F4U/F6F drag analysis

[F4UDOA]

Heya's,

Someone punted one of my post on carrier takeoffs making me dig back into some thrust versus drag stuff.

Questions.

Is this correct?

I used Zigrats spreadsheet for thrust drag and climb to to the grunt work for me. It is here if you want to check my math.

performance calculator

In any case I used these NAVAIR performance numbers for input to get my results. The spreadsheet does the math. The stall numbers come from the Flight Manuals of both the F4U-1 and F6F-3/5.

Mil power settings for both at sea level.

F4U-1D
HP-2000
wing span- 41'
wing area-314sq ft
1G stall at 11,300LBS=97MPH clean power on
max speed sea level mil power= 345MPH

F6F-5
HP-2000
wing span-43'
wing area-334sq ft
1G stall at 11,250lbs= 74MPH
max speed sea level mil power =315MPH

F4U
100MPH
cdi-1046
Total drag-1183

150MPH
cdi-455
Total drag-763

200MPH
cdi-257
Total drag-805


F6F
100MPH
cdi-926
Total drag-1105

150MPH
cdi-415
Total drag-817

200MPH
cdi-234
Total drag-948

You can see that at 100MPH the F6F has less induced drag and less total drag than the F4U. At 150MPH the F6F is slightly less draggy and at 200MPH the F6F has higher total drag because of increased paristic drag.

The best speeds for the lowest total drag for both A/C are the best  climb speeds for both.

150MPH for the F6F
165MPH for the F4U.

160MPH the drag of the F6F becomes higher than the F4U.

My question is this.

Does this mean that since both A/C have exactly the same HP and exactly the same propellor(same blade design number 6501A-0) that the F4U should begin accelerating faster than the F6F at exactly 160MPH?

[Widewing]

From what I recall, the F4U had a direct air inlet to the carb, allowing for RAM effect. Grumman routed intake air through the accessory section, and lacked the added power afforded the F4U. This, I believe, is the reason the F4U was faster on the deck.

My regards,

Widewing

[HoHun]

Hi F4UDOA,

Two points that can mess up calculated results:

- Calculations often don't deal with the increased lift due to the propeller slipstream.

- Stall speeds in the manual are mostly IAS, which usually is quite a bit lower than the calibrated air speed at the stall due to position error.

I'd also think that speed of best climb is not perfectly identical to minimum drag since thrust slightly increases with speed, especially if you figure in ram effect. On the other hand, best climb speed could also be below the manual's best climb speed as higher speed improves cooling so you might get a better sustained climb.

I haven't checked whether these points apply to your calculations, but it's probably easy for you to tell.

Regards,

Henning (HoHun)

[F4UDOA]

Widewing,

I have read Corky Meyers interview where he states that the reason he F4U was 25Knots faster at sea level was because of RAM Air. In part he is correct as RAM seems to work like a RAM jet. The faster you go the more efficient it is. However my little research here is really about low end acceleration below 200MPH. I had a long thread about RAM air a while ago where I was informed that it would not effect low end acceleration. I am interested in the point in which it reverses. The F4U's biggest drawback was it's induced drag from ot's lack of wing taper and low aspect ratio. Surprisingly it actually has more total drag due to this below 160MPH.

HoHun,

I am aware of the increased lift and lift drag. The calculations are based on lift generated with a rotating prop. I say this because the stall speed and weight are for an A/C in clean condition at engine idle. This increases Clmax and also increases lift drag. If I would have calculated based on Clmax with no prop it would have lowered the lift drag(cdi).

Also I am aware of IAS and CAS. Each flight manual gives a correction chart. In the case of the F4U and F6F-5 it makes very little correction at those speeds. In both A/C subtracts about 3 to 5MPH from Indicated Air speed. If I was using the F6F-3 it is almost a 13MPH correction, not the case with these A/C. Use of flaps also causes a large error but I did not use flapped settings in my test.

I wasn't really trying to figure out best climb speed, I just thought it was interesting that those speeds are exactly what is reccomended for best climb. It does say in the manual that to increase cooling increase climb speed and it won't have much effect. I think this is common in radial engines.

My question is really at what speed does the total drag of the F4U become lower than the F6F?

I can also do some offline testing to see when AH has this reverse point by setting climb speeds to higher settings to see when the F4U has a better sustained climb.

[HoHun]

Hi F4UDOA,

>I say this because the stall speed and weight are for an A/C in clean condition at engine idle.

At full throttle, the stall speed at the same weight is even lower as the propeller slipstream contributes some lift. However, the greater the speed, the less difference does it make.

It doesn't seem to matter for your application, though :-) Sounds like you've avoided the possible pitfalls quite well!

Regards,

Henning (HoHun)

[hitech]

F4UDOA: The answere to your theortical question is yes if those are the drag numbers and if each prop is producing the same net thrust (note you also have to factor weight into the climb/acc equation but the 2 planes are close) then yes, but with the difference you are showing in drags, your margin of error in the caculations becomes greater than what you are trying to prove.

HiTech

[F4UDOA]

HT,

I'm not exactly sure what you mean by the marging of error becomes greater than what I am trying to prove. I am trying to find out at what point the higher induced drag of the F4U reduces enough to offset the higher parasitic drag of the F6F.

