p40b climb issue

[ergRTC]

I am copying this from the CT forum.  Cause I wanna know the answer.

You guys notice something strange with that?  Maybe I am just crazy, but it appears that decreased fuel has little effect on climb rate.  


Am I nuts?

check it out.
both are autotakeoff in same direction, picture taken at 3000 feet (and 3500 cause I was watching tv).  Controls untouched during test.  First pic is at 25%  the second is at 100% fuel.
Can this be right?

[hitech]

Run the same test again, except when off the runway level out to best climb speed then hit the auto pilot for climb. The auto take off can take a while to get to best climb speed.

Also make sure you check at exactly the same alt.

You have to run the math on fuel load weight V plane weight to predict the climb rate change.

P40b fuel load is 147 gals. = 882 Lb.
so 25% = aprox 600 Lb less

so with 600  LB less on a 7000 LB air plane, you would expect less than a 10% increase in climb rate, so it would go from around 2000 to 2200 which is about what your gauges show.

HiTech

[ergRTC]

hmm...

I redid the test with your suggestions, and also checked another plane with like weight and fuel load (f4f).  Looks like your explanation makes sense.
Only issue I see now is that the f4f weights more, carries very slightly less fuel, and is impacted a little more by the decreased fuel load (at least it looks like it, it is very close though).  This of course may be the plane design.

f4f---
7,975 lbs.
144 gallons

p40b
7,352 lbs.
147 gallons

[hitech]

Now your into a different level of precision. To figure these out at 5k you would also have to account for fuel burn rate, and  the gauge precision it self.

HiTech

[ergRTC]

cc I know you cannot compare empty p40 and full f4f, only within plane differences.  

You do not expect the difference to be larger than it is shown though right?

[dtango]

Adding to what HT's said, you're starting to get into a little more complicated territory with aircraft performance.  

The question of what difference you should expect depends on a variety of other variables besides weight.  

For instance you're comparing the P-40B and the F4F- aircraft with comparable fuel loads and therefore weights at the same fuel load.  

That's just part of the picture.  A couple of other variables you have to factor in to make your prediction as to the relative performance differences are the thrust and drag (mainly induced drag) of each aircraft.

RoC = (T - D)* V / W

Looks simple enough but T and D aren't simple to obtain.  If you can calculate the above then you can make a prediction as to what you expect performance differences to be.

Tango, XO
412th FS Braunco Mustangs

[ergRTC]

I know.  I just noticed that when empty the p40b behaves much like it does at 3/4 a tank.  Other aircraft, such as the yak (which also has a smaller fuel tank), goes completely ufo when it reachs near empty.  

I was just hoping htc would say "yep, thats because the plane is a dog, and losing 600 pounds wouldnt help it a bit."

[F4UDOA]

Dtango,

I don't buy the induced drag portion of that arguement for 1 second.

The F4U-1D had an aspect ratio of 5.4 and the F6F-5 had a ratio of 5.5. Now look at the climb rates in the AH charts. The F6F is far superior even with higher wingloading the same power/engine/prop and higher parasite drag. The differance in induced drag is insignificant.

Also look at the best climbing prop fighter ever. The F8F had the lowest aspect ratio of them all. I have read an airplane climbs on it's prop many times and I believe it. The best way to determine climb ability is power to weight. And the lower the weight the faster the climb.

The only possible exception is the P-51 which is climbed at higher speed to use it's laminar wing to full effect with a high speed climb roughly 180Mph IAS.

I have the charts from the pilots manuals of the F6F and F4U which show significant increase in climb with weight loss of 600lbs. I would expect lighter A/C to have at least as significant a gain with the same weight loss.

[dtango]

Hi F4UDOA:

(1) Aspect ratio itself isn't the only variable to consider regarding induced drag.

CDi = CL^2 / pi * span eff. * AR

(2) Higher aspect ratio will be a factor in reducing induced drag.  (And if you break it down further it's really greater span length that makes the difference but that's beyond the scope of this discussion! ).  So in your example of the F4U-1D vs. the F6F-5 assuming equivalent coefficient of lift and span efficiency the F4U-1D would have greater induced drag not the other way around.

(3) Regarding the F8F, this proves my point exactly about the importance of the following equation:

RoC = (T-D)*V/W

...and why you have to evaluate aircraft climb performance based on that and not on a single factor such as just weight, or aspect ratio.

(4) I'll admit that my statement of "(mainly induced drag)" is misleading.  My point was that induced drag is an important factor to consider for an aircraft climbing at best rate of climb.  I was trying to avoid the specific explanation that at velocity for best rate of climb for a given aircraft the induced drag is roughly equal to the parasite drag. ~40-45% of the total drag is related to induced drag at best rate of climb.

Tango, XO
412th FS Braunco Mustangs

[F4UDOA]

Dtango,

I agree the F4U had higher induced drag than the F6F. But I disagree that the induced drag was a factor in climb differential considering the much higher parasite drag of the F6F plus the addition weight. I also think HTC is using Grumman factory charts which tends to blur the equation even more.

My larger point is that the power to weight ratio tends to be the deciding factor in climb unless there is some larger circumstance to change things.

BTW what do you mean span more than aspect ratio?

