Comparison in Diving Performance

[wuzak]

That is my understanding, I believe that in a theoretical situation with an unlimited atmosphere as we have at sea level and a force of 1G a tallboy would never break the sound barrier. Take one up to 100 miles high in the real world and it will on the way down then hit the earth before it has slowed down to its terminal velocity in air again. Unfortunately no Lancasters flew to 100 miles high and since the tallboy and grand slam were dumb bombs even dropping from 40,000 ft would mean you just didn't get any close enough.

The Tallboy was designed to be dropped from 35-40,000ft, not the 18-22,000ft that the Lancasters were capable of achieving.

Wiki says: "Tallboy was designed to be dropped from an optimal altitude of 18,000 ft (5,500 m) at a forward speed of 170 mph (270 km/h), hitting at 750 mph (1,210 km/h) "
Tallboy (bomb) - Wikipedia

750mph is only slightly lower than the speed of sound at sea level.

The altitude that it says was "optimal" I don't believe to be correct. Barnes Wallis even proposed the Victory Bomber to carry the bomb (actually the 10t "earthquake" bomb) to 40,000ft.

It would not surprise me that the Tallboy did not reach its terminal velocity from 18-22,000ft. It would surprise me that it would not from 40,000ft.

The terminal velocity is when the force due to gravity (= mass * gravitational acceleration) is equal to the drag force (= 1/2 density * drag coefficient * area * velocity squared). The acceleration in the upper atmosphere is irrelevant, unless the speed is faster than the terminal velocity in the lower atmosphere, in which case the bomb would slow as it fell. I believe that is why the V-2 could hit the ground at M2 - it started its descent at a much higher speed.

 

[BiffF15]

The USAF man that jumped from a balloon at 100,000+ feet in the early 60's exceeded the speed of sound.
He even had a small stabilizing parachute trailing him to prevent tumbling.
Can't remember his top speed , but somewhere around 600 mph, corrected, I think.

Red Bull Stratos: Meet Joe Kittinger

Met him in college, mid 80s. Seemed like a nice guy.

 

[XBe02Drvr]

The terminal velocity is when the force due to gravity (= mass * gravitational acceleration) is equal to the drag force (= 1/2 density * drag coefficient * area * velocity squared).

As I understand it, this drag formula works right up until you go transonic and start to push a compression wave, then your effective drag starts to increase more rapidly. Considering that 18,000 feet is roughly the half atmosphere level in terms of density, and the atmosphere is exponential, not linear, 40,000 ft should be somewhere south of one quarter the density at sea level. In that thin air, where the speed of sound is lower, as is aerodynamic drag, any of these monster bombs with their wind cheating shapes should have no problem getting well past Mach before they get into air dense enough to seriously impede them. I'm not much of a mathematician, so if any of you engineering types out there can explain in plain English why this would not be so, I'd like to hear it.
Cheers,
Wes

 

[pbehn]

As I understand it, this drag formula works right up until you go transonic and start to push a compression wave, then your effective drag starts to increase more rapidly. Considering that 18,000 feet is roughly the half atmosphere level in terms of density, and the atmosphere is exponential, not linear, 40,000 ft should be somewhere south of one quarter the density at sea level. In that thin air, where the speed of sound is lower, as is aerodynamic drag, any of these monster bombs with their wind cheating shapes should have no problem getting well past Mach before they get into air dense enough to seriously impede them. I'm not much of a mathematician, so if any of you engineering types out there can explain in plain English why this would not be so, I'd like to hear it.
Cheers,
Wes

Exactly as I understand it. If heard some scientists discussing exploration of other planets, with a very dense atmosphere entry is similar to earths but much father away from the surface. The nearer you get to the surface the more the probe stops falling like an object in air and starts drifting down like a waterlogged coconut in the sea.

 

[Zipper730]

People routinely discuss supersonic speeds, a passenger plane just crossed the Atlantic at "supersonic speed" it had a 200+ MPH tailwind. The V2 got to 100miles above the earth, easy to tell how fact it was going when it hit the ground, if we had no atmosphere, which is the point.

It's not really supersonic: Supersonic means it's moving faster than the speed of sound. I guess you could say "supersonic equivalent", but that is not the same.

Dive – In comparative dives there was nothing to choose between the two aircraft, save that the P-38 is limited by buffeting in high speed accelerated flight.

So up until compressibility is hit, they are so close as to make comparisons purely academic?

That's why I posted the calculations are above my pay grade. The atmosphere becomes thicker and thicker, then you hit the ground.

