XB-15 Projected Performance

[KJ_Lesnick]

From what I remember, the XB-15 was initially intended to use 4 x V-3420's of either 1,600 or 2,600 horsepower but they weren't available, so they went with the R-1830's instead which lead to the plane being underpowered.  With the V-3420's, how fast were they planning that aircraft to be!

BTW: I'm not sure if they were planning to use V-3420's with 1,600 or 2,600 horsepower as the V-1710 came in variants that produced between 750 and 1,000+ hp so either is possible.

[wagnersm]

Another Boeing 4-engine aircraft, the B-314 Clippers, used a the same wing with 1,500 HP Wright R-2600 Cyclone 14 (also called Twin Cyclone) engines.

It had a cruise of 188MPH and a top speed of 204MPH.  Perhaps that would be a good engine and performance estimate.

Source for both are articals in the Wikipedia.

Steve

[wuzak]

The XB-15 was originally scheduled to have 1000hp V-1710s. The V-3420 and R-2600 weren't around when the XB-15 was proposed.

[KJ_Lesnick]

Okay,

Firstly: How fast exactly was the XB-15 as it was
1: Unarmed
2: With a 2,000 lbs bomb-load
3: Full 12,000 pound bombload

Secondly, as a rule of thumb...
1: How much extra performance would an inline with a reasonably good radiator add to that?
2: How much extra performance would a turbocharger add to that figure?

[Hobbes]

The engines you mentioned were already supercharged. Adding a turbocharger isn't going to add much in the way of power, but it may make for a more efficient engine (since you're reusing energy that would otherwise be wasted out the exhausts).

Engine power is limited by the intake air temperature at the fuel injection site: more compression means a higher temperature until the fuel starts combusting prematurely (i.e. backfire), unless you add an intercooler to lower the air temperature.

You could replace a two-stage supercharger with a single-stage supercharger +turbo setup, or do away with the supercharger altogether.

At first glance, I'd expect an inline with radiator to have about the same drag as a radial of the same power: in both cases you need to dump the same amount of heat into the airstream, so both will have the same cooling surface area. The cost of the inline is in the extra complexity and weight (and vulnerability to leaks).

[jcf]

The most mis-understood thing about the USAAC turbosupercharger work on bombers is that
they were not looking for increased speed. The point was to be able to maintain the specified
cruise speed at all altitudes, the reduced drag at high altitude resulting in increased range.
Any increase in top speed was of course welcomed, but it was not the driving force.

[KJ_Lesnick]

The engines you mentioned were already supercharged.

The engines used on the XB-15, or the V-1710 to be used?

At first glance, I'd expect an inline with radiator to have about the same drag as a radial of the same power: in both cases you need to dump the same amount of heat into the airstream, so both will have the same cooling surface area.

Oh, I was under the impression that the V-1710 would have delivered some reduced drag in that the nacelle would be longer and more streamlined, the radiator drag might be the same but the nacelle would be less.  The XB-38 had a higher top-speed than the B-17E

The most mis-understood thing about the USAAC turbosupercharger work on bombers is that they were not looking for increased speed. The point was to be able to maintain the specified cruise speed at all altitudes, the reduced drag at high altitude resulting in increased range.

Well, turbos add greater efficiency as well as greater horsepower (particularly at altitude) so generally extra speed came out of the picture as well whether intentional or fortunate.

[wuzak]

The engines you mentioned were already supercharged.

The engines used on the XB-15, or the V-1710 to be used?

Both.

At first glance, I'd expect an inline with radiator to have about the same drag as a radial of the same power: in both cases you need to dump the same amount of heat into the airstream, so both will have the same cooling surface area.

Oh, I was under the impression that the V-1710 would have delivered some reduced drag in that the nacelle would be longer and more streamlined, the radiator drag might be the same but the nacelle would be less.  The XB-38 had a higher top-speed than the B-17E

Also the XB-39 had a higher speed than the B-29 (with a little more power).

In both cases the engine nacelles were adapted to fit the size and shape of the existing radials, which meant that very little of he streamlining benefit could be utilised.

The most mis-understood thing about the USAAC turbosupercharger work on bombers is that they were not looking for increased speed. The point was to be able to maintain the specified cruise speed at all altitudes, the reduced drag at high altitude resulting in increased range.

Well, turbos add greater efficiency as well as greater horsepower (particularly at altitude) so generally extra speed came out of the picture as well whether intentional or fortunate.

The turbos during WW2 were used to maintain the pressure at the carby inlet at sea level pressure. So the aim of their use was to maintain power at altitude - which benefits both cruise and top speed.

I must say, that turbos appeared to give much greater benefit to bombers than they did fighters.

[wuzak]

Okay,

Firstly: How fast exactly was the XB-15 as it was
1: Unarmed
2: With a 2,000 lbs bomb-load
3: Full 12,000 pound bombload

Secondly, as a rule of thumb...
1: How much extra performance would an inline with a reasonably good radiator add to that?
2: How much extra performance would a turbocharger add to that figure?

I don't have any numbers for the projected performance with V-1710s.

For actual XB-15 performance you could try here: http://www.joebaugher.com/usaf_bombers/b15.html
Or try Wiki http://en.wikipedia.org/wiki/Boeing_XB-15

[KJ_Lesnick]

Wuzak

Regarding turbos: Looking at the B-10, approximately 29 mph was added and their speeds were similar to one another.  I should note that the turbo used on the B-10 was different than that used on the Y1B-17 which might have added more speed
Regarding inlines: Approximately 10 mph was added by using the inline over the radial, however the B-17 was considerably faster than the B-10/B-15 and as a result the differences might have been more pronounced on those designs especially depending on radiator type