Well's Somthings not right in mudsville.

[funked]

Kats, based on flight test data, the difference should be about 30 mph.  But here the G-10 is good for 450 plus.  

DOA, try some acceleration tests at different altitudes.  I already did a bunch of stuff for P-38 and P-51 here:  http://bbs.hitechcreations.com/smf/Smileys/default/Forum9/HTML/000516.html

The way I did these was to put the plane on autolevel a couple hundred feet above the target altitude, then idle the throttle.  The airplane would sink and the TAS would drop below my start speed right as the altitude hit the target altitude.  Then I would firewall the throttle and engage WEP.  The plane would hold altitude, then as the speed increases to my start speed I hit the stopwatch.

[This message has been edited by funked (edited 06-13-2000).]

[wells]

 

A. The F4U and F6F. Both had the same rated HP. 2,000HP. Both had
                  approximately the same drag coefficient. F4U=.0267 F6F=.0272 Both had
                  approximately the same wing span, area and Propeller. Then why was there
                  such a disparity in performance and how would this be factored into AH
                  using the current FM design points?

I think you are being too general in your observations.  The wing areas are not as approximately the same as you might think when it comes to performance, nor are the drag coefficients.  That's why they are measured to 3 significant figures.  Example:

F4u = 0.0267 * 314 = 8.38 < drag index (note typo in AHT)
F6f = 0.0272 * 334 = 9.08 < drag index

So it can be estimated that the F6f is about 8% draggier or 4% slower than the F4u.  If an F4u can hit 360 mph on the deck, then we should expect 346 mph from the Hellcat.  That is pretty much true going by AHT.  It looks like the Hellcat is even a tad better than that at 350 mph.

Or, looking at it in reverse, you could say that the F4u should be capable of 365 mph.  If one factors in the 3% extra wing surface area on the F4u due to it's gull wing design, one could say that it accounts for about 1.5% loss in speed, which happens to almost exactly make up for the missing 5 mph.

[This message has been edited by wells (edited 06-13-2000).]

[F4UDOA]

Wells,

The F6F-5 was supposed to be able to do 350 on the deck. But if you look at the Navy's speed performance you will see that it only does about 330Mph at sea level. Another source that gives the same result would be the Fw-190, F4U, F6F test were again the F6F can only achieve 334MpH at sea level and the F4U reaches 363Mph. Also the F4U in AHT records a top speed of some 20Mph faster at altitudes above 20K. Even if Grumman's numbers are right the disparity at higher altitude still remains.

Do you know how to calculate thrust output from different propellers? With a 2000HP engine what would be the diffeence in thrust from say a 12'2" 4 blade prop vrs a 13'1" prop. I am looking for pitch and chord info on them but I'm curious about the thrust differential.

Thanks F4UDOA

Scanning those Grumman docs for you.


[This message has been edited by F4UDOA (edited 06-13-2000).]

[wells]

There was a 20 knts instrument error in early F6f's (found by flying with a Corsair in formation), that was later corrected, after the -3 model, I believe...and maybe retrofitted to some earlier planes.  The Hellcat is a 400 mph airplane.

As for propellers, it is ideal to use an airfoil that has a good Lift/Drag ratio as well as high aspect ratio for the blades without having such a narrow chord that the low Rn (Reynolds number) adversely affects drag.  Any drag equates to torque, requiring more power to turn the same RPM.  The power required to turn a 12'2" prop at the same RPM as a 13'1" prop varies to the 5th exponent.

(12'2" / 13'1")^5 = 70% power required

Propeller efficiency is a function of disc area vs power.  The larger the disc area for the power, the more efficient (ie, helicopter).

If 2000 hp is turning the 13'1" prop, then about 1400 hp would turn the 12'2" prop.

Maximum efficiencies at say 200 mph are 93.7% for the large prop/engine combo vs 94.9% for the smaller prop/engine.  If 2000 hp is turning the smaller prop, efficiency drops to 92.8%.  Real world efficiencies are about 85% of these values due to rotational and tip losses.  I should point out that there is a larger difference in thrust efficiencies at lower speeds.  At 100 mph, for example, maximum efficiencies are 65.2% vs 61.7% for the 13' and 12' props respectively.
 
Likewise, the number of blades affects the power and diameter requirements.  Less blades is more efficient up to a point.  Tip speeds need to be kept reasonable.  

Going from 3 blades to 4 would require a reduction in diameter to 94.4% that of an equivalent 3-bladed prop (3/4)^1/5.

