Author Topic: Turn rate and radius questions (Read 3904 times)

dtango · « **Reply #75 on:** May 19, 2003, 08:42:19 PM »

If the airspeeds are IAS then V-n diagrams for the different alts would overlap each other on accelerated stall limits. 3g stall speed is the same IAS at alt x as it is at alt y.

Tango, XO
412th FS Braunco Mustangs

F4UDOA · « **Reply #76 on:** May 19, 2003, 10:32:17 PM »

Dtango,

Honestly, Have you ever seen any accelerations chart that looks any different? I'm not sure what you are saying but if you could show me one chart that looks like what you describe maybe that would help.

I have the F8F, F7F, P-51 and P-38 charts. They are all in the same format and all say IAS.

Do you think the chart from the pilots manual is a better representation of the F4U or AH?

The difference in Clmax is probably due to prop wash since the 1.49 number come from the NACA report with a windmilling prop at 60MPH and acceleration test are done at power with at 164MPH.

BTW, Even with that I would be glad to have the F4U acclerated stall match the NACA 1.49 as long as every other A/C in AH did also.

F4UDOA · « **Reply #77 on:** May 19, 2003, 10:49:03 PM »

Urchin,

You may be right about the 190A5. I try not to lobby HTC to "pork" AC. I would rather work on getting the undermodeled ones fixed. I happen to have a ton of data on Navy birds so it makes it easier to concentrate on the F4U, F6F etc.

I would like to see a little more feedback from HTC. I am hoping for some real changes in AH2.

Funny thing when you point out flaws in here people attack you like your insulting there religion.

These types of conversations about warbirds are much easier to have on WW2 boards because you are not insulting the ego's of engineers.

Zigrat · « **Reply #78 on:** May 19, 2003, 11:00:22 PM »

yeah dtango i was wondering the same thing.. i would expect VERY MINOR differences due to differing reynolds number but not that severe.

F4UDOA · « **Reply #79 on:** May 19, 2003, 11:15:23 PM »

Heya Zigrat,

I still have no idea what your talking about.

I took the F6F-5 stall chart and superimposed the F4U stall chart onto it with the circles at the corresponding weights.

Try to take a F4U or F6F up and match the stall speeds at those weights power off clean or power on flaps and gear. You can't.

dtango · « **Reply #80 on:** May 19, 2003, 11:19:31 PM »

The P-51D V-n diagram you posted is a perfect example. There's only 1 V-n curve using IAS. 3g stall speed at alt X = 3g stall speed at alt Y when you're talking IAS.

The F4U-1D charts you posted have different V-n diagrams for different alts. If it was IAS it would just be 1 curve not many so something is strange about it.

Regarding the CLmax issues..remember your own quote:

Quote

Also I asked the same question about prop slip stream to Zigrat. According to him (In my understanding anyway) it doesn't make nearly as much differance in accelerated conditions as it does in 1G flight because the AOA reduces the slipstream effect.

The CLmax figures are based on 163mph 3g stall which would negate or reduce the impact of prop induced velocity.

Finally if you we used the Clmax of 1.48 then at 11,000 lbs:
1g stall speed = 96 mph
3g stall speed = 166 mph

Clmax of 1.49 then at 11,000 lbs:
1g stall speed = 95.5 mph
3g stall speed = 165.5 mph

That's hardly a dramatic difference between the 3g 170mph , 1g 97mph figures.

Keep in mind CG also plays a part in changing CLmax so the difference could be attributed just to that alone.

Tango, XO
412th FS Braunco Mustangs

Regurge · « **Reply #81 on:** May 20, 2003, 02:43:54 AM »

I'm not fully understanding the F4U diagram.

On the 10,000ft and below the left boundary is from accelerated stall, top is structural limit, and right is dive speed limit. All that makes sense to me, but what I don't get is what causes the curved upper right hand boundary.

I've read that in some planes pulling G's at at speeds close to critical mach caused buffeting. But would that be happening below 10,000ft?

The P-51 diagram looks like what we see in AH, where you can pull enough G's to blackout at speeds right up to critical mach.

Another thing about that P-51 diagram. I understand that it should be applicable for all alts wrt stall limit, but I doubt the dive limit was 505IAS at all alts. IIRC the P-47 dive limit was 500IAS below 10,000ft, subtracting 50mph for every 5,000ft above that.

Guppy · « **Reply #82 on:** May 20, 2003, 04:18:52 AM »

Quote

Originally posted by Regurge
Another thing about that P-51 diagram. I understand that it should be applicable for all alts wrt stall limit, but I doubt the dive limit was 505IAS at all alts.

You're right about the dive limits. The chart I have shows the following:

505 @ 5,000 ft.
480 @ 10,000 ft.
400 @ 20,000 ft.
330 @ 30,000 ft. (I think - the second digit is a little fuzzy.)
260 @ 40,000 ft.

dtango · « **Reply #83 on:** May 20, 2003, 08:47:20 AM »

Yep, you guys are correct regarding the max speed limits. My statement of "having 1 V-n curve" is not accurate. Stall limits would be the same at IAS for various alts with different max speed limit bound on the right side of the curves.

Tango, XO
412th FS Braunco Mustangs

F4UDOA · « **Reply #84 on:** May 20, 2003, 01:30:59 PM »

Dtango,

Quote

The CLmax figures are based on 163mph 3g stall which would negate or reduce the impact of prop induced velocity.

