Author Topic: Macchi C.202 performance (Read 2811 times)

Zigrat · « **Reply #15 on:** April 16, 2000, 05:47:00 PM »

warning: following is technical

about aspect ratio,

well the difference would really not be big enough to warrant the difference. Using finite wing theory, CL varies with AR in the following manner:

slope of CL vs alpha(finite wing) = slope of Cl(infinite)/((1+57.4*Cl(infinite)/(Pi*e*AR))

where e is an efficiency factor dependant on the wing (typical values of .65 to .9

this is in terms of degrees aloha where the slope of the CL-alpha line is .1097 for an infinite wing.

e is dependant on taper ratio etcera, and would depend on if there was twist in te wing and the planform area distribution. I dont know anything about the macchi wing reaally (if it had wash out) but the spitfire wing was one of the best in terms of WW2 planes because of its elliptcal shape (which leads to less vortex induced drag)

niklas · « **Reply #16 on:** April 17, 2000, 08:07:00 AM »

Does somebody know the disadvantages of a high aspect ratio for a fighter?? I mean, there MUST be some disadvantes. When a AR of ~8 like in a P38 has only advantages, why have all other top ww2 fighter AR of ~6 or even lower(109: 6, 190:6, P51: 5.85, La5: 5.5 ...)

niklas

RAM · « **Reply #17 on:** April 17, 2000, 08:14:00 AM »

ok, I admit it...I dont know what is aspect ratio

can someone explain it to me?
thks

Pyro · « **Reply #18 on:** April 17, 2000, 09:07:00 AM »

Gatt wrote:

The real C.202 climbed to 19,700ft in 5'55" and her max speed was 373mph at 18,300ft. Data from C.202 official manual CA 670/1 Air Ministry (yes, guess what, we had also an Air Ministry )
-----------------------------------------

Gatt wrote:

The real C.202 climbed to 19,700ft in 5'55" and her max speed was 373mph at 18,300ft. Data from C.202 official manual CA 670/1 Air Ministry (yes, guess what, we had also an Air Ministry )
-------------------------------

I've seen the official specs from the Air Ministry, and as I alluded to before, they don't add up. For example, they cite a time to climb to 3280 feet of 39 seconds or an average of 5046 feet per minute. This performance is cited at a weight of 6459 pounds.

Let's examine those numbers in detail. It takes about 988 excess THP(thrust horsepower) to attain that performance at that weight. They cite brake horsepower ratings of 1040 and 1075 for climb and emergency power settings respectively. Let's go with the high figure and say 1075 BHP. In order to make the 202 climb like they claim, you need a completely dragless airframe and airfoil, and a propeller capable of 92% efficiency. Both of these are outside the realm of reality, and therefore the data is invalid no matter how official and accurate the source appears.

------------------
Doug "Pyro" Balmos
HiTech Creations

Perfect plans, aren't.

Kieren · « **Reply #19 on:** April 17, 2000, 09:28:00 AM »

Ram-

Aspect ratio is the comparison of wingspan to its chord (width). Most WWII aircraft incorporate some taper, so figuring the exact aspect ratio involves some math.

The disadvantage to high aspect ratio is drag. You have a lot of wing exposed to the direct airstream, more than a low aspect ratio wing. Add to that the lack of sweep on WWII wings and you can see how air might pile up quickly. Airfoil selection would be critical.

Planes like the P38 pull this off because they were designed for higher, thinner air. There the high aspect ratio is an advantage (look at the Ta152H, or U2).

Zigrat · « **Reply #20 on:** April 17, 2000, 10:18:00 AM »

well actually, there is no drag penalty in having a higher aspect ratio compared to a low aspect ratio wing as long as both have the same area, actually high aspect ratio is better almost all around. The thing that limits aspect ratio is structural mechanics, ie wings of high aspect ratio are harder to build and support in flight.

Kieren · « **Reply #21 on:** April 17, 2000, 10:22:00 AM »

Once again, I defer to those with more expertise!

gatt · « **Reply #22 on:** April 17, 2000, 11:11:00 AM »

PYRO, thx for the reply. You should consider that data are from 202's of IV-VIII series, so I think they probably must be referred to the normal C.202, the one with only 2x12,7mm that is. This one weighted some 70-80Kg less than the "more armed" one. But I dont think this is enuff for you.

