Making a B-29 go faster

Factual Questions
1

(I am again reading one of those time-traveller-goes-to-1940 stories again. They bring silly questions to my mind.)

The top speed of a B-29 was 310 knots. How could we make it faster? If we took off a couple of tons guns and stuff, I presume it would go faster, but this line of thought reaches a point of silliness. If we reduced the weight to zero, would it fly infinitely quickly?

More to the point:

  1. What part of a propellor plane sets the upper speed limit?

  2. Other than reducing weight, what else could be easily done to increase speed?

  3. Roughly, how fast could any WWII propellor aircraft go in level flight? WOuld there be any way to guess how fast a 310KIA B-29 could be made to go?

Please forgive the silly question.

2

[QUOTE=Paul in Saudi]

  1. What part of a propellor plane sets the upper speed limit?

[/QUOTE]

Well, drag is expressed as D = C x ((density) x (velocity)^2)/2 x (reference area), where C is an experimentally-derived coefficient and density/velocity are properties of the air.

As an aircraft’s speed increases, the speed of the air hitting it will increase, and the drag will increase. Eventually, the drag will be equal to the thrust of the engines, and the aircraft will not be able to go any faster. This is dependent on the shape and size of the aircraft.

So I’d say the shape and engine thrust set the upper speed limit. The effective thrust of a propeller-driven aircraft is dependent on a number of factors relating to the propeller, none of which I understand too well.

But as an aside, the fastest propeller-driven aircraft in the world is the Tupolev Tu-95 Bear strategic bomber. With four sets of contra-rotating turboprops, the maximum speed of the Tu-95M is 925 km/h. Granted, they’re not the piston engines you’d get on a B-29, but that should give you the top speed. (Though if you somehow stuck them on a B-29, the crew would go deaf in short order - sweet baby Jesus are those engines loud.)

3

How odd, 925 KPH is just about 500 knots. Funny how that works out.

Of course a turboprop engine is out of the question for a number of reasons related to the WWII time frame. But perhaps we could improve the aerodynamics and reduce drag with better manufacturing techniques?

4

[QUOTE=Paul in Saudi]
(I am again reading one of those time-traveller-goes-to-1940 stories again. They bring silly questions to my mind.)

The top speed of a B-29 was 310 knots. How could we make it faster? If we took off a couple of tons guns and stuff, I presume it would go faster, but this line of thought reaches a point of silliness. If we reduced the weight to zero, would it fly infinitely quickly?

More to the point:

  1. What part of a propellor plane sets the upper speed limit?
    [/quote]
    I’m not an aerodynamicist but I think propellor tip speed is important.

I suppose using all the fancy wing profile alterations that are common now would help. That is, the wing could be a low-drag profile for cruising and it’s camber and chord could be changed with moveable flaps for low speed operation as is done now. And aerodynamicist could tell you how much flexibility a propellor driven plane would have in that area, and one will probably be along. WWII fighter planes were getting into the low transonic range in a dive if the pilot wasn’t really careful.

If you want a WAG, mine is about 425 mph. With modifications I don’t see any reason a B-29 couldn’t do as well.

5

Thank you. It is bedtime here. I will check back in the morning.

6

[QUOTE=David Simmons]
I’m not an aerodynamicist but I think propellor tip speed is important.
[/QUOTE]

I believe once the tips hit Mach 1 horrible things start happening unless some very clever indeed is applied.

[QUOTE=David Simmons]

WWII fighter planes were getting into the low transonic range in a dive if the pilot wasn’t really careful.
[/QUOTE]

This often resulted in the plane coming apart since the planes weren’t robust enough or the right shape to cope with the sonic shockwaves.

Certainly taking off all the gun turrets, stripping out any excess weight, and giving the thing a good polish would be a good start. Blueprinting, tuning and adding things like nitrous and so on to the engines would help a bit more. Beyond that you’re probably getting into some fairly substantial redesign. But according to this page in 1946 a b-29 got up to 321 knots over 1000km with 2200 hp engines, whereas a Grumman Bearcat has done 459 knots with a 3800hp version of pretty much the same engine, so it should be possible to squeeze more speed out of the big bird.

