9, Ontario, Canada
From the P-51D/K POH (V1650-7)
"The supercharger blower will automatically shift into high speed at between 14,500 and 19,500 feet. This change will be accompanied by a momentary power surge and increase in manifold pressure until the manifold pressure regulator catches up."

Therefore speed change would be insignificant and settle back to the speed before the switch.

Also, any exact speed change would be pretty tough to figure out without knowing the configuration and weight & balance of the aircraft at that time.
ie; fuel weight and location (which tank), external stores, coolant and cooler flap settings, etc.
Air temperature would greatly effect power output too!

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Chewbacca84
PostPosted: Thu May 24, 2012 2:47 pm 
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Joined: Thu May 24, 2012 2:42 pm
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My guess is 10 miles per hour or 8.69 knots
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JoeS475
PostPosted: Thu May 24, 2012 2:55 pm 
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Posts: 987
Scott, my answer is 13 mph.

Skycat, love what you did there.

Joe

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Kiwi00000
PostPosted: Thu May 24, 2012 2:59 pm 
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Airman

Joined: Mon Nov 14, 2011 9:37 pm
Posts: 13
:-) good luck everyone.

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Last edited by Kiwi00000 on Thu May 24, 2012 3:16 pm, edited 3 times in total.

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crashaz
PostPosted: Thu May 24, 2012 3:03 pm 
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Airman Basic

Joined: Thu Dec 29, 2011 2:52 pm
Posts: 7
Mickey wrote:
crashaz wrote:
394 MPH to 407 MPH

I looked at this one quite closely, but how did you arrive at the 'jump' of 394 to 407 MPH. If I trace the the 360 MPH TAS line all the way up, at approx 17,500 feet, it branches out into a separate 61" MP trace. But, there is no increase in speed AT 17,500 feet, right?

I must be missing something very obvious here!


I took the 46" and 67" plots... which I read are both at 2700 RPM.... and 3000 RPM is how to get to 75in.

the 61,67in HG plots run on the same line at 17,000 feet so I took that intersection and followed it down to 407 MPH at the bottom.

How I got the difference for 67" HG.... took the difference between the graphed 46" and the 61"...15 /2 = 7.5 took the difference with the speeds at the plots 407 (67") and 387(46") = 20 /2 = 10 mph

they are linear in measurement.. so then 46" + the 7.5" = 53.5" which is another 10 mph faster. = lol... 397 MPH OOPS... so I should say 397 to 407 MPH increase....

probably the wrong answer as I just worked with the information given.... not which Merlin model... nor which model of the P-51 (B or D, H)

Nevertheless.... fun activity to talk shop amonst friends, no? :mrgreen:
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Scott - A2A
PostPosted: Thu May 24, 2012 3:14 pm 
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Joined: Wed Feb 11, 2004 12:55 pm
Posts: 9979
Location: USA
Guys, I will say there is a correct answer in here. I just did a short youtube video explaining it, as it was just easier to talk and point. It's uploading atm. It's a great topic.

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