Glide performance of AH fighters

[Zigrat]

i dont understand how a p51 with a windmilling propeller has a L/D of 14.5:1. At a weight of 9000 lbs that is saying that it only has 620 lbs of drag at 175 mph. Even if you use the cd0 calculated by using top speed where ther  p51 gets a big benefit because of thrust from the radiator this means that the windmilling propeller is adding less than fifty pounds of drag. Kinda hard to swallow.


As a reference a schweitzer sailplane gets a L/D of 22 and it has a massive aspect ratio, is obtaining that L/D at like 60 mph, and doesnt have to worry about propellers.

I see what the chart says buts its just kind of unbelievable.

[MANDOBLE]

Originally posted by AKSWulfe
I never called you a whiner.

As far as I remember, agree with that. In any case, hope my post have not been the trigger for you to ask for some evidences here.

[AKSWulfe]

I won't ask for any evidence, but if they want to get something changed- they will have to provide some evidence.

HTC just doesn't look at things because "they might be wrong", they need to know it's wrong so they can go look for it. Otherwise they waste valuable devel time on trying to find a bug that may or may not exist.
-SW

[CJ]

Within reason, L:D has almost nothing to do with weight or wing loading.  The best glide airspeed simply increases as you add weight internally (not counting external ordinance).   Because of the increase of airspeed, and since the aircraft is traveling along the same slope, the vertical speed does increase.  You could make a glider that would be absolutely useless for soaring, but that got a 50:1 L:D simply because it's best glide speed was 300 knots.  If a draggy old Cessna 172 can get a 9:1 glide ratio, why then couldn't a clean, retractable, with a prop that can go to very low pitch, and laminar flow wings do somewhat better?  Pyro posted the chart right from the manufacturer, and that's not good enough?

[CJ]

That was supposed to be L (Colon) D, but I guess I should have written it L/D since this program knows what I want to say better than I do myself.  Gotta love "artificial" intelligence...

[Badboy]

Originally posted by beet1e
I read this board from time to time, and notice posts concerning flight performance of various planes such as rudder drag, but this is piffling stuff compared to some glaring misrepresentations of various flight characteristics.

Am I alone in shaking my head in disbelief at the glide performance of the fighters? I was recently piloting a Spitfire 9, and attacked a B26. I escaped under the belly of the B26, but my radiator was pinged out and my engine quit shortly afterwards. (Stupid for water cooling to be used in fighter aircraft!) I have no idea about the best glide speed for the Spitfire 9, so I trimmed out at about  120-130mph. Well, just as in Warbirds, the glide performance of fighters is ridiculously overmodelled!  I was able to glide all the way back to home base, which took about 10-15 minutes!  I don’t recall the altitude at the start of that glide, but it wasn’t huge. Assuming still wind conditions, that’s at least 20 miles!!!  I don’t have any figures for the wing loading factor of a Spitfire 9, but I do know from my RL flying days that no way could such a glide be possible in real life. In a 22 metre open class sailplane, maybe. But in a high performance plane with high wing loading and low aspect wings? No way.

Actually, what you have described doesn't sound wrong at all. If you were flying at 120mph for about 10 minutes and managed to glide 20 miles, that fits perfectly with what one might expect. For example, if your starting altitude had been 13200ft you glide ratio would have been only 8:1 and at 120mph the descent would have taken you just over 10 minutes. Now that isn't a particularly good glide ratio and is less than you would expect from a clean airframe, and could be due to prop drag. The real Spitfire would have been able to glide that far from under 8000ft, and that also falls within the 120-130mph and 10-15 minute parameters you quoted. So what you have described, actually looks ok to me.

Badboy

[Badboy]

Originally posted by funkedup
I've got it in my head that P-51 had a glide ratio of 10:1.  I can't remember where I read it though.  Anybody have any sources?

PS Aircraft and Vehicles forum is a better place to post this question.  There are some pretty smart guys who frequent that forum but never read this one.

