Some New Data Carts to chew on

[eddiek]

Hiya F4UDOA,

I've submitted requests for copies of the F6F-3 tests conducted at the Naval Air Test Center in Patuxent. Maryland.
Whether or not I am able to get copies remains to be seen.
At the very least I am hoping they can redirect me to someplace that might still have copies.

[wells]

My issue is that climb has more variables than just Thrust-Drag/ Mass.  Climb has Lift coefficients.

Lift coefficients only play a part in induced drag.  Climb rate *is* (T-D)/W * V, whereas acceleration is (T-D)/Mass.  Note that weight is related to mass by the gravity constant.

   

And the P-38 has a high Max Cl because of high aspect ratio where as the F4U has a relatively moderate Max Cl(no Flap with prop in case Niklas is reading) of 1.48. This reduces the climb of the F4U in relation the P-38. So the 3100FPM vrs the 3700FPM are really not directly proportion to acceleration.

You're right, but not because of lift coefficients, because climb speeds are different.

   

So if you calculate Thrust-Drag / Mass at a given speed say 150MPH I am will to bet that F4U does almost as well as the P-38 despite not being able to climb.

Acceleration is directly related to climb rate *at the same speed*.  Let's say the F4u makes 3100 fpm (52 ft/s) @ 155 mph (227 ft/s) and the P-38 makes 3700 fpm (62 ft/s) @ 180 mph (264 ft/s).  

To convert those climb rates to accelerations, you divide by V and multiply by gravity constant of 32.2 ft/s^2

P-38 @ 180 mph: a = 7.56 ft/s^2 (straight and level)

F4u @ 155 mph: a = 7.38 ft/s^2 (straight and level)

Whether the F4u could out-accelerate the P-38 at 155 mph depends on whether the P-38 can climb 3100 fpm at that speed.

[ 01-04-2002: Message edited by: wells ]

[bolillo_loco]

thank you for the site and info F4UDOA, going to check it out. I am just curious, but why is it that everytime I download and then open a document it says it is "too large for note pad to open would you like word pad to read this for you", and then it is a bunch of characters that I do not understand? in reference to F4UDOA's document on "F4U comparative analysis" thanks

[ 01-04-2002: Message edited by: bolillo_loco ]

[Daff]

I suspect that these test datas are from the planes used at the Fighter Conference?
I seem to remember having similar discussion with F4U about the P-47's HP.
The P&W R2800-63 was used on the D-11 to D-20 and was from factory set to max 2300HP.
However, it was common to increase the max MAP allowed, which is very likely what have happened to this one, so that it was producing 2600HP, while using WEP.
From P-47D-26 onwards, this was done at the factory as well.

Daff

[Hooligan]

bolillo_loco
 
If I am correct your problem is that you are trying to read the document with the wrong program.  For the .pdf file you need to download the free version of adobe acrobat.  Otherwise for any other file type I would suggest Microsoft Word.

Hooligan

[bolillo_loco]

yes thank you hooligan that worked for me   I will be back to wine later now that I can read it  

[lapa]

I was gonna ask where Wells got his ROC equation, it looked a bit weird to me. Then I noticed that I was just confused with the imperial system (used to metric.) Duh.

Anyways, F4UDOA: do you have the frontal area numbers available that were used in calculating the drag coefficient? Since the drag is the product of the coefficient and the area, I guess the Cd and power alone would not tell the whole story about acceleration. Actually it would be real cool to see a comparison of those fighters based on the total drag and power, but I have never stumbled upon that kind of a table.

BTW, here is a nice link:
Beginner's guide to aerodynamics

Nice site, loads of good stuff about planes if you are like me and don't know the first thing about aerodynamics =)

Lapa

[wells]

Lapa, those drag coefficients are based on wing area.

[HoHun]

Hi F4UDOA,

>Climb has Lift coefficients. And the P-38 has a high Max Cl because of high aspect ratio where as the F4U has a relatively moderate Max Cl(no Flap with prop in case Niklas is reading) of 1.48. This reduces the climb of the F4U in relation the P-38.

Maxmimum lift coefficient in a climb would be reached at 1 G stall speed. That's much slower than best climb speed, and it's less efficient for climbing because the maximum lift is achieved at very high drag.

The best climb rate is reached at maximum excess power.

A first approach to find out what that means would have to look at the maximum ratio of lift to drag where the aircraft flies with least drag. This is reached at much smaller angles of attack than maximum lift.

However, since propeller efficiency increases with speed (in the low speed range at least), the best climb rate will be achieved at even higher speeds than that of best lift to drag, and accordingly at a yet smaller angle of attack.

So, the second approach will be to determine the point where the difference between usable power and speed-dependend drag is largest - that's the speed of best climb.