7

I am under the impression that the method adopted in wartime was to fly them in the jetstream or other high-speed tailwind.

Sailboat

8

Hmmm…semi-swept wings to add to speed.

Canards to give additional lift at takeoff, & compensate for the swept wings.

Winglets to boost speed & effeciency.

9

Well, the Speed of Light, not infinitely fast. But close enough.

In any event, even if you do clever things with the propellers, you’re not going to be able to pass Mach 1. Supersonic planes must be designed radically differently, from the ground up, from subsonic planes, so you’re not going to be able to go supersonic just by modifying an existing plane. I would not, however, be surprised if you could get very close to the speed of sound.

10

[QUOTE=Chronos]
Well, the Speed of Light, not infinitely fast. But close enough.

In any event, even if you do clever things with the propellers, you’re not going to be able to pass Mach 1. Supersonic planes must be designed radically differently, from the ground up, from subsonic planes, so you’re not going to be able to go supersonic just by modifying an existing plane. I would not, however, be surprised if you could get very close to the speed of sound.
[/QUOTE]
The site on the Reno air races states that a heavily modified P-51 won a race at an average speed of 634 mph which is about Mach 0.9. I have to assume that the modifications included some computer controlled, power assisted controls. I would think that at 634 mph the normal control system would be so stiff the pilot could hardly move it. In addition, at Mach 0.9 any slight control mistake would be disasterout and the airframe must be close to instability.

It also gives the plane a top speed of 450 mph with the original Merlin engine.

11

[QUOTE=David Simmons]
The site on the Reno air races states that a heavily modified P-51 won a race at an average speed of 634 mph which is about Mach 0.9. I have to assume that the modifications included some computer controlled, power assisted controls. I would think that at 634 mph the normal control system would be so stiff the pilot could hardly move it. In addition, at Mach 0.9 any slight control mistake would be disasterout and the airframe must be close to instability.

It also gives the plane a top speed of 450 mph with the original Merlin engine.
[/QUOTE]

That’s a misprint. Last years winner in Unlimited was John Penney in “Rare Bear” at an average speed of 466.298 mph. No way are they averaging 634 mph.

Link to searchable database Here

12

Sorry for the double, wanted to see if the results were saved and hit submit.

Anyway, a search for top speed of any year gives the record to Skip Holm in “Dago Red” at 507.105 mph (440 knots) in 2003.

13

Don’t they fly at fairly low altitude and turn constantly at Reno? Top speed in level flight would generally occur at fairly high altitudes, I should think.

Here are some contenders for the WWII level flight speed crown:

Spitfire Mk XIV - 439mph
link

Tempest Mk V - 435mph
link

P-51D - 437mph
link
The H model could hit 487mph, but was just coming into service at the end of the war.

Fw 190-D9 - 440mph
link
The Ta-152 would have been a bit faster, but I don’t believe it made it into service, though I could be wrong about that.

Bf109 K-4 - 460mph
link
Very late model, the much more common Gustav variant was rather slower.

The field, as you can see, is pretty tight, and all the numbers are arguable. I’m not aware of any Soviet aircraft that was quite in this league - the Yak-9 was a hair slower, for example, but I might be forgetting something. Been a while since I played Il-2. :slight_smile: I believe there were unarmed reconnaisance models of some of these aircraft that would have been somewhat faster as well. But, the answer to the question, how fast did the fastest WWII prop planes go is roughly 440mph, though it should be noted that many of these models were only just coming into service in '45, and for much of the war the top speed of most fighters in level flight was somewhat slower.

14

[QUOTE=Crescend]
But as an aside, the fastest propeller-driven aircraft in the world is the Tupolev Tu-95 Bear strategic bomber. With four sets of contra-rotating turboprops, the maximum speed of the Tu-95M is 925 km/h. Granted, they’re not the piston engines you’d get on a B-29, but that should give you the top speed. (Though if you somehow stuck them on a B-29, the crew would go deaf in short order - sweet baby Jesus are those engines loud.)
[/QUOTE]
Don’t forget that the Tu-95 has swept wings, which probably accounts for some of the performance improvement.

