Explain me this coefficient of drag on P47 and F4

[fscott]

According to "America's 100,000" the P47 has a significantly lower drag factor than the F4. Considering the fact that both have a 2000 horsepower engine at 1000 feet, why does the F4 have a faster on the deck speed?

I assume the drag equation takes into consideration all things that induce drag includign the type of propellor...

fscott

[Vermillion]

Reread the section of AHT about flight dynmaics.

There are two components too drag, parasitic drag and induced drag.

------------------
Vermillion
**MOL**, Men of Leisure

[F4UDOA]

Fscott,

This is a question I have researched extensively. I live very close to the author of "America's Hundred Thousand" and have visited him and E-mailed over the past year. His name is Francis "Diz" Dean, he 75years old and writing another book even as we speak on VTOL aircraft. He is a retired Aerodynamic engineer and a very nice man.

In any case I wondered the same as you about those drag coefficients because of one thing.

1. P-47D30
HP=2600
Weight=14,500Lbs
Max Speed at sea level=345MPH
Wing area=300sq ft

2. F4U-1D
HP=2250
Weight=12,000lbs
Max speed at sea level=359MPH
Wing area=314

So why is the F4U almost 15mph faster than the P-47 if the drag is higher? Answer?
It can't be as draggy. At least at high speed.

At low speed the wing of the F4U produces more lift than the wing of the P-47(Lift goes up and drag goes up together except with A/C with a high aspect ratio or an elliptical wing ie. Spit or P-38).This is called lift drag or induced drag represented as Cdi. However as speed increases drag from lift decreases and drag from the frame of the A/C increases. This is called Zero Lift drag or just Cd. These two combined give you total drag which is Cdo. When the Cdo(total drag) equals the available thrust of an A/C it's stops accelerating and you have the top speed.

So again in AHT (America's Hundred Thousand) when he refers to the CD of the P-47 and F4U he is doing the equation at a low or moderate speed, about 200mph. At that speed the F4U produces more lift and drag than the P-47(Lift and drag are directly proportional). If the same equation is done at 300mph the F4U fairs much better. Second however to the P-51 at high speed. The one part of drag that is constant and does not change is the Flat plate drag area of an A/C. This is also a calculated number based on a number of factors. Wing area, Aspect Ratio,(wing length divided by chord) Lift Coefficient and others.

I have looked at the data in that book extensively and there are only two discrepancies I have found. The lift coefficients in the back of the book on turn radius are not all with no flaps as listed so they change the turn radius comparisons and the drag coefficients are deceiving if you don't know what your looking at.
If you want to check lift coefficients check this NACA document from 1944.
 http://naca.larc.nasa.gov/reports/1945/naca-report-829/naca-report-829.pdf

Here is a copy of the E-mail he Frances Dean sent me on that subject.

 

Hi Mark--
It is some time since i got back to you-have been working hard on a new book
concerning VTOL aircraft for which I have a contract, so first things first!
I spent a couple hours today looking up some zero lift drag coefficients
based on wing area..
First, the drag coefficient for the P-47 was somewhat unfair-the value of
.0213 came from an Army Air Corps Technical Report of Sept.11, 1941 for an
XP-47B. The test was of a 1/14th scale model in a five foot tunnel and of
course there were none of those humungus wing store stations there.. Plus
don't know how they made out with all the tunnel corrections.
Calculating the drag coefficient is a dicey business-here are some references:
NACA MRdated Dec.14 1943:
       A/C            Calculated  Cd0   Flight Cd0
      P-38J              .0264                .0290
      P-39N             .0177                 .0175
       P-47C             .0194                ..0193
       P-51B             .0177                .0171
       P-63A             .0166                 .0165
In NACA War Report ACR L5A30 Feb 1945
        A/C              Cd0    (service condition)
         YP-38          .0293
           F4U-1         .0284    Another place .0267
            F6F-3          .0291    '"            "     .0272
            P-63             .0221   "            "     .0203
           P-51B           .0208
   QUEST FOR PERFORMANCE  NASA by Loftin
         A/C               Cd0
         P-51D            .0163
          P-38L           .0268
          F6F-3            .0211
  NACA ACR Oct. 1940
         A/C              Cd0
        XP-40           .0242
        XF4F-3          .0253
   NACA Report ACR 5.D04
        P-39             .0217
   DEAN Estimates a few years back when writing book
        P-47           .0251
         P-38F        .0269
        P-38J          .0283
        XP-47B        .0217
         P-47D BUB. .0251
         P-47M         .0235
         P-51B          .0155  (all Dean estimates for no jet thrust}
         P-51D          .0177
         P-51H          .0159
         F4U-1D        .0174
         F6F-3           .0211
         P-63C          .0153
    BOEING in-house chart
         P-38            .025
         P-38J           .0269
I suggest you look in Larry Loftin's book, which I understand is on the net
under NASA somewhere--Appendix C where it tells the aero relationship he (and
I) used in calculations.  To calculate the Cd0 one has to know the HP the
propeller efficiency (which one has to pretty much guess at-I worked for
Curtiss propeller for 18 years- the density ratio, the wing area, and of
course the speed-incidently the Cd0 is inversely proportional to the cube of
the speed- the wing loading, Oswald's span efficiency factor, the aspect
ratio, and there are some constants in the equation  One doesn't know either
the Oswald factor-usually around 0.8, or the prop efficiency, so one usually
guesses about 80 percent
I am getting too old and tired to go thru all the calculations so have your
fun!

I have allot more info on the subject. Mostly from emailing Wells from these boards. He has done an Excel spread sheet to determine Cdi, Cdo, Cd as well as climb. It is criptic but functional. If you want to see it email me.