Finally if you we used the Clmax of 1.48 then at 11,000 lbs:
1g stall speed = 96 mph
3g stall speed = 166 mph

Clmax of 1.49 then at 11,000 lbs:
1g stall speed = 95.5 mph
3g stall speed = 165.5 mph

That's hardly a dramatic difference between the 3g 170mph , 1g 97mph figures.

I agree, it's not a huge difference if you compare the 11,000LBS AH results with the 12,000LBS chart. The difference is the weight is lower by 1,000LBS to achieve the same acceration results. If you recalculate what the chart determines for 11,000lbs the differance is much greater. You would achieve a 3G stall at 150MPH so the differance is actually 20MPH not 5MPH.

And to quote myself

Quote

Also I asked the same question about prop slip stream to Zigrat. According to him (In my understanding anyway) it doesn't make nearly as much differance in accelerated conditions as it does in 1G flight because the AOA reduces the slipstream effect.

I do believe that. But as I said, not nearly as much of an effect. I didn't say that it had no effect at all. Just look at how NACA did their Clmax test. At 60MPH prop installed and windmilling. Well in and accelerated stall there will be increased airflow over the wing. At what speed I have no idea. I'm guessing more than 60MPH less than 164MPH. But that difference can account for an reasonable increase in Clmax enough to bring the 3G stall from a calculated 173MPH down to 164MPH at 12,000LBS.

Again this is my opinion based on not only the chart I provided and my very limited understanding of how things work but on two other pieces of source documented flight test that show the F4U-1/1D/4 accelerated stall to be at least as low as what is represented on the chart.

One is a British document on 4G acclerated stalls and the other is from a modern evaluation of the F6F-5, FG-1D, P-51D and P-47D.

gripen · « **Reply #85 on:** May 20, 2003, 01:38:34 PM »

The shape of the flight envelope is caused by two things. Critical mach number decreases when g load increases and relative Clmax decreases (in most cases) when mach number increases. 1 g stall speed in IAS stays quite constant but at higher speeds also accelerated stall limits decrease for given IAS speeds. See NACA reports mentioned above.

I also compared AH 1.06 to 1.14: No noticeable difference in the stall speeds of the F4U-1D. At 10k and 200mph IAS it stalls 3,7 g on pull up test, testing is just a bit more difficult because accelometer appears to be more sensitive when internal view is used for films.

Stall speeds appear to be same in the turns and pull ups for given g load. In the real world airplanes reach higher Clmax on pull ups because plane yaws a bit in level turn. As an example see mentioned NACA reports.

gripen

frank3 · « **Reply #86 on:** May 20, 2003, 03:29:16 PM »

dang! Im getting dizzy here....

Zigrat · « **Reply #87 on:** May 21, 2003, 12:01:13 AM »

wow gripen i didnt look at that stuff till now very interesting articles!

the reduction in clmax with mach number at low (<.6) mach is very surprising to me, prandtl glaurt rule says lift curve slope will increase due to compressability and i guess i just assumed that alpha(stall) didnt change much so the OPPOSITE trend was true. i'm gonna read over these papers more carefuly tomorrow.

f4u for the reason why the prop wash isnt as important at high velocities i will try to sum it up briefly

the reason you get artificially higher clmax from prop is due to the fact that local flow velocity is higher than free stream. the section lift coefficient isnt really higher, but you are getting more lift since loval vinf is higher, and when you use the reference flight velocity for calculations it sppears that clmax is higher than reality.

this effect degrades with velocity for 2 reasons:
1) propeller thrust decreases with velocity. power = thrust * velocity so for constant power as velocity increases thrust decreases

since thrust is decreasing , the change in momentum imparted to the air by the propeller decreases (the delta v is smaller) so this leads to smaller increases in local velocity in the prop wash

also the % change as a fraction of freestream gets smaller as freestream increases, so you can see that these 2 effects combined mean that prop wash effects will rapidly decay with speed. they will, however, be quite strong in low speed configurations (ie lets say a missed approach by a f4u)

dtango · « **Reply #88 on:** May 21, 2003, 12:40:33 AM »

Yeah, I've been looking over those reports pointed out by Gripen. I was puzzled by them before. After reading 1044 a couple of times however I think I understand.

That report was a start of the analysis to indicate why Re effect on increasing Clmax is bound. So in other words it begans to try to explain why the following is true.

"We note that as Reynolds number increases, the maximum lift coefficient increases. But this does not occur indefinitely; when flows become very turbulent, the maximum lift coefficient begins to drop and so does the overall lift coefficient."

In other words prandlt-glauert compressibility correction stops at a certain point. [edit: actually thinking about it now I'm not sure if this is an accurate statement!]

1044 goes about this by treating mach number or reynolds number as independent variables. The gotcha is it varies one while holding the other constant.

Check out NACA Report 1299 done the following year:
Naca 1299

1299 has charts with Clmax plotted with Re and mach as dependent variables. Makes more sense to a simpleton like me!

1299 has real nice page on concluding remarks that spells things out nicely.

Tango, XO
412th FS Braunco Mustangs

gripen · « **Reply #89 on:** May 21, 2003, 03:38:10 AM »

IMHO a good article on compressebility corrected envelopes can be found from the "Aero Digest" magazine, September 1, 1945 ("Computation of Compressibility Effect on the Flight Envelope" by H. R. Foottit). Example calculations in the article are for the P-47. I believe most air museums and technical universities have Aero Digest volumes in their collections.

gripen