[dtango]

F4UDOA:

F6F-5 - F4U-1D COMPARISON:
The induced drag in F6F-5 vs. F4U-1D comparison is the factor that contributes the most to the performance best ROC performance differences of these two aircraft.  Think about what you just said- F6F-5 has greater parasite drag vs. the F4U-1D.  They are both roughly the same weight, both have the same powerplant, so why does the F6F-5 climb better if it's parasite drag is higher?  Lower induced drag.  The higher a/c weight of the F6F-5's contribution to its induced drag is offset by it's greater wingspan and wing area.

POWER TO WEIGHT RATIO:
We are actually agreeing regarding your larger point.  (T-D)*V / W IS power to weight ratio.  You have to be careful with that phrase though.  When you hear power-to-weight ratio being mentioned it doesn't mean simply engine BHP to aircraft weight.  It means aircraft excess-power to weight, excess power being the difference between thrust and total drag times velocity.

(T-D)*V = Excess Power

Plug this into the RoC equation above and it simplifies to:

RoC = Excess Power / W ---> voila, power-to-weight ratio!

WING SPAN FACTOR IN INDUCED DRAG:
If you boil the induced drag equation down it becomes:

Di = 2W^2 / (pi*air density*span eff.*v^2*wingspan^2)

Notice the effect of weight and wingspan on Di since Di is directly proportional to W^2 and inversely proportional to Wingspan^2 and how aspect ratio drops out of the equation.  Reduce weight and your induced drag goes down.  Increase wingspan and your induced drag goes down.  This relationship is playing out in the F6F-5 example - how can it have a lower induced drag vs. the F4U-1D when it's slightly heavier?  Answer- greater wingspan.  (Span efficiency also plays a part in it all but let's just keep the analysis at this level for now.)

Another way to look at it is as you increase wingspan you're also increasing aspect ratio.

Tango, XO
412th FS Braunco Mustangs

[F4UDOA]

First I apologize for hijacking this thread. It seems from looking at the indicators shown in this thread that the P-40 should climb better than what has been shown here although I have not been able to test it.

Next,

Dtango,

I should probably just start another thread but...

Everything you have shown is very plausable but the difference between the two wongspans is only 2'. Also the aspect ratio is very close. When calculated the total drag of the F6F is still higher than the F4U. I am not trying to prove one way or another which one climbs better at this point. I am just saying that I beleive it is one of two other larger factors that make these climbs rates seem odd (to me).

Two possibilties.

1. The test data used by HTC is factory data and not Navy data. The NAVAIR test of the F6F-5 does show a much lower rate of climb.

or

2. I have a British report that says the F6F's climb rate is improved compared to the F4U because of it's superior cooling capability allowing it to be climbed with Cowl flaps closed where as they used half open clowl flaps on the F4U.

My use of the F8F was only to show that by itself Cdi would not prevent an Aircraft from being an exceptional climber as long as other factors such as power to weight were good.

[dtango]

Hiya F4UDOA:

Yeah, Actually I think I got us off the track with my "(mainly induced drag)" statement.  I unintentionally did what I was trying to point out to avoid - that is we have to look at the whole and not just a part.

The main point I was trying to make is if people want to compare rate of climb performance then we have to look at the whole relationship, not a part of it.

RoC = (T-Di-Dp) * V / W

Everything we have been talking about is a piece of the above relationship - AR, span, power, weight, blah blah blah.  We can't take just take bits of the above and make a performance conclusion but have to evaluate the above equation as a whole.

And your F8F is a great example why we can't do that!  You're right,  it may have a higher Di but that doesn't tell us the whole picture.

Tango, XO
412th FS Braunco Mustangs

[joeblogs]

I wonder if the difference is in the airfoil, with the F6f generating more lift per unit of area than the F4u?

While they are close, the F6f wing has a bit more area than the F4u.

My math is rusty, but I seem to remember that drag enters into lift and speed equations somewhat differently, which may explain why one plane can climb a bit better while having a slower top speed than another with the same power plant and rather similar weight.

Originally posted by F4UDOA
First I apologize for hijacking this thread. It seems from looking at the indicators shown in this thread that the P-40 should climb better than what has been shown here although I have not been able to test it.

Next,

Dtango,

I should probably just start another thread but...

Everything you have shown is very plausable but the difference between the two wongspans is only 2'. Also the aspect ratio is very close. When calculated the total drag of the F6F is still higher than the F4U. I am not trying to prove one way or another which one climbs better at this point. I am just saying that I beleive it is one of two other larger factors that make these climbs rates seem odd (to me).

Two possibilties.

1. The test data used by HTC is factory data and not Navy data. The NAVAIR test of the F6F-5 does show a much lower rate of climb.

or

2. I have a British report that says the F6F's climb rate is improved compared to the F4U because of it's superior cooling capability allowing it to be climbed with Cowl flaps closed where as they used half open clowl flaps on the F4U.

My use of the F8F was only to show that by itself Cdi would not prevent an Aircraft from being an exceptional climber as long as other factors such as power to weight were good.

[LtMagee]

Hi guys,

...the P-40 should climb better ...

You should ask LongDist about his flight in the P-40-?. I know all the guns, ammo, pilot armor etc are removed (more than likerly) but he says the P-40E in AH is very under modeled.