Vought SBU and Curtiss SOC

[Markus Becker]

The combat radius was a mere fraction of a plane´s nominal range. An SBD had a range of 1,100 miles but the CB-radius with a 500lb bomb was just 225 miles, roughly a fifth.

An SOC would have a CB-radius of 180 miles, wouldn´t it? The SBU biplane had a range of 550 miles. Could it make an attack on a known target that´s ~200 miles away?

[Bill Walker]

There are ranges, and then there are ranges. Without knowing the details that each quoted number is based on I can only guess. I assume the "range" number you quote is based on maximum fuel load. The "radius" number may be based on a reduced fuel load which makes it possible to lift the bomb without exceeding maximum gross weight.

Given the primary scouting role of both aircraft, it would make sense that the designer provided tanks large enough to maximize range without a bomb load. Carrying a bomb would only be possible then if the fuel load was decreased.

[JoeB]

Just to benchmark the SBD in a primary source, the "Aircraft Characteristics and Performance" sheets for SBD-5 dated June 1 1944 give the a/c's range with 1000# bomb as 1115 statute miles, radius 240, same nominal range but 260 mile radius with 500#. The conditions for the radius are specified in detail, including warm up, take off, forming up, cruise, combat, return, reserves, etc. (the sheets I have for earlier SBD's don't give radius). It's a bit different than you quoted but same idea of a quite small %, and it's considerably affected by the assumptions. (editing to add, those figures assume full internal fuel load plus the bomb).

AFAIK the USN didn't officially specify combat radius on pre-WWII types like the SBU, and as you know SOC was usually employed as a cruiser floatplane during WWII, though it could also operate on conventional landing gear. The problem with directly extrapolating the radius/range as a % is that parts of the assumed fuel consumption like warm up, form up, and particularly end of flight reserves are constant: a short enough legged plane would theoretically have a zero combat radius because it used up all the fuel doing those things and had none to cruise out to a target.

A less extreme real version is the official combat radius for the F4F-4 with no drop tank, as given in its ACP of July 1 1943: only 105 statute mile radius despite max range of 830. Part of this was also the assumption of much longer time at combat power for a fighter (20 min v 5 min for the SBD-5) but it's also the effect of the fixed components of consumption taken out of a lower max range. OTOH Lundstrom said in "The First Team" that the F4F pre-drop tanks was viewed by the operating units as having a practical radius of around 175 miles.

But even based on the latter figure, it wouldn't seem that SBU (548 st mile range in Swanborough/Bowers "USN A/c Since 1911") or SOC (675 miles same source) had a practical combat radius near to 200 miles. In an extraordinary situation with target location well known, smaller formation (or single SOC from a cruiser, perhaps), important enough to accept more risk of fuel exhaustion loss, maybe?

Joe

[Graham Boak]

Interesting: from my more recent experience I'd have said one third as a factor rather than one quarter. However, that's without allowing for forming up and I suspect lower reserves. Can you quote a percentage for these two factors?

[Bill Walker]

One third is the modern factor for converting range into radius, commonly used on turbine powered aircraft. Perhaps the older aircraft had a less direct relationship between higher power and higher speed. With two wings and lots of struts and wires you may reach a point where more power just burns fuel and makes more noise, without moving you much faster.

Working with USN specs since the 1980s I know that they now have very detailed conditions defined for calculating both range and radius. The radius calculation does, as Graham and Joe indicated, include more time for form up, more time at high power, plus time over the target (i.e., burning fuel but not going anywhere). They probably had similar detailed definitions back in the 1930s, which could explain the numbers we are seeing.

In reality, all these published numbers are just guidelines. The actual usable numbers would be worked out flight by flight, based on the circumstances. I find it interesting that back in the 1930s and 1940s it was common to perform fuel consumption tests on individual aircraft at operational units. This suggests to me that there could be significant aircraft to aircraft variation in range and radius numbers, even within a given type.

[Graham Boak]

That sounds like a description of the Jet Provost - increasing the throttle increases the noise rather than the speed! More seriously, I don't see it as a drag factor, but a matter of allowances, and there are good reasons to suppose that Naval allowances could be higher than Air Force ones,

[Jim Oxley]

Fascinating topic. I've always been quite confused as to why the 'combat' range of an aircraft was so much lower than it's cruise range. The above explains the factor's that affect it well.

But a question.

Given that combat range of carrier aircraft is roughly a quarter of cruise range, would the same apply to land-based aircraft? I would have thought that land-based aircraft would have slightly longer legs, given that the take-off and landing procedures (for them) are much faster than for carrier aircraft.

[Bill Walker]

Jim, it all depends on the definitions used for each mission. US government specs require you to use standard assumptions when calculating the numbers: so many minutes warming up, at a given power; so many minutes formating, at a given power; so many minutes in combat, at a given power; and so on. You need to know the assumptions before you compare numbers. Obviously, if you have the same procurement agency (AF or Navy for example) at about the same time, you probably have the same assumptions. The problem arises when you start comparing different agencies and/or different time periods.

As I said before, all this is academic when it comes to planning an actual mission. There have been a few examples of well trained and well disciplined crews exceeding published range data. Think Doolittle's raiders, or the FAA attacks on Norway from UK bases. I suspect there are some examples of other crews not making the published numbers, but they probably weren't boasting about it.

Just another thought on the original question: I suspect that the external bomb load would lower your gas mileage for half the mission, due to drag. So, the radius with a bomb load would then become some smaller fraction of range without a bomb load.

[Jim Oxley]

Thanks for the explanation Bill.