Author Topic: Climb and acceleration(engineers please) (Read 1169 times)

wells · « **Reply #30 on:** July 13, 2002, 05:03:19 PM »

Naudet,

Here's what I came up with for a Fw-190D-9. This is based on mass of 4270 kg, 1680 PS at 6600m (1800 PS at sea level), where the plane did 686 km/h. The results are (assuming 20% losses to the propeller efficiency):

Best climb (initial): 17.4 m/s @ 265 km/h ias, reduce climb speed to 250 km/h @ 6600 m (14.3 m/s).

The zero-lift drag coefficient (Cd0) is about 0.022.

First one, thrust and drag curves...

sea level

6600m

Then, climb performance vs speed:

sea level

6600m

Finally, glide ratio vs speed:

sea level

6600m

dtango · « **Reply #31 on:** July 13, 2002, 05:12:25 PM »

D'oh! Thanks for catching that Badboy. Corrected above now.

Wells- I was just about to fire off an email to consult with you on the D-9 graphs

using your little program.

Tango, XO
412th FS Braunco Mustangs

Naudet · « **Reply #32 on:** July 14, 2002, 04:58:26 AM »

Great curves wells.
But i could need a little explanaton, especailly for the 1st two diagramms, cause i don't know which of the four curves illustrates what. Which colour has the drag curve, which colour is the thrust curve and so on.
The climb curves i understand.
And the glide ration i don't even ever heard off.

Sorry that i am such a aerodynamics novice.

Btw would it help you if you would have a power curve for the JUMO213?
I have one if you want i will attach it here, so you can take exact power outputs for all settings (1700/1900/2100PS).

dtango · « **Reply #33 on:** July 14, 2002, 11:08:26 AM »

Thrust - drag curves...

White - thrust available
Yellow - induced drag
Blue - parasite drag
Green - total drag (thrust required)

To convert this to Pa / Pr - multiply thrust available by velocity over the range of velocities to get Pa and multiply total drag by velocity to get Pr.

Tango, XO
412th FS Braunco Mustangs

F4UDOA · « **Reply #34 on:** July 15, 2002, 10:19:34 PM »

Back to the original subject.

Question.

Why would Vought engineers build a wing with so much induced drag? It had the lowest aspect ratio of any of the 5 major American fighter types as well as the lowest taper ratio?

What gives?

GRUNHERZ · « **Reply #35 on:** July 15, 2002, 10:38:50 PM »

F4UDOA you are overthinking this.

I

hihgly doubt Vought cared so much about this as the past 60 years of development and plain time to think or argue about the finer points of aerodynamics have. You could just the same ask why they did not incorporate laminar flow wings, or why they didn't cover the outer wing in metal instead of fabric, or why they went with a bent weng-we know other navy planes with R2800 didnt need them. And so on and so on.

Or one can say they simply were not as smart as the NA enginners who designed the P51 wing, or Kurt tank who designed the very taperd very high aspect ratio Ta152H wing.

But seriously the only logical reason I think it was to maximize wing area while minimizing wingspan.

dtango · « **Reply #36 on:** July 15, 2002, 10:40:40 PM »

F4UDOA:

I guess you missed my post above on induced drag - see just a couple of posts up.

(1) AR is not the primary factor determining induced drag. Weight and wingspan are.

(2) AR, taper ratio have more to do with span efficiency - e which usually is between .7 - .9. WW2 wing planform improvements to improve span efficiency makes only a minor impact on induced drag.

Tango, XO
412th FS Braunco Mustangs

F4UDOA · « **Reply #37 on:** July 16, 2002, 09:36:26 AM »

Grunherz,

The F4U was porbably fying before the first Laminar wing was built. Putting a laminar wing on a carrier fighter was probably not anyones thinking in 1939.

What I can't understand is why with some seemingly minor changes to the wing design the F4U could have climbed much better. I realize that everything is a trade off of one feature or the next. I am just trying to understand the why. The P-51 and TA-152 are purpose built and it is easy to see what the designers where thinking at the time. I am trying to get inside the head of Rex Beisel who has been dead a long time. BTW Rex is the guy who put the oil coolers in the wings of the F4U making the cowl opening smaller than the F6F and P-47. This design was copied in the F8F Bearcat. Obviously Rex was the influence behind the Bearcat not the FW190.