Later
F4UDOA
 



[This message has been edited by F4UDOA (edited 11-29-2000).]

[fscott]

Hey thanks for a great response! I don't understand though.

You say that these drag numbers are from low to moderate speeds, ok, the F4 produces mroe lift at low speeds and therefore produces more drag.  My question is how can an aircraft go faster then? It seems contradictory if you have high drag at low speeds, how could you increase speed? OK, so crank out more horsepower via the throttle, and overcome the drag.  By that equation a P47 will use less horsepower to travel at a moderate speed than the F4. So the F4 requires more horsepower to travel at a moderate speed....

I dunno, It sounds good, but I don't quite get it. Maybe I do and I just can't get what I'm getting.  Perhaps the gull wing was an amazing design whereas you get less drag as you increase speed. Wow. Is there any other aircarft in the lineup that does this also?

fscott  

[Zigrat]

wow thats some great data, thanks!

[F4UDOA]

Fscott,

Just to let you know about 4 months ago I was asking the same questions as you. But I started to do some hard core reading and with a lot help from Wells got to understand this a little better. But I am a newbie to Aerodynamics myself so bare with me.

While at low speed the F4U produces more lift/drag than the P-47. But at high speed the lift drag goes down. The Cdo is supposed to be a constant but the value does change with speed. I do not completely understand this because in the calculations the Cdo definitely varies with the speed curve.

In any case if you notice in the range calculation section of AHT the P-47 profile drag is considerable higher than in the drag section. Also the flat plate drag area is listed as being lower than the F4U. This is also a calculated number that I would question.

Your question is how can an A/C go faster if the drag is higher at low speed. The answer is this. The total drag Cd is not higher at low speed. Just the lift drag. The total drag is actually lower. It becomes higher until the A/C hits its max speed where it becomes equal to the total thrust/HP.

Also if an A/C has 2250HP to use at sea level it is only using roughly half that HP to cruise at 200MPH. If you notice the P-47 cruises more efficiently at 200+MPH and the F4U cruises at approx 180MPH. I believe this is also a function of the lift from the larger wing of the F4U generating more lift/drag. I believe the F6F also displays similar characteristics with high drag at low speed and a cruise speed of 160mph.

Another piece of test data to consider is this. In a modern evaluation of the F4U, P-51, P-47 and F6F done in 1989 with stock aircraft at military power, 10,000ft they did an acceleration test. The P-51 was the best and the F4U was the worst at low speed. But when the A/C crossed 220knots IAS the F4U became superior to all but the P-51 in speed and acceleration. I believe this is a result of the airframe of the F4U being more efficient at higher speed. However in test done with the F4U-1A and the P-51B the F4U was superior in acceleration so I am also confused.

Here is a link to the F4U vs P51B flight test
 http://members.home.net/markw4/index2.html  

But notice that the F4U-1 is in a condition representitive of the F4U-4 although without the extra HP. The F4U-1A is a service A/C. Also the F4U-1A as well as the F4U-1 in the test is faster than any F4U in Aces High.

Later
F4UDOA

[This message has been edited by F4UDOA (edited 11-29-2000).]

[fscott]

Ok it becomes more clear now.  I am wondering if the F4 wing design and lower drag at higher speeds was something the team planned or if it was just a benefit of making the landing gear shorter using a gull wing.  I've always been awed by the Corsair's super clean profile and was amazed to see that it's drag facotr was higher than a P47. Thanks

fscott

[F4UDOA]

Fscott,

I wish I knew weather that design was intentional or not. Unfortunately Boone Guyton the chief test pilot from Vought passed away a couple years ago before I could contact him. The F4U design does seem ideal for it's mission though just as the P-51 does for it's. The F4U has high lift at low speed for maneuvering and carrier landings and low drag at high speed.

One thing I have noticed from all I have read lately is that all aircraft are purpose built. Nothing is by accident. The P-38 has long wings for lift and range. The P-51 laminar wing is also designed for range as well as the P-47. But Navy birds are designed for maneuvering and low speed carrier landings so they tend to have more lift but less speed. Everything is a trade. The F4U was an anomaly and well ahead of it's time. It was described in one book this way. "The F6F was an aircraft of evolution and the F4U was an aircraft of revolution". But I am certainly biased.

BTW, the main reason for the reduced drag of the F4U is a much smaller cowl opening due to the air intake for the oil coolers being moved to the wing roots. The F6F and P-47 had very large cowl openings where the oil coolers are.

Later
F4UDOA

[This message has been edited by F4UDOA (edited 11-29-2000).]

[washedup]

Fscott,

Forgive me if I'm OT but this is my 2 cents.  Problem here is that you can't use absolutes for the drag coefficients.  The drag of an aircraft is actually a curve that looks like a parabola with a bucket in the bottom.  The shape of the curve varies because of the relationship between induced and parasitic drag.  Looking at individual numbers can be deceiving.

[Rocket]

Hi guys!
  I love all the tech talk.  I am actually starting to understand some of it with each post  
  On the gull wings what I have read on the F4U is that some of the original designs had straight wings but as noted the landing gear would be too long to withstand the carrier landings.  The gull wing was chosen because of design effiency.  The angle the wing meets the fuselage makes for a much more clean wing (not sure how).  C-V attributes the gull wing design for the amazing E retention that the hog has.
  If the guys from C-V are to be believed the drag issues with the gull wing are luck as the wing initial design was to lower the length of the landing gear with the benefits of less drag than the original straight wing design.

S!

Rocket