Sustained climb means flight at 1 G, so initial acceleration in level flight is indeed perfectly proportional to climb rate at the same speed :-)

The only way for a F4U-1 to accelerate away from a P-38J is to go into a steep dive to get the speed up into the region where total drag is more important than power-to-weight ratio.

Regards,

Henning (HoHun)

[Tac]

the p38 accels when past 250mph? wow!  

 

[gripen]

I compared this data to the AHT and some manual data I have. Generally the data about the Navy fighters (F4U-1, F4U-4 and F6F-5) appears to be bit more optimistic than the data in the AHT. There a little differences in the ratings; 59" vs 60" for the B series R-2800 and AHT's rating for the C series is probably at 64". But loadings are very similar (except some loadings for the F6F). Rammed critical altitudes are also a bit strange, they don't behave like they should.

The data for the Army fighters varies abit more. The AHT claims just around 13500lbs weight for the P-47D (up to D-25) at similar loading but a closer look to the AHT's data revealed that there is an error in the ammo loadings for the P-47D, ammunition should weight about 1057lbs not 664lbs. So weights for the P-47D are actually very similar in both sources. Generally rating for the R-2800-63 should be 2300hp@58" WET but 2600hp@64" WET rating certainly existed for the later B series P-47D engines and was used at least in the pacific. Again rammed critical altitudes are a bit strange.

The weight for the P-38J seems to be very low,  with full ammo and 300 gallons of fuel it should weight atleast 16800lbs (actually a bit over 17000lbs according to the manual). The performance apppears to be it on low side if the weight is correct; at 16415lbs the P-38J should be able to climb very close to 4000fpm also speed is a bit low. The critical altitudes are again a bit strange but these can be partially explained by plane's behaviour to reach it's top level speed below critical altitude.

The engine model and rating information for the P-51B are very strange. The claimed MAP is for WER and critical altitudes looks like the values of the V-1650-3. But text claims that ratings are MIL and that engine is the V-1650-7 also hp values are closer or exactly for the V-1650-7 at MIL. Low altitude speed and climb can be be for both engines, the V-1650-7 actually did about same hp at MIL rating as the V-1650-3 at WER at low altitude. Also the P-51 typically reached it's top speed at 61" (above rammed critical altitude).

The Radius of action comparison is quite questionable, the P-38J and P-51B should have a better radius than the P-47D.

Overall it's a interesting set of data but it appears that it is not a real test report but a collection of data and sources are not claimed.

Then couple notes about level flight acceleration calculations. IMHO there is no quick and easy way to calculate correct accelerations; we should know propeller efficiency for given speed range and altitude, Cl values for given speed range and weight and how drag changes when Cl changes. Actually it is pretty damn difficult... Some of the data is available for the planes like the P-51 (total drag, drag/speed/Cl relations) but generally propeller part is a problem. Also exhaust thrust should be counted.

gripen

[ 01-04-2002: Message edited by: gripen ]

[GRUNHERZ]

All those chart and number things are neat, but this is still just a plain old whine.  

[HoHun]

Hi Gripen,

>IMHO there is no quick and easy way to calculate correct accelerations

Acceleration from low speed can be estimated from climb rate with good accuracy.

Here's an excess power chart for a generic WW2 propeller fighter:

 

(Generic propeller efficiency curve, exhaust thrust figured in, ram effect neglected.)

The left hand limit is defined by the stall.

Where the total drag curve intersects the available power curve, the aircraft has reached its level top speed.

Where the difference between available power and total drag is the greatest (i. e. the excess power graph has its maximum), the best climb rate/acceleration is reached.

The interesting thing is that the excess power graph is rather shallow around the maximum, so that speed variations only have a minor impact on the climb rate/acceleration.

In other words, the relative acceleration of two WW2 propeller fighters at the same speed can be safely estimated from their relative climb rates even if these climb rates are achieved at different speeds.

Regards,

Henning (HoHun)

[wells]

Hohun,

Although prop efficiency increases with speed, thrust decreases, since

Thrust = Power * e / V

If you plot thrust instead of propeller efficiency, you will find that the greatest excess thrust is at the stall speed, where the best angle of climb is, not the best rate.  As you noted, the drag curve is flat at that point, so increasing speed will result in a higher climb rate up to the point where excess thrust is decreasing faster than speed is increasing.

[gripen]

HoHun,
English is not my native language so I might use wrong terms in following. As Wells noted we are interested about thrust/drag/weight(mass) relations not directly about power. Another problem is that climb rate/speed is a pretty static flying form but during acceleration relations between thrust and drag change when speed increases and in addition there are big differences between best climbing speeds like in the case of the P-38 (around 160mph IAS) and P-51 (around 200mph IAS).

It is pretty easy to create generalized models and I believe that at low speeds they give acceptable results but as real world test showed in the case of the P-38 and P-51, things are very different when speed increases also altitude can make a big difference.

gripen