From which aircraft?
Dive limits
- Thread starter tomo pauk
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From which aircraft?
I have read the same, I think it was from an aircraft Barnes Wallace would have designed himself
, it was a theoretical or preferred height for maximum effectiveness of the bomb.The Grand Slam and Tallboy were "earthquake" bombs, the idea was to explode deep underground and create a "camouflet" or void which undermined the target, to do this obviously needs a sort of armoured bomb and a minimum velocity depending on the target. In the case of the attack on the V3 cannons in the Pas de Calais the perfect strike was to miss narrowly all the concrete covering and undermine the whole construction, as with an attack on a bridge, not only was the above ground structure destroyed but the foundations were too so a quick reconstruction wasn't possible.Ahhhh....thanks Pbehn.
That is a great graph Graeme but, with respect, I don't believe it, for a start it is too damned neat.
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Wow, I didn't know it was 0.96... yikes that's fast for a propeller driven plane (hell, I've never flown that fast and I've only been in a propeller plane once)!
Were there any aerodynamic differences to the Spitfire from WWII to this point?
Regarding the Grand-Slam bomb, that was a bomber designed by Barnes Wallace known as the "Victory Bomber", it was a six-engined aircraft of about the same length as a B-29, with beautiful elliptical wings of high aspect ratio, and a fuselage that kind of looked like a baleen wale: From above it was clearly gorgeous, from the side it was ugly as sinGraeme said:
It's performance nonetheless was quite remarkable: While carrying a 22,000 pound bomb, it's radius of action was well in excess of 1,500 miles with a top speed of around 340-350 miles an hour. The massive surplus range would allow it to easily strike any target in Germany and, as a result, it would climb partially to altitude while in UK airspace; then it would continue to Germany: Bomb release would occur at between 40,000 to 45,000 feet and, being unburdened of 22,000 pounds of weight, it would continue climbing to altitudes of 50,000 feet or greater on the way back. It would start descending once out of enemy air space and land somewhere in the UK.
It was supposedly inspired by the high altitude Wellington proposals, and had some of the same problems. Due to the geodetic construction, they could not incorporate a pressure hull like the USAAF's B-29's. So they basically had pressure hull inserted into the fuselage with the forward fuselage wrapped around it (hence it's shape).
The pressure hull had a pair of bubble-canopies up top for the pilot and navigator, and a window aimed downward which was for the bomb-aimer (known everywhere else as a bombardier). From what I remember, there were problems on the high-altitude Wellington variants with frost forming on the windshield (this seemed to occur on the later Canberra as well). Since it's altitude was very high and, with a top speed approaching some fighters, it was felt to only really be vulnerable from behind: As a result, it only had a defensive turret in the tail. It was a quad 0.303 similar to that used on the Vickers Wellington but due to the pressure hull, it was remotely controlled. The system was sighted through a periscope mounted in a transparent enclosure in the tail.
Wallis wanted to get the design into the air as quickly as possible, so he pushed to design it around a single bomb: This didn't go over well with the Air Ministry, so he eventually agreed to propose other armaments (some went up to 29,000 or 32,000 pounds), but it was too late.
I believe the Spitfire had a higher theoretical mach number but also in to post war years the P51 Mustangs under lend lease had been returned written off or scrapped.
I guess I don't know what determines a plane's mach number, but wasn't the Mustang slicker (less drag) than the Spit?
Aerodynamics isn't my major but I have read here that although the Mustang had less drag at most speeds at the absolute limit the thinner wings of the Spitfire held an advantage. However at these speeds propellers start falling off and controls stop controlling.
DarrenW
Staff Sergeant
Can someone explain how the P-38 can have the highest acceleration during a dive but end up near the bottom of the list when comparing maximum dive speeds? I assume it's about the effects of compressibility but I'm unsure. If I'm correctly understanding what Dean is explaining in his book, at the start of a power dive the P-38 would initially leave the P-47 behind, but eventually the Thunderbolt would catch up and overtake it because it has a higher max dive speed and wouldn't have to throttle back as early as the Lightning to avoid structural damage. Is this it in a nutshell?
Also, why would he use data for a P-38G in one category and switch to a P-38 J/L model in another? Didn't the latter have dive flaps so it could have better control in a dive situation? I'm assuming the P-38G's dive limit is somewhat lower than the J/L model because it lacked these same flaps.
. Does this seem about right?
Also, why would he use data for a P-38G in one category and switch to a P-38 J/L model in another? Didn't the latter have dive flaps so it could have better control in a dive situation? I'm assuming the P-38G's dive limit is somewhat lower than the J/L model because it lacked these same flaps.
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Airfoil used in P-38 and how the junction between fuselage and wings was made were IIRC the main reasons of the low critical Mach number of P-38. The function of dive flaps was to enable the recovery from high speed dive IMHO they didin't rise the critical Mach number, simply allowed getaway from possibly fatal situation.
DarrenW
Staff Sergeant
Thanks Juha2 for your reply. I was wondering about the dive limit of the P-38. I have seen 420 mph IAS for 10,000ft and below but apparently this can be increased to 440 mph indicated with dive recovery flaps extended.