A continuous 1G fall with a starting speed of 225 mph at 18000 feet produces a time of fall of 24.73 seconds, and an impact speed of 1125.7 f/s which is Mach 1.008. I'm not sure to what extent this factors in air-resistance.

Back to my pet peeve, static ports! No way an aircraft designed for pre supersonic flight is going to give you reasonably accurate airspeed indications once you bump up against the transonic range.

Yeah, the airspeed gauge becomes completely erroneous.

As for Tallboys/Grand Slams/V2s and Mach busting; it seems to me that it's all a function of how much acceleration is achieved in the thin atmosphere before the drag rise of the lower atmosphere sets in.

Yeah, and there's also the issue of how much resistance the projectile produces, as there's a terminal velocity achieved. Humans don't have a very high terminal velocity, around 120-140 mph, but a projectile who's forward shape matches that of some artillery shells with the afterbody being highly streamlined might very well have a terminal velocity that would see it accelerating right up to impact (though maybe not terribly fast).

Based on an online calculator (so, take of that what you will as a disclaimer), fall time from 40000 feet with a speed of 300 mph, you'd see a fall time of 38.03 seconds, and an impact velocity of 1663.6 f/s or 1.49 Mach. That was the figures projected for the Victory bomber.

My experience in my one and only supersonic ride confirmed what I'd been told, that drag rises exponentially in the last 10% before Mach 1.

This was in an F-4 Phantom, correct?

That's when "Preacher", my pilot, said: "There we are, 1.15, welcome to the club! Notice the fuel flow and fuel remaining? We're flying AWAY from the fuel and about to penetrate Pensacola's practice area. Deselecting afterburner, now." WHUMPFF! We flew into a brick wall, and my head flew off the headrest and arms off the armrests.

Actually depending on engine and inlet efficiency, sometimes you'll see thrust and drag decouple from each other.

The F-104, the XF8U-3, and Concorde were all examples of this

• XF8U-3: Required burner to bust through Mach 1, but at around Mach 2, when the afterburner was disengaged, you'd be able to hold Mach 2 from what I was told.
• F-104: Supposedly the same, but I do remember, that the prototype could get through Mach 1 without using afterburner. It was also able to slip through Mach 1 at low altitude in shallow dives without afterburner. Consider the following...
  • The XF-104 used the J65 which was less powerful than the J79
  • The XF-104 also used simpler inlets with a pair of splitter plates: The YF/F-104 had the half-cones for inlets. The simpler inlet used on the XF-104 might have been better subsonic and transsonic. From what I was told, after reaching 1.4 to 1.6, inlet efficiency was increased. Pilots described it as "clutching" into gear.
  • The J79 had some strange characteristic where RPM would have to be run up somewhat to cope with airflow changes at certain mach numbers (1.8?)
• Concorde: Afterburners were needed to get through Mach 1 to Mach 1.7...

The USAF man that jumped from a balloon at 100,000+ feet in the early 60's exceeded the speed of sound.
He even had a small stabilizing parachute trailing him to prevent tumbling.

I had actually wanted to mention that earlier, but I guess there's no need

 

[wuzak]

That is true, drag rise increases rapidly when the object enters the transonic range.

Without drag, a bomb would surpass the sea-level speed of sound in about 20,000ft.

 

[pbehn]

A continuous 1G fall with a starting speed of 225 mph at 18000 feet produces a time of fall of 24.73 seconds, and an impact speed of 1125.7 f/s which is Mach 1.008. I'm not sure to what extent this factors in air-resistance.

The start speed is zero, the forward speed of the aircraft has no effect on the fall of the bomb. As discussed the various types of air resistance increase with the speed of the bomb and start to increase exponentially as mach 1 is approached.

 

[DarrenW]

So up until compressibility is hit, they are so close as to make comparisons purely academic?

There was an article called Flight Test Comparison...Ending the Argument, found in the June 1990 edition of the EAA's Sport Aviation magazine, and it seems that how an airplane behaves in a dive may be just as crucial as it's maximum safe dive speed and overall acceleration. Less pilot input (trimming) allows for a straighter trajectory and should improve acceleration during the dive. The rankings during these "modern day" dive tests put the P-47D in first place and the FG-1D (same as F4U-1D) in last :

 

[eagledad]

eagledad said:
Dive – In comparative dives there was nothing to choose between the two aircraft, save that the P-38 is limited by buffeting in high speed accelerated flight.
So up until compressibility is hit, they are so close as to make comparisons purely academic?