For questions about modifying a B-29, look to the B-50. It was essentially a B-29 with a taller vertical stabilizer and more powerful engines (3,500 hp each, compared to 2,200). Maximum speed, according to wikipedia, was 395 mph (343 knots).

15

[QUOTE=Gorsnak]
Don’t they fly at fairly low altitude and turn constantly at Reno? Top speed in level flight would generally occur at fairly high altitudes, I should think.

[/QUOTE]

True. Not sure how fast a Reno Unlimited would do at altitude in a straight line, but I’ll wager it’s shy of 634 mph by a good bit.

16

[QUOTE=Gorsnak]
Don’t they fly at fairly low altitude and turn constantly at Reno? Top speed in level flight would generally occur at fairly high altitudes, I should think.

Here are some contenders for the WWII level flight speed crown:

[/QUOTE]

The Dornier Do 335 had a top speed of 474 mph.

17

[QUOTE=Duke of Rat]
That’s a misprint. Last years winner in Unlimited was John Penney in “Rare Bear” at an average speed of 466.298 mph. No way are they averaging 634 mph.

Link to searchable database Here
[/QUOTE]
I thought it was funny that they said 634 while they gave the speed with the Merlin as 450. I puzzled over it for a while and then interpreted the thing as saying that they must have used a different engine. Now that I think about it I should have been suspicious because the training film for the P-51 warns the pilot flying a 51 with a Merlin to be easy on the brakes because the heavier Merlin (as compared to the original Allison) makes nose-over while taxiing quite easy. A more powerful engine would be even worse and might even upset the CG so much as to make the thing unflyable. And, of course, there is the control problem.

Glad you straightened that out.

18

[QUOTE=Paul in Saudi]
(I am again reading one of those time-traveller-goes-to-1940 stories again. They bring silly questions to my mind.)

The top speed of a B-29 was 310 knots. How could we make it faster? If we took off a couple of tons guns and stuff, I presume it would go faster, but this line of thought reaches a point of silliness. If we reduced the weight to zero, would it fly infinitely quickly?
More to the point:

  1. What part of a propellor plane sets the upper speed limit?

The Propeller acts as a big flat disc, and is a definite limiter as you approach Mach speeds.

  1. Other than reducing weight, what else could be easily done to increase speed?

  2. Roughly, how fast could any WWII propellor aircraft go in level flight? WOuld there be any way to guess how fast a 310KIA B-29 could be made to go?

Please forgive the silly question.
[/QUOTE]

There are two major drag components on an airplane: induced drag from lift, and drag from stuff sticking up into the wind and the frontal area of the airplane having to push the mass of air out of the way as you go through it.

Induced drag is a function of weight. If the airplane has zero weight, it would need no lift, and induced drag would be zero. So all you’d have left is the same kind of drag your car makes as it tools down the road.

I’m assuming that we’re trying to modify an existing B29, not redesign a propeller bomber from scratch. I also assume we’re not going to do radical things like design new wings with a different airfoil and sweep and all that.

So let’s talk about induced drag first. Lighten the airframe. Get rid of all the guns, the bomb racks, the turret hardware. But make sure you aren’t totally screwing up the weight and balance of the airplane.

Now, the wings and structure are strengthened to handle a huge bomb and fuel load, and the total lift the wings create is designed to be able to lift that load. So if you substantially lighten the airplane, you might be able to chop the wingtips back to reduce the span. However, that might just screw things up as well, because you’re reducing the aspect ratio of the wing. If you’re feeling really daring, you could try cutting some of the now-overdesigned structure out of the airplane in strategic places to lighten it further.