Yep, Francis Dean quotes a value of 14.92

Badboy

[Badboy]

Originally posted by CJ
Heres what I came up with for the Spit 9.  I used 50% fuel, and set my airspeed to 170 on auto.. shut the engine off, reduced the prop RPM fully, and waited for it to stabilize.  TAS was a little higher than 170, and the speed worked out to 249 ft/sec.  The vertical speed was around -1700 fpm, which worked out to 28.333 ft/sec.  L/D, which is the same as glide ratio = 249/28 = 8.89:1.  

This seems pretty reasonable to me.  

At 140 MPH I got closer to 10:1.  

Turns out that the maximum L/D for the Aces High Spitfire occurs at 140mph so that 10:1 ratio is the best you will get under those conditions. In order to improve that to the figure you would expect the airframe to be capable of, you need to fully feather the prop, which isn't possible, but it may be that a fully feathered situation is being modelled when the engine is damaged and the prop is unable to rotate.  

Badboy

[streakeagle]

Originally posted by Zigrat
i dont understand how a p51 with a windmilling propeller has a L/D of 14.5:1. At a weight of 9000 lbs that is saying that it only has 620 lbs of drag at 175 mph. Even if you use the cd0 calculated by using top speed where ther  p51 gets a big benefit because of thrust from the radiator this means that the windmilling propeller is adding less than fifty pounds of drag. Kinda hard to swallow.


As a reference a schweitzer sailplane gets a L/D of 22 and it has a massive aspect ratio, is obtaining that L/D at like 60 mph, and doesnt have to worry about propellers.

I see what the chart says buts its just kind of unbelievable.

Read it and weep. The P-51 may have had too high a wing loading to turn with a Spitfire, but aerodynamically, it is one of the most efficient aircraft of the war. It has one of the lowest drag coefficients, partly due to clean lines, but largely due to its wing cross section's advanced laminar flow shape. The fact is that even though it weighed 10,000 lbs loaded compared to the Spitfire's 7,0000 lbs, it was still faster despite having basically the same engine. Since power was essentially the same, that performance could only come from much lower drag.

One other thing to consider is how much of the P-51's weight came from fuel. The glide chart doesn't specify the fuel load. Perhaps it is assuming all fuel has been dumped? Or perhaps the usual 1/2 full = combat weight?

The P-51 might not have been the best all-round fighter, but neither the Spitfire nor the Bf109G10 could do what it did: fly from Britain to Germany and back at incredible speeds with fuel to spare. The United States has not ever really built lightweight point-defense aircraft unless you count the F-16. The best defense is not a short-ranged, tight-turning dogfighter; nor is it a short-ranged, rapid climbing interceptor.

"The best defense is a good offense"- Mel, the cook on the TV show Alice

I am proud to say, the F-15 represents this same strategy. It will never turn with the MiG-29 or Su-27, but it doesn't need to.

Back to the topic of glide slope: The F-16 has a pretty decent glide slope for its wing-loading/aspect ratio. I can't remember the numbers, but I think it is better than 8 : 1 at 170 kts. Back in the 1980s when F-16s used to be based here in Tampa at MacDill AFB, every time one would make a dead stick landing it would make the local news including data on its glide performance.

[Badboy]

Originally posted by Zigrat
i dont understand how a p51 with a windmilling propeller has a L/D of 14.5:1. At a weight of 9000 lbs that is saying that it only has 620 lbs of drag at 175 mph. Even if you use the cd0 calculated by using top speed where ther  p51 gets a big benefit because of thrust from the radiator this means that the windmilling propeller is adding less than fifty pounds of drag. Kinda hard to swallow.


As a reference a schweitzer sailplane gets a L/D of 22 and it has a massive aspect ratio, is obtaining that L/D at like 60 mph, and doesnt have to worry about propellers.

I see what the chart says buts its just kind of unbelievable.

The L/D ratio of 14.5 does not include the prop. In Aces High the P-51 has a L/D closer to 14 at 174mph with 50% fuel, but with the prop windmilling it can't do that.

Badboy

[beet1e]

Thanks for all the feedback, guys. I've done some of my own tests, and I think I piped up too soon. I think CJ hit the nail on the head - I was probably at 300mph when I got hit, and gently slowed down to about 130mph. That would have been a good glide.