Zipper730

One could conclude that between the P47C and P38F

Eagledad

 

[K5083]

The normal pitot tube and ASI system in a WW2 fighter were completely unable to work at high mach. Only aircraft with the kind of instruments used by a flight testing establishment, Farnborough, Wright Field, Rechlin etc ,would be any good at all.

63rd Fighter Group as mentioned above is actually 63rd fighter squadron, part of the 56th FG, the pioneer Tbolt group.

 

[Dawncaster]

There was an article called Flight Test Comparison...Ending the Argument, found in the June 1990 edition of the EAA's Sport Aviation magazine, and it seems that how an airplane behaves in a dive may be just as crucial as it's maximum safe dive speed and overall acceleration. Less pilot input (trimming) allows for a straighter trajectory and should improve acceleration during the dive. The rankings during these "modern day" dive tests put the P-47D in first place and the FG-1D (same as F4U-1D) in last :

View attachment 574976

It seems to have missed an dive acceleration part.

For the dive acceleration, the Thunderbolt outruns the Mustang even though it started diving at slower speed, and the Corsair was faster than the Hellcat, even though started at the same speed and Hellcat finished the test method more quicker.

 

[tommayer]

I'm curious what planes had the highest dive performance. In particular the P-47, P-51, Fw.190, the J2M, Ki-61, and Ki-84.

Suggest you go to Greg's airplanes and automobiles. Watch his long vid on P-47 dive performance. I think it might answer most of your questions. VERY short & VERY incomplete summary: No single bird was the best diver at all altitudes. 47 may have been best over all. But, for example, Spit was best for a few thousand feet in the mid 20 FLs. I find Greg really useful. He's a very experienced pilot himself, I suspect he's an aeronautical engineer, he's a serious warbird sim player. He tells you what his references are. He seems to me a first rate academic researcher, in addition to his real world and sim world expertise. Maybe 50% of the tech stuff that gets debated ad infinitum on this site, Greg has covered definitively in one of his vids. This dive question is a prime example. PS. I have never met Greg, do not know his last name, and emphatically am not being paid to advertise his site.

 

[XBe02Drvr]

The start speed is zero, the forward speed of the aircraft has no effect on the fall of the bomb. As discussed the various types of air resistance increase with the speed of the bomb and start to increase exponentially as mach 1 is approached.

Actually, the start speed is zero only in the vertical dimension but the aerodynamic drag speed is 225 mph, although initially working only in the horizontal direction. As the bomb gradually transitions to a more vertical flight path, that elevated aerodynamic drag becomes more of an impediment to vertical acceleration. It seems intuitively to me that this would generate a different acceleration profile than a free fall drop from a stationary platform at that altitude, as the aerodynamic drag starts out higher. The calculus behind this is above my paygrade, so could some of you math wizards jump in here?
Cheers,
Wes

 

[pbehn]

Actually, the start speed is zero only in the vertical dimension but the aerodynamic drag speed is 225 mph, although initially working only in the horizontal direction. As the bomb gradually transitions to a more vertical flight path, that elevated aerodynamic drag becomes more of an impediment to vertical acceleration. It seems intuitively to me that this would generate a different acceleration profile than a free fall drop from a stationary platform at that altitude, as the aerodynamic drag starts out higher. The calculus behind this is above my paygrade, so could some of you math wizards jump in here?
Cheers,
Wes

I think you need a ballistics maths wizard lol

 

[XBe02Drvr]

This was in an F-4 Phantom, correct?

Actually depending on engine and inlet efficiency, sometimes you'll see thrust and drag decouple from each other.

The F-104, the XF8U-3, and Concorde were all examples of this

This was in an F4J, which had a very sophisticated system of Variable Geometry Intakes and Variable Geometry Exhaust Nozzles, and according to the pilots who flew it, didn't suffer from those effects. While its prototype was a contemporary of the XF8U3, and a generation newer (in design terms) than the F104, it was by this time the beneficiary of a decade and a half of tweaking.
In the F4, speeds above Mach 1.2, while achievable, were of little practical value, as a plane in mission configuration would pay a prohibitive fuel penalty for doing so, and the same limitation applied to all of its potential opponents. Despite its Mach 2+ capability and it's polite supersonic manners, the F4 (USN/USMC, anyway) operated almost exclusively subsonic, as did its contemporary opponents. It was a draggy airframe, a high thrust brick.
Cheers,
Wes

 

[Zipper730]

This was in an F4J, which had a very sophisticated system of Variable Geometry Intakes and Variable Geometry Exhaust Nozzles, and according to the pilots who flew it, didn't suffer from those effects.