And I'm not sure if we are comparing apples to apples with regard to these max dive speeds of different aircraft because many placards will give a range of altitude (in the case of the P-38 they are in 10,000ft increments). If we are talking about the limit at 10,000ft one could either quote the figure given for 10,000ft and below (420 mph) or 10,000- 20,000ft (360 mph), but of course the limit would be 60 mph less if we use the latter approach.
And I'm not sure if we are comparing apples to apples with regard to these max dive speeds of different aircraft because many placards will give a range of altitude (in the case of the P-38 they are in 10,000ft increments). If we are talking about the limit at 10,000ft one could either quote the figure given for 10,000ft and below (420 mph) or 10,000- 20,000ft (360 mph), but of course the limit would be 60 mph less if we use the latter approach.
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- #76
The P-38G-L will have more power than P-47C-D, for about the same weight. Talk 2650-3200 HP vs. 2000-2600, on 130 grade fuel. Thus it will have better initial accelration.
P-38 will also out-climb the P-47C/D for the same reason.
The aircraft is fairly heavy and, until mach effects appear, fairly streamlined so air has little ability to impose significant resistance to keep the plane from picking up speed very fast with gravity pulling it down combined with the thrust of the aircraft's propellers and exhaust system (which is little as it has a turbocharger).
The propellers produce enough thrust to have allowed speeds of over 400 mph in the prototypes, around 395 in mass production models (extra weight, some drag produced by the bullet proof glass), and up to around 420 mph in later variants, gravity effectively provides (at least in a 90-degree dive) 1g of acceleration, turning every pound of aircraft into a unit of thrust pushing downwards at around 32 feet every second, with little resistance.
After the filleting of the wing/body junction, the critical mach number was around 0.65 (airflow hits mach 1 on some part of the wing) drag divergence mach is around 0.67-68 (airflow is now supersonic on some parts of the wing, shockwave strength reaches a point where it can drain enough energy out of the airflow to cause some separation), which causes an increase in drag, as well as a decrease in control effectiveness. The aircraft's loss of control occurs at around 0.74 mach. While gravity still is pulling it down, the drag from the airframe has gone up, and the efficiency of the propellers are affected by airspeed and mach numbers (which is why flying up high helps to a point, though mach effects do start to kick in and, past a certain point, it'll reduce the available net thrust) and by this point the net thrust of the propellers are less than the drag produced by the aircraft. I'm not sure if 0.74 is terminal velocity (a speed at which drag and thrust equal each other, gravity in this case is thrust as well), but going straight down without a means of recovery is quite terminal!
Since the speed of sound is affected by temperature, as the airplane reaches warmer air, some control effectiveness is restored, but propeller thrust also increases for the same reason (not much, but with gravity doing it's thing, it's enough to get you back to 0.74), so once the control is restored one would want to start pulling back, at first with everything you got, then progressively less (as the aircraft comes out of the dive, it will stop accelerating as gravity is being countered by lift to a degree, propeller thrust is fairly low, overall airflow over the aircraft is significant, and lift actually increases drag) to avoid over stressing the airframe (once it slows down enough, you'll regain full control and be well above the maneuvering speed, yanking the stick all the way back above the maneuvering speed risks snapping off the wings).
Long message short: The plane accelerates very quickly into mach-tuck! The P-38's problems with mach tuck had to do with two things
- Thick wings: This is largely due to the high aspect-ratio (8.26), which favors a thick structure to avoid excessive aeroelastic flexing
- The junction between the fuselage and the wings
- The wings are convex on top, which causes the airflow to accelerate over them
- The fuselage is convex in shape, causing the airflow to accelerate to either side it
- The two combine together to produce an unusually high velocity airflow (I'm surprised this isn't thought of as an early example of area rule issues)
An airplane like the Spitfire has less mass, which means it won't pick up speed quite as fast, but it can ultimately tolerate a higher mach number, so it will accelerate a lot longer before it runs into mach tuck or airframe destruction: While this does sort of contradict what we were all taught in school that all objects fall at the same speed, this fails to factor in
- Air resistance: A heavier object is not affected as much by the mass of the air. If not raindrops would probably kill us.
- Gravity: This only applies with objects of significant mass, but technically they would both attract each other, and as a result would hit faster than if only one attracted the other.
BTW: Edited to properly include units that are consistent
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DarrenW
Staff Sergeant
Thank you Zipper for the outstanding explanation. So I guess it depends on the situation that you find yourself in. If you just need to make a quick break away during a dogfight and are not being followed too aggressively than a plane like the Lightning would be what one would select. But if you are alone and fighting your way back to safer skies, with the enemy in chase and bent on your ultimate destruction, than a plane with a higher critical Mach number is needed in order to eventually outdistance your pursuer. If you start with enough of a lead of course.
DarrenW
Staff Sergeant
Thanks for the nice summation Tomo. Now it makes perfect sense to me.
Glider
Captain
The one that gets my attention is the F4U1-D. It wouldn't take much of a dive to exceed 443 when things get risky
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