What is RAM AIR?

[funkedup]

My question is how do two A/C with the same engine, same rated MAP and same rated HP at a given alt. (under 8K is the main blower stage for both)have a 25knot difference in performance?

How would high pressure air affect performance to that degree without changing rated HP, MAP or RPM?

Read my post again.

[funkedup]

Ahhh NM Shiva splained it very well.  He said the same thing I did, but in English instead of Nerdlish.  Well done.  

[J_A_B]

"I don't ask questions without reason. You said yourself that the F6F is larger and draggier than the F4U. So then why does the F6F accelerate so much better than the F4U in AH?? "

I think the guys above explained it fairly well.....at speeds where these planes reach peak acceleration, the F6F has more excess power available....the F4U is using more of its power just to stay airborne at low speeds.  As speed increases the F4U will surpass the F6F in acceleration.

That said, I have always thought the F4U in AH didn't perform terribly well....perhaps it's because of the versions of the Corsair we have in AH.   How did the -1A's compare to the -1 and -1D?  Were any of them equipped with the paddle-blade propeller?  If so, I'd imagine it could climb and accelerate better than the -1D since it'd be lighter.    Really, IMO we have the worst overall versions of the F4U....the early F4U-1 and the heavier, ground-attack F4U-1D.

J_A_B

[HoHun]

Hi F4UDOA,

>At some point the F4U becomes faster because of a parasitic drag advantage for sure but what is the result of the sustained affect of Ram Air through different speed ranges?

Actually, the ram effect increases with speed. The result is that the point of equal climb of F6F and F4U is shifted towards a lower speed than without ram effect.

>How can you say which will reach 150MPH first? Then 200MPH, 250MPH etc.?

That's a good example for a complex situation as you start at 100 MPH and traverse different flight situations until you reach target speeds.

Let's say America's Hundred Thousand's comparison of climb is based on climb rate at speed of best climb at sea level. That's a simple situation with the aircraft being judged by a single frame on the imaginary film I mentioned.

Acceleration is hard to measure without averaging over a time span. For example, one could clock the time an aircraft needs to accelerate from speed A to speed B. This would mean you're going to have to take some length of film to record the test, with a great number of different frames.

If you select the speeds interval 100 mph to 200 mph, the F6F would certainly beat the F4U in acceleration as that's centered around the speeds of best climb where the F6F has the greater specific excess power (as evident from its climb rate).

If you make it 200 mph to 300 mph instead, the F4U would probably out-accelerate the F6F as at high speed, aerodynamics and ram effect combine to make the F4U better.

If you make it 150 mph to 250 mph - well, hard to say! Everthing could happen :-) Still, the winner of this acceleration test would not necessarily accelerate faster than the loser in a different speed range.

That's why it's important to know the exact test conditions if it comes to acceleration. Climb tests are pretty uniform, there's little confusion about them, but I know of no standard procedure for acceleration tests.

Looking at climb rate and top speed will give you two reliable indicators of acceleration though: At speed of best climb, it's proportional to the climb rate at that altitude, at top speed it's zero. And in between it's in between :-)

(I think the contradictions you run into result from the comparison of complex situations with simple situations, or even with other complex situations like time to altitude comparisons.)

Regards,

Henning (HoHun)

[Shiva]

Ride any new sports bikes alas a GSX-R 1000 if you want to really feel what Ram Air is like. The howl of the intakes and air box along with the amazing rush of ever increasing hp numbers and speed is a mind blowing experience.

Over the speed ranges that street bikes get to, you're not getting much ram air effect -- and in fact, your comment about the 'howl of the intakes and air box' proves it. What you're hearing is the resonant oscillation of the pressure in the air box and intakes, the same way you can make a musical tone by blowing across the top of a beer bottle. In fact, doing this with a beer bottle is a good illustration of how the tuning effect works -- as you drink more of the beer, the volume of the air box (the space above the beer line) increases, so the resonant frequency -- the tone you hear -- will get lower.

If sufficiently careful attention is paid to the design of the intakes and air box, they will have more than one resonance point within the engine's RPM range -- the natural tone and its harmonics. The lower the resonant tone, however, the less pressure variation is occurring, so the design will typically put primary resonant frequency at the RPM where the biggest torque boost is wanted; the harmonic points, if within the engine range, are lagniappe.

[Pongo]

this thread has degenerated from the simple elegence of shivas post...it all sounds like blah blah blah now.

[niklas]

RAM effects give u more power when you run the engine with unlimted boost.

But WW2 aircraft engines were limited, so RAM effect does REDUCE power a bit below critical altitude.
It just shifts the critical altitude a bit upwards, aroud 1500-3000ft.
Below the critical altitude the pressure gain due to RAM effects is wasted, because the blower could do the same. What happens is a raise in temperature, and this reduces the cylinder filling with same pressure.
You can see this effect of a slight performance loss in german engine and aircraft charts.

The R-2800 has for the 1st gear a very low critical altitude, so when the stage was not sufficient to keep pressure equal up to 2k feet than RAM effects will make the difference. The F4U didn not have more power, the Hellcat had less, if u understand what i mean.
Actually F4U performance charts show clearly that the 1st gear was not sufficient to build up full MAP for climbs at sealevel. But this changes already in 1-2k where the 2nd gear get activated.

niklas

[F4UDOA]

Niklas,

I'm glad you joined this conversation.

How do you mean the Hellcat had less power??

Could you please give me a simple calc for acceleration that I can apply to the Hellcat and F4U.

[Holden McGroin]

Aw hell...

I thought this thread was about St. Louis' passing game.

[niklas]

FU4DOA , you have great data about the Corsair, you should begin learning to read them correctly

Simple calculations, ok i try.

Lets assume your maximum MAP is 60", corresponding power shall be 2000PS (just an example) for 1st gear


EDIT: i took wrong  conversion factor, now correct ones for inches of mercury

Let´s assume a supercharger compresses the air with a given ratio, in the 1st gear we assume a very low ratio, only 1.5:1.

Air pressure near ground is ~36".

36"*1.5 = 54"

This is below maximum MAP, though the throttle is full open you wont have full power. Maybe only 1800PS

Now, another aircraft with good exploit of RAM effect at high speeds.

If we assume a full transfer of dynamic pressure into static pressure we can assume at 300mph a gain of around 2.8"
Total pressure in front of the supercharger would be 38,8" now.
so 38.8*1.5 = 58.3 ", still not maximum, but very close, 2150PS maybe.

So in this special case, where the stage of the supercharger alone would not be sufficient to build up full MAP, the RAM effect helps you to get more MAP.  And this was the case for the R-2800 of the corsair and hellcat at sealevel in 1st gear, just look at the engine power charts where they distinguish between climb power and topspeed power curves.

niklas

Edited for correct units