Position Error & Compressibility Error Correction: WTF?

[Zipper730]

I was looking at data on WWII Aircraft Performance and I was sifting through the aircraft and there was often a list of position error and compressibility error correction: Does this simply mean that I add or subtract these numbers onto the IAS/ASI reading?

For example if ASI is 150 and Position Error Correct is +2.0 and Compressibility Error Correction is -0.5, does that mean I just add 1.5 to the speed and get 151.5 mph ASI?

drgondog S Shortround6 D Deleted member 68059 W wuzak
X XBe02Drvr

 

[Zipper730]

I'm asking because I'm looking through performance charts, and I'm adding the numbers and I'm getting figures that make absolutely no sense whatsoever. I'm curious how they factored in these varaibles.

Here's some examples...

F.36/34
Notes: Hawker Hurricane Prototype
Weight: 5672 lbs

Climb Trials
ALT ......R/C..............TAS................IAS................PEC & CEC...CAS

0'.............2550 fpm.....151.5 mph.....150.0 mph.....+1.5.....0.0.....151.5?
1000'.......2600 fpm.....154....mph.....150.0 mph.....+1.5.....0.0.....151.5?
2000'.......2650 fpm.....156....mph.....150.0 mph.....+1.5.....0.0.....151.5?
3000'.......2710 fpm.....158.5 mph.....150.0 mph.....+1.5.....0.0.....151.5?
5000'.......2810 fpm.....163.5 mph.....150.0 mph.....+1.5.....0.0.....151.5?
6500'.......2880 fpm.....167....mph.....150.0 mph.....+1.5.....0.0.....151.5?
7600'.......2950 fpm.....170.//mph.....150.0 mph....+1.5.......0.0.....151.5?
10000'.....2680 fpm.....173.5 mph.....148.0 mph.....+1.6....-0.5
13000'.....2370 fpm.....177.5 mph.....144.0 mph.....+1.9....-0.5
15000'.....2150 fpm.....181....mph.....142.0 mph.....+2.0....-0.5
16500'.....2000 fpm.....183....mph.....140.0 mph.....+2.1....-0.5
18500'.....1840 fpm.....186.5 mph.....139.0 mph.....+2.2....-0.5
20000'.....1620 fpm.....189.5 mph.....136.5 mph.....+2.3....-0.5
23000'.....1310 fpm.....195....mph.....133.0 mph.....+2.5....-0.5
26000'.......990 fpm.....199.5 mph.....129.5 mph.....+2.7....-0.5
28000'.......790 fpm.....204....mph.....127.5 mph.....+2.9....-0.5
30000'.......570 fpm.....208....mph.....125.0 mph.....+3.0....-0.5

Speed Trials
ALT.......\.TAS................IAS................PEC & CEC.....CAS
0'.............253....mph.....258....mph.....-4.9 -0.0...........253.1 mph?
1000'.......257.5 mph.....259....mph.....-5.0 -0.0...........252.5 mph?
2000'.......261.5 mph---.259....mph.....-5.0 -0.0...........254....mph?
3000'.......265.5 mph.....259....mph.....-5.0 -0.3...........253.7 mph?
5000'.......274....mph.....258.5 mph.....-5.0 -0.6...........252.5 mph?
6500'.......280.5 mph.....258.5 mph.....-5.0 -1.0...........252.5 mph?
10000'.....295.5 mph.....257.5 mph.....-4.9 -1.7...........250.9 mph?
13000'.....308....mph.....246....mph.....-4.2 -1.9...........239.9 mph?
15000'.....314....mph.....243....mph.....-4.0 -2.2...........236.8 mph?
16200'.....315....mph.-.-239....mph-.-.-3.7 -2.4........\\232.9 mph?
16500'.....315....mph.....237.5 mph.....-3.7 -2.4...........231.4 mph?
18000'.....313.5 mph.....231....mph.....-3.3 -2.5...........225.3 mph?
20000'.....311....mph.....222....mph.....-2.7 -2.5...........216.8 mph?
23000'.....306....mph.....207.5 mph.....-1.9 -2.5...........203.2 mph?
26000'.....298.5 mph.....192.5 mph.....-1.0 -2.5...........189....mph?
28000'.....291.5 mph.....185....mph.....-0.4 -2.5...........182.1 mph?
30000'.....282.5 mph.....170....mph\\\.+0.3 -2.4...........167.9 mph?

I'm not sure if these PEC & CEC correction figures are right... CAS was not listed, so I'm just applying the numbers as I estimate them to be. However, I'm not sure if I'm applying the numbers wrong

 

[Zipper730]

Is this number by any chance in percent?

 

[Zipper730]

I've been looking at this and I found a graph that seems to indicate the speed-trial data Hurricane Mk.I @ 6316 w/ a constant speed prop @ 3000 rpm, and presumably the climb-trials under the same condition (though the Mk.I climb-trials with the constant speed prop did have an engine problem, and the trials were just done at 2600 rpm)...

Now, I compiled the speed-trial data on the aircraft on the same condition...

ALT.......TAS..///PEC & CEC
0'...........N/A......N/A
1000'.....N/A......N/A
2000'.....N/A......N/A
3000'.....N/A......N/A
5000'.....275......-10.0 & -0.8
6500'.....280.5...-10.0 & -1.0
10000'...291.5...-10.0 & -1.7
13000'...301......-10.0 & -2.2
15000'...307......-10.0 & -2.6
16500'...312......-10.0 & -2.9
17750'...316......-..9.9 & -3.2
18000'...316......-..9.9 & -3.2
20000'...314.5...-..9.8 & -3.4
23000'...310.5...-..9.2 & -3.4
26000'...305......-..8.3 & -3.4
28000'...299.5...-..7.4 & -3.3
30000'...292.5...-..6.6 & -3.2

... and, while the position & compressibility errors were revised, I'm just trying to figure out how they're computing the position error and compressibility errors. I don't really know what the numbers mean. I mean for speed, that's in mph -- how many miles one covers in a 60 minute window, RPM is how many revolutions of the shaft every minute. Understand?

I'm trying to create large accurate graphs of aircraft performance, and that's where my interest in this lie.

 

[pbehn]

Zipper, why not actually get some of the many books on these subjects that people have recommended to you over a long period of time so that you have the answers in your head.

 

[Zipper730]

Zipper, why not actually get some of the many books on these subjects

I got no response on this subject ever as far as I know.

 

[Zipper730]

When it comes to the revised PEC estimate: I'm curious how they came up with the revision?

That said, I get the following figures for the revisions

ALT.......TAS...
0'..........264 mph
1500'....269 mph
3000'....274 mph
5000'....280 mph
11000'..300 mph
13500'..308 mph
16375'..321 mph

 

[thunderbird]

I've read enough posts here that I should have known better than to get involved. The answer to the followup question was in the original post, if one could be bothered to read even two paragraphs.

 

[Zipper730]

The questions you're asking weren't taught at my college aerospace engineering curriculum. I learned most of this from company documents and OTJ training. But I seem to recall that most of this information is available in FAA pilot training books. I'll only cover the basics here.

Just one simple question...

If the correction table list CEC as -10, that would mean you would take off 10 mph off the IAS?

I'm basically trying to take the data from World War II Aircraft Performance and convert it into CAS data that's accurate. The British aircraft seem to list these tables a lot, and I think a few of ours did.