I know it happens somewhere in the speed curve I just didn't know where.

My original intention was to find an acceleration calculator on the web. I could not find one but then I remembered Zigrats spreadsheet comes close by giving me cdi, cdo and cd. So I took both A/C and plugged in the Flight manual stall speeds and weights. The assumption I am making is that Zigrats numbers are correct. One thing I am sure of is that this happens before 200MPH. I know this because at the same cruise settings the F4U is approx 20knots faster at all alts. Approx 160knots to 180knots at the same MAP.

The AH flight model does essentially the same thing with a graphic representation. I have not tested to see when this drag crossover takes place but I know it does. How close the results are should be interesting.

I guess if there is a point to doing all of this offline testing that must be it.

What I would really luv to know is why Rex Beisel designed an A/C wing with so much induced drag. It has an extremely low aspect ratio and almost no wing taper. The other two factors that could lower cdi are wing sweep and the F4U has none and wing twist which I am just starting to understand.

Do you know if WW2 fighters used wing twist? If so how much?

[GRUNHERZ]

F4UDOA I think you are seriously overanalyzing when you ask why the designer made a plane with so much induced drag....  
 
Maybe he just like that wing shape and used it before in other designs, just like Kartveli used the same basic wing shape on many of his planes.

[F4UDOA]

Grunherz,

Your right, I am over analyzing. But that is as much of a hobby to me as the simm itself.

I have a book by Ray Whittford on The Fundamentals of Fighter Design but he doesn't really get into the why of wing design other than the P-51 Laminer.

I did read on a engineering web page though that the reasons for having have rectangular wing instead of an elliptical.

They were

1. An elliptical wing adds weight.

2. A rectangular wing has better fuel storage.

3. Easier to get the desired wing area.

4. better Clmax

There were others but they stuck in my head.

Also to much wing taper causes bad stalls.

But why have such a rectangular wing I do not know.

The one thing about the F6F/F4U comparison is that they have the same engine and propeller so the comparison is easy.

I tend to think of climb and accleration as a function of power to weight but in the case of the F4U F6F this is not the case. The F6F was 500Lbs heavier however depending on what you read it climbed better. I have plenty of data that says it doesn't but it is annecdotal.

If anybody has Corkey Meyer's phone number I would luv to speak to him.

Hey HT, how about contacting him for the next AH convention??

[GRUNHERZ]

Oh one thing I read about F4U drag was its use of fabric covered outer wings. When they switched to all metal skin on the -5 I think it was said drag went down a lot.

[dtango]

F4UDOA:

Congrats!  I think you're getting a better grasp of the concepts!  Just a couple of comments.

As HoHun has mentioned Zig's spreadsheet doesn't factor in the effect of propwash.  In the end it doesn't make a big difference but if you're interested in small degrees of accuracy then you would probably want to account for it.

The induced drag difference is simple.  The biggest contributors are wingspan and weight.  What you are seeing in the induced drag difference between the F6F and the F4U is the extra 2 ft of span and 14 sq ft of wing area of the F6F.

F6F -->longer wingspan and lower wing loading --> lower angle of attack at equivalent airspeeds vs. the F4U --> lower induced drag vs. the F4U at equivalent airspeeds.

Lastly "climb/acceleration as a function of power to weight ratio" - keep in mind that this is true aerodynamically because it's really excess power to weight ratio meaning you have to subtract the "power required" due to induced and parasite drag from "power available" produced by the engine.  

In otherwords you can't just look at the engine HP's and weights of a/c to compare climb and acceleration since this totally ignores the "power required" due to total drag.  

Tango, XO
412th FS Braunco Mustangs

[HoHun]

Hi F4UDOA,

>I did read on a engineering web page though that the reasons for having have rectangular wing instead of an elliptical.

In the case of Navy aircraft, the wing shape often was determined by storage requirements. (As in the case of some landplanes - just think of the Stirling's 99 ft span so it would fit into a 100 ft hangar, and its box-like fuselage so it would be rail-transportable).

So a Navy aircraft's wings have to meet geometrical requirements, they have to provide a lot of lift at low speeds to facilitate carrier take-offs and landings - and high drag at low speed might actually be a bonus for low-speed controllability.

The F4U's folding mechanism might have determined the maximum wing span, and the minimum speed might have determined the necessary area. The gull wing was a stroke of genius to shorten the gear, increase wing span and reduce interference drag all at once, but still, the wing shape was a compromise.

The F6F's backward folding mechanism might have given it a bit of extra span, but the difference to the F4U isn't that great.

An radical contrast is provided by the Me 109T - small wing, high aspect-ratio, slats, and most importantly: spoilers. The big difference simply was that the Me 109T was a lightweight fighter so it could employ a very different wing shape under similar restrictions.

Regards,

Henning (HoHun)

[fats]

F4UDOA,

Fw 190 wing root is at 3 deg incidence while the tip is at 0. In case that's what you were looking for.


// fats

[dtango]

Just FYI- Wing twist is used to control stall characteristics of a wing.  The idea is to "equalize" the coefficient of lift across the wing with twist so that you control at which region along the span of the wing stalls first.

Tango, XO
412th FS Braunco Mustangs