Dtango,

I must have really been out to lunch because I completely missed your post. However now that I see the equation for Cdi, I am really confused.

This is why.

The Cl max of the F4U is very low compared with the F6F or any of it's contemporaries. It is 1.49 power on no flaps. The F6F by comparison is closer to 1.8. I am not sure what E represents in your equation but it would seem that the number will be much smaller in the F4U because of the lower Clmax.

dtango · « **Reply #38 on:** July 16, 2002, 11:15:01 AM »

If you go back to the induced drag equation in level flight you will notice that Cl doesn't factor in. The related variable to Cl in induced drag is the airspeed [edit: and weight]. Cl is really a measure of AoA which is a function of the airspeed and weight of the aircraft.

Clmax is the maximum AoA a wing can obtain before an aircraft stalls. I may have to think through the following statement to make sure it is technically accurate but the F4U having a lower CLmax value just says that it can't generate as much lift at slower velocities vs. other a/c with higher Clmax values. Clmax occurs in level flight at the stall speed of the aircraft.

Clmax isn't related to span efficiency - e.

Tango, XO
412th FS Braunco Mustangs

Badboy · « **Reply #39 on:** July 16, 2002, 02:11:32 PM »

Quote

Originally posted by F4UDOA
What I can't understand is why with some seemingly minor changes to the wing design the F4U could have climbed much better.

Because aircraft don’t climb on their wings, they climb on their engines. During a sustained climb, the wings only support a little less than the weight of the aircraft, the climbing is done by the engine’s excess power. The more excess power, the higher the rate of climb, the more excess thrust, the steeper the climb. That’s slightly simplistic because of course the excess power is partly determined by drag and thus by the wing, but even so, a designer trying to increase the climb rate would be more likely to look towards the engine and propeller combination for improvements. For example, when paddle blades were retrofitted to aircraft during WWII, their higher activity factors meant that much more of the engines power could be absorbed and pilots noticed a very dramatic increase in climb rate.

Quote

The Cl max of the F4U is very low compared with the F6F or any of it's contemporaries. It is 1.49 power on no flaps. The F6F by comparison is closer to 1.8. I am not sure what E represents in your equation but it would seem that the number will be much smaller in the F4U because of the lower Clmax.

The number normally represented by the character e in induced drag calculations was originally known as Oswald’s efficiency factor, and his original paper is available for download from the NACA report server. More commonly it has a component of parasite drag lumped in with it and is just called the airplane efficiency factor and can be estimated depending on the aspect ratio, taper ratio, and sweep angle. Theoretically the Spitfire would have an efficiency factor close to 1, meaning that it will have a coefficient of induced drag close to the theoretical maximum. You might expect most WWII fighters to fall somewhere above 0.8. Aircraft with higher values will produce less induced drag, which is why the Spitfire holds its energy so well in a turn, aircraft with lower values will produce more induced drag so they tend to bleed energy more quickly during turns. The value for the F4U is lower than the Spitfire, but only because its wing shape has a less elliptical lift distribution, not because of the difference in lift coefficient.

The efficiency factor, and the lift coefficient both have an influence on the amount of induced drag when the aircraft is maneuvering, but independently of each other. For example, even if an aircraft has a high efficiency factor, it may still produce more induced drag than one with a lower efficiency factor if it produces more lift. That means that an aircraft may lose energy much more rapidly than another when maneuvering close to their maximum coefficient of lift, and yet lose energy less rapidly when they are both in level flight.

Hope that helps.

Badboy

Nawc · « **Reply #40 on:** July 16, 2002, 03:09:57 PM »

DOA,

There is a really great book put out by AIAA, the Author's last name is Raymer I believe. It's an introductory book to aircraft design.

Many decisions are made when designing an aircraft. It's more or less a balancing act. If you want a certain performance aspect in an aircraft, other parts may suffer, or may not be needed. LIke the wing of the corsair. It affords speed and maneuverability, but may induce more drag, but in the overall scheme of things, that drag may be negligible.