Next up: form drag. You want to minimize the frontal area of the aircraft. So any turrets sticking up should be removed and plates riveted over them. In other aircraft, you could do things like chop the canopy down (a lot of Reno racers do this). In the case of a B-29, it’s already smoothly faired into the fuselage, so that won’t help. But in general, make the airplane as small to the airstream as possible.

Now there’s parasitic drag. That’s drag due to all the little things sticking off the airplane. Antennas, machine gun barrels, gear legs, pitot tubes, tie-down rings, yada yada yada. Get rid of them. These things can cause more drag than you’d imagine, as they can trip up the nice smooth laminar airflow over the fuselage and wing and cause it to become turbulent, which is a really energy sink. Make the airplane as smooth as you can.

Then there’s cooling drag. Here’s where some modern aerodynamics might be able to help. NACA ducts and other modern techniques to help cool the engines might allow you to tighten up the cowls a bit and reduce drag around the engines.

Finally, trim up the airplane for your speed run. That means moving the Center of Gravity back as far as you safely can, to reduce induced drag. It also means getting out your duct tape and taping over all open seams around the doors, access hatches, etc.

That’s probably the best you could do without re-engineering the airplane. If you’ve got a test program going you could trying fooling around with things like vortex generators, dialing in a bit of reflex into the flaps, or other tricks.

In the end, all this probably won’t gain you very much. That airplane was a pretty good design, pretty clean, and designed to fly at high altitudes to minimize drag. You might get it going another 50-75 knots.

19

Wasn’t the B-29 considered pretty damn fast for a piston-engine heavy bomber? Even the B-36 (without jet engines) and B-50 couldn’t get a cruising speed of 400 mph.

20

Ok , not much of an aero engineer (in fact not one at all) but I would postulate that drag and weight are directly independent

Drag should be a function of air density and viscosity, aeroplane speed and geometry(, foward facing surface area, vortex generating profiles, stickyness/smoothness of surfaces, number of ariels, guns turrets sticking out etc etc.
Basically take a plane of identical geometry made out of titanium filled with helium with all requisite surface finishing and throw it throuh the air at say 400mph - it will be dragging the same mas of air behind it as say - an identical plane made out of pig iron stacked to the turets with depleted uranium with the same cool paint job. Total internal mass will not affect drag.

So why bother lightening the fairyplane to make it go faster? (apart form removing all the external sticky out things hence altering geometry) My guess would be lift and altitude. Lift generated is related to wing profile, forward velocity (relative to air) and air density. How high you get is a celebritry death match of lift vs gravity tag teamed with mass of aeroplane. A light plane for the same lift (geometry) is going to get higher in the sky compared to a heavier one.
The higher you get, air density goes down, probably viscosity also, so drag reduces hence for same forward thrust you should be able to get a higher net forward velocity.
That said the law of diminishing returns (is tha a law or just the way stuff works?) will hapilliy pop up and point out the higher you go as air density decreases you will get less lift so each additional kilo you loose to gain some altitude will have less and less altitude gain associated with it. Add to that as the air thins out the oxygen content decreses so the engines will be less offective (actualy not so sure - as oxygen content goes down do the engines produce less power or can it be compensated for by adjusting fuel air mix or do they just become less efficent in terms of fuel consumption? not much of a mechanical engineer either).
So cutting out mass doesn’t reduce drag directly, but certainly will help you get some altitude, which will help reduce drag.

Anyway for what its worth - another way to get a B-29 faster would be to add those little vertical wings at the end of the main wings, This stops the vortex shedding at the end of the wing which effectivly cuts down the useable wing area available for lift generation, hence that chunk of wing at the end is only adding to drag and total mass of aircraft but not providing any lift .

The jet stream adding to speed - may help if you were flying from US to europe - but I didn’t think B29 were involved in the european theater in WWII - I thought they only made an appearence in the pacific front. If so (I suck at history as well) jet stream isnt much good flying from US East to japan, although I supose the airbases in the islands which were to the south would not have help from the jet stream short of blowing the areoplanes off to the west ( and I doubt I need to advise you of my SE Aisa geography skills)

Blondchap