The planes still FEEL that they're gliding ever so well. Thanks, Pyro, for that P51 glide data.

In my RL flying days, would you believe I once flew and owned a share in a German 109? Yep, that's absolutely true. It was a Grob 109a motor glider  I think its wingspan was 15m (54 feet) and THAT thing really could glide for miles. hehe - I used to restart the engine by "bumpstarting" it in the air - unfeather the prop and then accelerate to 95mph (remember to turn on ignition ). The absolute hardest thing for me when converting to Power planes (Cherokee 140) was getting used to the fact that they wont glide very well. I remember during one lesson, my instructor cut the engine and said "pick your field". And I said "oooh, let's see now - how about that one over there?" (about 5 miles away). And he yelled "we'd never get there! it's a Cherokee, not a glider. Your choice of fields is that one, that one or this one" - pointing to various fields almost directly below. And sure enough, we really were coming down like a brick. The later Cherokee Warriors had a bigger wingspan, and an improved wing design. Those old slab wings were OK when fuel was cheap...

While we're on the subject, I attach a pic of my first EVER field capture in Warbirds - in the days when you simply had to land at a closed field. I had been shot and my engine was dead. I was flying a ki84. Rather than bail, I thought I would glide to safety. I must have covered 20 miles! And I saw a field but it was an enemy field. I didn't know it was closed so I landed anyway - got the capture! See how many names you recognise.

[beet1e]

doh! - screwed up prev. post - here is pic

[Badboy]

Originally posted by streakeagle


Back to the topic of glide slope: The F-16 has a pretty decent glide slope for its wing-loading/aspect ratio. I can't remember the numbers, but I think it is better than 8 : 1 at 170 kts. Back in the 1980s when F-16s used to be based here in Tampa at MacDill AFB, every time one would make a dead stick landing it would make the local news including data on its glide performance.

Glide slope for the F-16 (according to my analysis) is about 13.5:1

Badboy

[streakeagle]

Pilots wish F-16 had a 13 : 1.

Here is a link to a PDF of an Air Force manual describing flameout procedures (how to glide in deadstick ).

http://www.fas.org/man/dod-101/sys/ac/docs/16v5.pdf

To make a long story short:

When the engine quits, jettison stores and turn toward the nearest suitable runway. Establish best range speed of 210 KCAS (plus fuel/stores). Trade excess airspeed for altitude.... The gear may be extended when the aimpoint is between 11º and 17º and landing is assured.
If EPU fuel depletion is a factor because of range to the runway, consider a 10º gear-up glide when the best range glide has given you a 1:1 ratio between altitude in thousands of feet and range to the runway (i.e., 20,000' AGL at 20 NM).

1 NM (6076.1 feet) : 1000 feet ===> 6:1 which is close to 10 degrees.

The 11 degree angle is for gear down.
The 17 degree angle is for gear down with airbrake deployed.

[Badboy]

Originally posted by streakeagle
Pilots wish F-16 had a 13 : 1.

The 11 degree angle is for gear down.
The 17 degree angle is for gear down with airbrake deployed.

Yep, but I think those glide angles have been arrived at with the intention of generating the speeds for the correct landing approach, because a little later, it mentions "In a nutshell, if you flame out, regardless of altitude or distance out and the recovery field is below the 7º pitch line, you immediately know you can get there." and 7 degrees is a little better than 8:1 and that would probably include an allowance for the steeper approach and a possible turn required in the landing pattern, all with the gear down and not less than a 5% margin above the stall. Not forgetting that's 7 degrees with the additional drag caused by the need to drive the generator with airflow through the engine, a drag situation similar to the windmilling prop in Aces High. Also, in order to make a statement such as "you immediately know you can get there" that 7 degree glide slope would have to have a comfortable margin of safety. Taking all that into account, and accepting that perhaps the 4.3 degree glide slope suggested by a 13:1 ratio might be a tad optimistic, I still don't think it is far off. All of which is a million miles away from Aces High

Badboy