Thrust

[Widewing]

 

Originally posted by juzz:
OK, so they both do the same speed at sea level, but the G-10 has about 300hp more.

What does this mean? The G-10 propellor is only 85% as efficient as the P-51D's? The G-10 airframe has 17% more drag? A combination of the two? How can we tell?

I doubt that you will find deviations between different props that exceed +/- 3%. I think the G-10 suffers from a great deal higher drag. I would expect to see a drag coefficient up around .0280 or more. Just look at the aircraft. Bulges, scoops, radio masts, non-retracting tail wheel and the main wheels are only partially covered. You also have aileron counter weights out in the wind, a nearly vertical windscreen and the radiator and cooler inlets are in the boundary layer. Despite switching to a true cantilever tailplane, allowing the deletion of the braces, the G-10 probably has a higher total drag than the Bf 109B-1 of 1937.

Comparing it to the ultra-clean P-51 with its lower drag laminar flow wing, the G-10 shows it age. The Mustang also had the advantage of its thrust generating Meredith
Effect, energy recovering radiator duct design. North American's Lee Atwood thought that the radiator duct design was the key to the Mustang's performance> A few years ago, he wrote the following:

Atwood explained, "Both the British and German engineers at the time thought you could test a scale model in a wind tunnel. But the wind tunnel models didn't generate the engine-heat factor, which we successfully controlled within the air scoop to create positive thrust. They were all looking at the Mustang's laminar flow wing, which was noted for reducing air friction over the surface of aircraft wings."

"The laminar flow wing is great for jet airplanes or in a high-speed dive but had relatively little effect on the P-51's overall performance envelope. You have to attribute the speed increase to the radiator energy recovery (positive thrust), not the characteristic of the wing itself. The wing did help in a dive -- not in level flight. I never mentioned this to anyone during the war."

Atwood credited F.W. Meredith of the RAE Farnborough, U.K., whose August 1935 report known as the Meredith Effect greatly influenced his work on the P-51 cooling radiator and duct design.

My regards,

Widewing

[funked]

F4UDOA:  I never said anything about power/weight or thrust/weight being related to top speed.  I'm not sure what you are talking about.  Also you mentioned something about a "chart" above and I don't know what you are referring to.  Confused here.  

Widewing:  Yes if speed is not changing, then thrust equals drag.  I just wanted to make it clear that the max thrust speed is not the same as the max drag speed if the engine is operating at full power.

Juzz:  "What does this mean? The G-10 propellor is only 85% as efficient as the P-51D's? The G-10 airframe has 17% more drag? A combination of the two? How can we tell?"

You can't really tell.  
The best you can do is get drag estimates for the two planes (either from wind tunnel or CFD or other analytical  tools), then figure how much thrust would be needed to achieve the top speeds.  If you have a good power figure, then you can work out the efficiency of the prop.  

Obviously the drag estimate may be different from the actual in-flight drag.  Also the engine may produce more or less power when installed in the airplane compared to on the test stand, due to a number of effects.  Finally the exhaust stacks on some WW2 planes generated a significant amount of thrust.  All these factors could show up in your "prop efficiency", even though they have nothing to do with the performance of the prop.

Another way to do it would be to deal with the prop directly.  Either use analytical tools to figure out how much thrust is generated for a given rpm/airspeed/pitch combination, or some sort of testing.  I've seen photos of prop/engine combos rigged up with strain gauges (which can measure thrust load in the prop shaft) which were presumably being used for this sort of testing.  Again there are some problems though, because the performance of the prop maybe different in flight due to aerodynamic interaction with the airframe.  And there is still the exhaust thrust to deal with.

But all of those techniques can definitely get you in the ballpark, a lot closer than the "0.8" fudge factor.  

[Jimdandy]

Thanks Widewing. I think we're getting a handle on it now. I'm still trying to figure out (the original question by fscott a few days ago)  what is the cause of the N1K's "exceptional" performance in the loop example is. I don't see in these figures where the N1K should out perform the P-51D. Is it because the thrust to weight is a bigger factor at low speeds (drag increasing with v^2) where the drag is less of a factor? I hope you can pop a comment on before tomorrow afternoon because I will be off line for a week or two. Thanks again for the info Widewing.

[funked]

"Is it because the thrust to weight is a bigger factor at low speeds (drag increasing with v^2) where the drag is less of a factor? "

Bingo!!!

[wells]

You can get a spreadsheet I made here.  It will calculate thrust, induced drag, profile drag, climb rate, glide ratio, sustained turning speed, stall speed, air density, prop efficiency..etc..
 http://www.iaw.com/~general6/flight_performance.htm

I've only got one plane (The f4u-1) as a template right now, but I will put others up for comparison soon!

[Widewing]

 

Originally posted by wells:
You can get a spreadsheet I made here.  It will calculate thrust, induced drag, profile drag, climb rate, glide ratio, sustained turning speed, stall speed, air density, prop efficiency..etc..
 http://www.iaw.com/~general6/flight_performance.htm

I noticed that your link does not work. At least I can't access it via our MCI T1 line.

My regards,

Widewing

[Jimdandy]

 

Originally posted by funked:
"Is it because the thrust to weight is a bigger factor at low speeds (drag increasing with v^2) where the drag is less of a factor? "

Bingo!!!

Well damn it!   We are back to what I was harping about in fscott's post. At the speed he was talking about thrust to weight is the factor to look at. I admit that hp/wt may be a very good estimate at those speeds but maybe the Japanese had a nice prop to go with a great engine and the thrust figures my be more telling than it might first appear. Could this be a reason for some of the higher hp estimates out there? I also remember something about the Japanese paying attention to thrust augmentation from the exhaust. The wing may have been lacking the high speed performance but the prop, engine, wing combo made for a wicked low to medium alt fighter.

[wells]

 

I noticed that your link does not work. At least I can't access it via our
                MCI T1 line.

Hmm, anyone else have a problem?  I've been having problems accessing certain websites myself, but maybe it's on my end?  I can try e-mailing it if you want?  Let me know!

[Jimdandy]

I was able to get in wells. I've been having trouble with some web sites and links too. Several of the posts have had pictures that only show as the little red x the last couple weeks. I don't know if it's me or them or the web.

[This message has been edited by Jimdandy (edited 02-23-2001).]

[Widewing]

 

Originally posted by wells:
Hmm, anyone else have a problem?  I've been having problems accessing certain websites myself, but maybe it's on my end?  I can try e-mailing it if you want?  Let me know!

Still no luck from the office. I will try again later from home. Fridays are a half day for the Engineering Department. Dress down day too. I spent my morning designing a roll-over sensor for a jet ski. Get it upsidedown and it kills the ignition. Naturally, they won't let me do the field testing..

My regards,

widewing

[Jimdandy]

 

Originally posted by Widewing:
...Naturally, they won't let me do the field testing..  

My regards,

widewing

I live on the beautiful Columbia River. Jet Ski barge wake heaven. Bring it up here and I'll test it.