An elliptical wing is the best wing design for a low Cdi (induced drag) coefficient. But, if you do some quick back of the envelope analysis you can approximate that Cdi very closely by using a tapered or sectionally tapered wing. So close in fact, you can argue that the difference is negligable in most cases. Again depending on how you 'optimize' your design. Again, aircraft design is give and take, you change one equasion...and low and behold all the others are changing to reflect it. So you really have to become creative on how you tweak your design and what is truly desirable.

I highly recommend the above mentioned book, it may be pricey but you can get it used pretty reasonably. It's been awhile, my aero is a bit rusty, I haven't done any basics in awhile. But I work on aircraft everyday and it's a great field to have an interest in.

Also for anyone interested, Bruce Bohannon is doing some crazy time to climb record breaking in his Exxon Tiger. Check it out.

NAWC

wells · « **Reply #41 on:** July 16, 2002, 04:49:57 PM »

Quote

Originally posted by F4UDOA
Back to the original subject.

Question.

Why would Vought engineers build a wing with so much induced drag? It had the lowest aspect ratio of any of the 5 major American fighter types as well as the lowest taper ratio?

What gives?

They probably figured that they needed a certain wing area to get the takeoff and landing speed/distances they wanted. Then the span had to be such to fit into carrier elevators and hangers and things. Vought chose to fold the wings up instead of back, like on the F6f.

dtango · « **Reply #42 on:** July 16, 2002, 05:27:41 PM »

Great explanation Badboy.

On a tangent Badboy is hitting a principle to keep in mind when he mentions that e and Cl have independent impact on induced drag while an aircraft is MANEUVERING. This is a different state vs. "level-flight" or "constant velocity climb".

The principle is- aerodynamics is the field of dynamics meaning you have to be mindful about the various states or parameters of flight you are analyzing because they change depending on the state of flight. Things are tricky indeed.

For instance back to Clmax, Clmax occurs at a specific condition- in the case of level flight at the stall speed of the aircraft which occur at different velocities for different airframes. Comparing Clmax values doesn't make sense unless you are comparing the Cl values at the same velocity.

Tango, XO
412th FS Braunco Mustangs

GRUNHERZ · « **Reply #43 on:** July 16, 2002, 05:54:27 PM »

Actually it was a copied off the commie Yaks. IIRC the Bearcat doesnt actually have the oilcoolers in the wings, it just has the air intakes for them there, unlike all the Corsairs except AU1 who actually have the coolers in the wingroot.

Why were the outer wings covered in fabric untill the -5?

F4UDOA · « **Reply #44 on:** July 16, 2002, 10:32:53 PM »

Grunherz,

Most fighters had fabric covered control surfaces to the bitter end. The P-51D had them until the D-15 I think it was. The F6F, Spit and many others. Why fabric? Why not I guess. I have no idea.

Gents,

I do really appreciate all of the explanations. The bizarre thing about them however is the ambiguity of terms.

BadBoy mentions that an A/C climbs on it's engine(If this is true the F4U-1 should climb with the best American A/C). So this would lead me to believe that power loading is the key to climb. Wrong====>Wells says that aspect ratio is the key to climb. Wrong====>Dtango says the Cli is the ticket but wait, you can't really measure it unless you know what speed the climb is taking place and the AoA. I'm sorry about the sarcasm but it seems like one big circle.

I was almost satisfied with Wells answer of an index # with A/R being the determining factor. But then seeing the equation for Cli makes me believe otherwise. However without knowing Oswalds number I may never know the answer. Is Oswalds number engineer code for an unexplainable situation? Like a boondoggle?

Frankly I'm begging to believe that the way engineers really find out how an A/C fly's is say "Hey Tony, go take the one at the end of the line" and then make up equations to explain the results.

BTW, Spanloading. I listed some numbers for spanloadings of several A/C. How do I interpret these numbers. What is optimal?

1. F4U-1D======= 294
2. F6F-5======== 291
3. P-47D========345
4. P-51B======== 251
5. P-38J======== 316