However, what makes me thinking about the C.202's FM is the final result: IMHO, 7'15" to get to 19,700ft with WEP is definitely too much for a light fighter with a good wing and a good engine like the DB601Aa. Moreover, the max speed is so different from the official one .... In other words we have no one of the famous features of the "Folgore", we have only her light armament to deal with.

Regards,
GATT
4°Stormo CT

v-twin · « **Reply #23 on:** April 17, 2000, 01:19:00 PM »

Hi Pyro!
I have some doubts that is possible to determine the real performances of an aircraft doing simulations starting from knowed parameters, whatever they are.
I work 4 a truck-factory and my job is to simulate the performances & the behaviour of the future vehicles.
I noticed that I can put all the possible and most accurated parameters in the sim-softwares, but when we test the vehicle, the results are always different than the predicted results of the sim, because it is impossible to consider all the variables wich affect the behaviour of a truck.
Ok, planes ar *different* from trucks, even if a 190 is quite similar...

, but the idea remains.
I have the official Macchi 202 Service Manual, wich is the ONLY official manual written by Macchi (a flight manual never existed), and this manual reports absolutely NOTHING about performances, only weights, dimensions, service & maintenance instructions etc.
I never saw the manual of the Air Ministry, but it seems reasonable to me that the data on this manual comes from the tests performed by the Air Ministry, because AFAIK the results of the Air Ministry tests were the only valid 4 the evaluation of an aircraft and the resulting data were the only accepted and ratified as official data (4 example the official max speed of the Mc 205 Veltro is 642km/h because this was the lowest max speed obtained during the test).
I think that is reasonable that official data of whatever plane have to come first than the simulations.

v-twin

ps: I apologize 4 my bad english, hope I was able to explain my thougts

Kieren · « **Reply #24 on:** April 17, 2000, 02:36:00 PM »

I do think it fair to say that actual performance figures are almost always less than the design figure for exactly the reasons you state. It is impossible to take into account everything that relates to performance. The unknown usually degrades, rather than enhances, performance.

funked · « **Reply #25 on:** April 17, 2000, 03:06:00 PM »

"I have some doubts that is possible to determine the real performances of an aircraft doing simulations starting from knowed parameters, whatever they are."

V-Twin, I agree that it is difficult (working on such a project right now) but the people who build airplanes can do this quite accurately.

v-twin · « **Reply #26 on:** April 17, 2000, 04:06:00 PM »

To funked

Sorry, I couldn't explain well: with no doubt people who build AC can simulate very well the behaviour, I ment that maybe the few data available 4 the 202 aren't enough to obtain a good simulations of the AC.
But I work on trucks, not on planes

, so maybe I'm wrong...
Anyway, if I simulate a truck and the sim says it should do 100km/h and I find it running 120km/h, the real speed is 120km/h not 100km/h.
I think the same can be applied to planes too.

v-twin

[This message has been edited by v-twin (edited 04-17-2000).]

funked · « **Reply #27 on:** April 17, 2000, 04:33:00 PM »

V-twin, you have a good point, I understand what you mean.

I am currently trying to design a robot controller in the same fashion. I find the only successful technique is to combine the direct parameter measurements (mass, inertia, etc.) with estimates of the parameters based on experimental performance. It's hardly scientific, but I get good results if I use both sets of information.

But if I get one set of experimental data that is distant from direct measurements and other experimental data, I am inclined to ignore it, and I think Pyro is doing the same.

[This message has been edited by funked (edited 04-17-2000).]

v-twin · « **Reply #28 on:** April 17, 2000, 05:22:00 PM »

Yes funked, but I think Pyro is ignoring the direct measurements...

I'm quite sure that the data on that manual are measured performances of the AC, but at the moment I don't know how to verify this.
AFAIK the ratified data of the ACs didn't come from the factories, but were given by the Air Force after the evaluation tests.

v-twin

Pyro · « **Reply #29 on:** April 17, 2000, 06:56:00 PM »

Hey V-Twin,

I understand what you're saying. When I first started doing this I'd see one report and think I knew how precisely how an airplane performed. But then I'd see more reports and all the sudden I no longer knew.

What I'm saying though is that in this case, it's not a little bit here, a little bit there, it's completely out of what I can make happen. I can't make a dragless plane with a propeller that has a 92% efficiency. I can only bend things to make them fit, I can't break them.

------------------
Doug "Pyro" Balmos
HiTech Creations

Perfect plans, aren't.