Where paddle blade props are concerned, more blade area is concentrated out towards the tips, where the majority of thrust comes from.  Also, with a constant speed prop, the tips are almost always operating at an optimum angle of attack throughout the speed range, while the roots may stall at lower airspeeds, so it's beneficial to have more blade area towards the tip under those conditions.



[This message has been edited by wells (edited 06-13-2000).]

[F4UDOA]

Wells,

I read that to about the Hellcat pitot tube config. The Grumman engineers were screaming for years about it. It depends on who you listen too. Even after the mistake was corrected in the F6F-6 which was the comtemporary of the F4U-4 with again the same engine and prop it was still 25mph slower that the F4U-4. That is a conversation for another day.

Anyway I think I follow you on the efficiency's but how does that translate into thrust? Why not use a smaller prop if there is no advantage to using a larger one.
Another engineer friend of mine told me that a larger prop delivers more thrust. Is that a true statement?

Later
F4UDOA

[wells]

 

Anyway I think I follow you on the efficiency's but how does that translate
                  into thrust? Why not use a smaller prop if there is no advantage to using a
                  larger one.
                  Another engineer friend of mine told me that a larger prop delivers more
                  thrust. Is that a true statement?

Yes, a larger prop is more efficient.

The difference in thrust will depend on airspeed, so at 100 mph, there might be a difference of 300 lbs between the 2 props.

[-lazs-]

funked the Hog had the wrong prop and was running lean and used wep for only 3min.... Under those conditions..... Yes I believe the 190 should be faster at 15K.
lazs

[F4UDOA]

Wells,

I am reading "The Illustrated Guide To Aerodynamics" and it discusses optimizing propeller efficiency ranges to suit the needs of an aircraft IE if an A/C needs to cruise at 200mph then the prop would be optimized for 200mph max efficiency. And If you need an A/C to climb steeply you would optimize it's range at 150mph. By following this logic I am looking at the P-51 and P-38 which were purpose built for long range cruising as well as their best climb speed being around 200mph. I am also assuming that their best prop efficiency is in the 200mph range. By contrast you can say that the F4U and F6F which have a best cruise at 170mph and 150mph respectively and also have a best climb speed of approximately 140mph would have propellers that were at max efficiency at 150mph. Due to the purpose build of a carrier fighter having a need to take of in a short distance and have the ability to wave off of landing with extreme low speed acceleration I think it is a reasonable assumption. In that case the standard power loading tables would have to be re-evaluated. See if you agree with my concept. My numbers are not accurate since I am not sure of the calculation for prop efficiency.

All numbers for sea level max fighter weights
P-38L combat Max horsepower  100%effic.  3200
150mph max brake horse power  75%effic.  2400
Power loading 100% effic.17699lbs/3200hp=5.53
Power Loading 75% effic.17699lbs/2400bhp=7.37

F4U-1D Combat Max Horsepower 100% effic. 2135
150Mph Max Brake Horsepower   90% effic. 1921
Power Loading 100%effic. 12289lbs/2135hp=5.75
Power Loading 90%effic. 12289lbs/1921bhp=6.39

This would mean that the F4U would accellerate at a higher rate at low speed relative to those aircraft that are purpose built for a high speed cruising condition. I think what is taking place in the current FM is that all prop efficiency's are modelled at 80% throughout the speed range causing certain aircraft to loose characteristics that they were reputed to have. This would also effect such A/C such as the N1KI and the FW-190A8 as well as the F4U and any future modeling of the F6F or P-47D-25.

Wells if I snatch the pebble from your hand may I leave the temple??

Later
Grasshopper

[wells]

A propeller's design (twist) is optimized for a certain advance ratio (ratio of forward speed to RPM) to give optimum angle of attack at all points on the blade.  For the most part, designing for speed is also designing for cruise.  Example:  300 mph @ 3000 rpm is the same as 200 mph @ 2000 rpm.  That's one area of performance that I have yet to do much research with WW2 planes, not having very good access to them.  I would like to measure the twist of the propellers used on these planes!

[funked]

How would you measure it Wells?  I'll be in Dayton visiting my grandparents soon...

[wells]

I dunno funk,

Some kind of incidence meter placed at various intervals along the blade, say 0.25r, 0.5r, 0.75r and 1.0r (the tip).  I'd imagine that it would be very clumsy to try and measure it while it's on the plane (if they'd even let you?).  Maybe they have the specs for the propeller hidden away somewhere in some kind of manual?  

[funked]

Too bad I don't work for Ham. Std. any more.