I don't mean thrust increased out of proportionality: I meant thrust did...

While its prototype was a contemporary of the XF8U3, and a generation newer (in design terms) than the F104, it was by this time the beneficiary of a decade and a half of tweaking.

The XF8U-3 was a better fighter in almost every respect, except the twin-man crew (which was a big if). It could also stay on station longer and fly further when supersonic (the interception profile could entail an outbound dash of 750 nm followed by either intercept, or loiter for 15 minutes, then cruise back).

In the F4, speeds above Mach 1.2, while achievable, were of little practical value

In typical CAP missions? Sure.

 

[Zipper730]

Actually, the start speed is zero only in the vertical dimension but the aerodynamic drag speed is 225 mph, although initially working only in the horizontal direction.

That's a good point. That said, the bomb transitions from horizontal to vertical. While drag increases as speed increases, the force of gravity is operating in the same direction of the projectile, and would help maximize vertical acceleration. I don't know the math behind that.

S Shortround6 are you good at predicting ballistics? You know a lot about guns, so I figured you've fired quite a lot of 'em.

 

[XBe02Drvr]

The XF8U-3 was a better fighter in almost every respect, except the twin-man crew (which was a big if

AND it's pointy little nose didn't have room for the big dish high powered radar required for its design mission, which was interdicting high speed bombers beyond the range of their standoff missiles, and beyond the radar horizon of the fleet. A Bear or Badger size target could be detected at 300+ NM and locked up by 250. Remember, the design work was done in the age of electron tubes and analog computers, and the sky was not full of spy satellites keeping tabs on everything. And at the time, the Phantom was not perceived as a bomb truck, so was envisioned as operating in a much cleaner configuration than happened in the real world.
If you look at flight deck photos from the early 60s (the "pure fighter" days), they tend to be pretty clean, with two or four Sparrows embedded in the fuselage, sometimes a single small drop tank, and no pylons, no bomb racks and no Sidewinders. By the mid 70s, they were bomb trucks with pylons, MERs, TERS, Sidewinders, and either a large centerline tank or two smaller pylon mounted tanks. Now dig out your F4J NATOPS and look up the drag indexes of all these items and plot its performance on the zillion graphs in the performance section. Have fun!
Cheers,
Wes

 

[XBe02Drvr]

In typical CAP missions, sure.

By the time I was in, CAP missions of the traditional type were mostly a thing of the past, aside from a brief BARCAP between Yankee Station and Indian Country during launch and recovery operations. In the combat world of the 70s the prevalence of AWACS coverage could generally detect threats in time to launch the alert birds to deal with them. A good thing, as their endurance wasn't optimum with all that stuff hanging from their wings.
Cheers,
Wes

 

[Zipper730]

AND it's pointy little nose didn't have room for the big dish high powered radar required

That's not entirely true, they were able to move the radar further back in the nose. Some electronic boxes had to be repositioned, and that did displace some fuel, but it was within an acceptable amount.

The range and endurance of the XF8U-3 did come up shorter than anticipated, but so did the XF4H-1/F-4B, so an F-4B with drop-tanks was about the same as the XF8U-3 on internal fuel still.

A Bear or Badger size target could be detected at 300+ NM and locked up by 250.

With the APG-72?

the Phantom was not perceived as a bomb truck

That's actually right, as originally intended, it kind of was a series of fighter designs called the Super Demon that had a secondary A/G capability. There was some version that had a modular forward fuselage, so one or two crew members could be used, depending on role. The modular cockpit idea was rejected, and the Super Demon was ultimately rejected in favor of the F8U-1.

Evidently, specifications came in for an all-weather interceptor at some point and, either because of this, or simply as an ends-justify-the-means way of some officers to fund a design they liked, they stuffed the plane into the attack-budget, as the fighter budget ran out. With the designation AH-1, it was fitted with more pylons (I'm not sure if any modifications to armor or add fire-suppression equipment were made, but the F4H didn't seem to handle air-to-ground missions as good as some aircraft), but wasn't well received compared to the A4D. When money went back into the fighter-budget, the interceptor spec was applied to the design, and that included 4 x AAM-N-6 (AIM-7C), and 4 x AAM-N-7 (AIM-9B) were added. The guns were removed out of a misguided belief that they were unneeded (I've heard there were volume limits).

Ultimately, you ended up with an interceptor that retained some vestiges of the momentary attack designation.