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 Post subject: P39 level speeds
PostPosted: Sat Sep 14, 2019 10:56 pm 
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Posts: 167
Hey guys...
I've been looking all over for data to support (or more likely, refute) the SL speed we get out of the p39D with wep.
According to the POH, the maximum (War emergency power) RPM at sea level is 3000 RPM.
The same manual shows a top speed at SL of 259mph at 2600 RPM. So... I'd expect it to be around 30-40mph faster at sea level. I've seen spec sheets that show a max speed of between 305 and 315mph at SL. (The performance graph is a bit tough to read). Those numbers seem a lot more likely for plane like the P39D.
I'm wondering how we got to the very high speed we see in WB.
Anybody have data that are in agreement with the WB numbers?
The airacobra speed puts it in a class with the Typhoon and FW190- this seems a bit optimistic.
Thanks
finn


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 Post subject: Re: P39 level speeds
PostPosted: Sun Sep 15, 2019 3:32 pm 
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Speaking of rpm concerning the 190, I may have missed the explanation elsewhere but can anyone clear up why boost 1 drops several percent when it reaches 5k alt and then resumes at 9k alt? Makes for interesting fights between those two alts. Is this meant to be?

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 Post subject: Re: P39 level speeds
PostPosted: Sun Sep 15, 2019 3:44 pm 
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My grandpa said P39D could fly at 350mph at low alt! Period!


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 Post subject: Re: P39 level speeds
PostPosted: Sun Sep 15, 2019 5:53 pm 
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Both Finn and Qweqwe are correct.

The P-39D manual states the original Take Off/Military boost which is 42" Hg at 3000 rpm. At this setting the Allison V-1710-35 engine has a power output of 1050-1190 hp, depending on altitude. The 305-315 mph speeds are the data we can find from real life trials, and our P-39D flightmodel's top speed is 308 mph at sea level, at 42" Hg, 3000 rpm. This is quite in line with the ww2 data.

Qweqwe's Grandfather was also correct about 350 mph at sea level. This since the pilots soon found out that the Allison V-1710-35 engine could be pushed to 56" Hg, 3000 rpm for WEP without damage to the engine, while engineers at the start had limited it's usage to 42" Hg. At 56" Hg the engine had a power output of a whooping 1470-1520 hp, depending on altitude. The dragco of the P-39D flightmodel is set to match 308 mph at sea level at 42" Hg, and this dragco along with the power output at 56" Hg gives our P-39D flightmodel a top speed of 348 mph at sea level, which is quite on par with the 350 mph top speed that Qweqwe's grandfather stated.

The same goes for the P-40E and Mustang Mk.I, which both are using the Allison V-1710-39 engine. Their pilots also soon found out that the rugged Allison engine could be pushed to 56" Hg boost for 5 minutes of WEP, which was much higher than the original factory recommendations.

<S> and I hope this clears it up :-)
/Robert


Last edited by Robert on Sun Sep 15, 2019 9:55 pm, edited 2 times in total.

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 Post subject: Re: P39 level speeds
PostPosted: Sun Sep 15, 2019 6:17 pm 
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You da man Robert.


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 Post subject: Re: P39 level speeds
PostPosted: Mon Sep 16, 2019 2:32 am 
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Pakrat wrote:
Speaking of rpm concerning the 190, I may have missed the explanation elsewhere but can anyone clear up why boost 1 drops several percent when it reaches 5k alt and then resumes at 9k alt? Makes for interesting fights between those two alts. Is this meant to be?


Should be the full throttle height peak, then lowering power until the shift point of the 801 single stage 2 speed super-charger. The higher the boost the lower the peak power alt.


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 Post subject: Re: P39 level speeds
PostPosted: Mon Sep 16, 2019 2:40 am 
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Joined: Wed May 31, 2017 2:22 pm
Posts: 167
Robert, that does clear it up... but I'd still like to see that data source that confirms that.
The allison engine certainly was rugged.. so I can believe that your numbers are possible.
It certainly makes the plane more fun in the arena.
finn


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 Post subject: Re: P39 level speeds
PostPosted: Mon Sep 16, 2019 2:41 am 
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Another thought...
Did this apply to the P38?
Interesting!
finn


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 Post subject: Re: P39 level speeds
PostPosted: Mon Sep 16, 2019 8:25 am 
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I can't remember in which ww2 document this subject was discussed, but it was a ww2 report. I might find it later. Below is an engine chart though that shows the 56" Hg manifold pressure, 1470 hp, at War Emergeny Power for the P-39D's V-1710-35 engine. I've marked the rows for the -35 engine in the chart, with the 56" Hg WEP marked in red.

Hmm, I need to check about the P-38F & G. It might be that their engines were also allowed a higher boost setting than the officially stated 44.7" Hg. Searching some ww2 trial reports I've found indications of tests made at 51" hg, so will definitely bring this up with the FM Team for discussion. Thank you for the heads up, sir!


The V-1710-35 engine chart:

Image


<S>
/Robert


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 Post subject: Re: P39 level speeds
PostPosted: Mon Sep 16, 2019 1:13 pm 
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Joined: Wed Jan 11, 2017 2:41 pm
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grumpy wrote:
Pakrat wrote:
Speaking of rpm concerning the 190, I may have missed the explanation elsewhere but can anyone clear up why boost 1 drops several percent when it reaches 5k alt and then resumes at 9k alt? Makes for interesting fights between those two alts. Is this meant to be?


Should be the full throttle height peak, then lowering power until the shift point of the 801 single stage 2 speed super-charger. The higher the boost the lower the peak power alt.


Thanx Grumps..Wasnt sure this was intended..I hadnt ever felt this before and recently started to get acquainted with the 190's again.. Good to know its achilles heel..

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