What is the cause of possible crash in an accidental low RPM takeoff?

Question

I've read from quite a few sources that during the World War II, it was not that uncommon a mishap for a fighter pilot to take off with a "coarse pitch" / "low RPM" prop setting. This would typically happen as a result of pilot fatigue after a prolonged period of intense activity, aggaravated by the fact that in a single-seat fighter there were no other crewmembers to warn about the incorrect setting.

Sometimes, though apparently not always, the result was a crash. Even if there was no accident, writers depicting these incidents would usually use a tone indicating that they considered such a takeoff a first-rate blunder and an indication that the squadron as a whole was overtaxed and in an urgent need of rest. So, clearly it was a serious pilot error.

I started to think about what would go wrong in such a takeoff and found out that it's not at all obvious to me. I know how constant speed propellers operate, including the analogy of blade pitch with automobile gears, and understand that the engine isn't able to provide maximum performance on a low RPM setting, even at full throttle. However, since there were successful takeoffs as well, and especially because the lack of performance seemingly wasn't so severe as to make the pilot realize that something was wrong and to abort the takeoff run, it doesn't look to me like the reduced power alone would readily explain the mechanics of a crash in that situation.

So, what was the fundamental reason for a low RPM takeoff to end up in a crash? Did the aircraft simply roll past the runway's end or stall upon rotation due to the lack of power? Or would the engine fail catastrophically because of misuse? Surely the pilot would see from the instruments that airspeed wasn't building up as intended if it was just about a deficit of power?

Answer 1

There are two effects from taking off with the propeller levers at MIN RPM.

Firstly, horsepower is limited by the propeller governor, preventing the engine from developing takeoff RPM. It's not unlike a car with an automatic transmission stuck in high gear. Acceleration will be sluggish, and if you're on a hill with a trailer and the car needs to downshift but it can't, you're going to bog down on the hill.

So a pilot taking off with MIN RPM set will simply have a pretty severe performance penalty, with the engine only able to make the HP it can make at redline Manifold Pressure at minimum RPM. A supercharged radial making 2000 HP at a takeoff RPM of 2700 might make only, say, 1100 hp if it's limited by the prop governor to, say, 1600 RPM (or whatever the MIN RPM setting on the prop control commands).

If you're flying a Corsair fully loaded with ordinance, and the engine is limited to 1100 or 1200 HP, you're probably going into the weeds at the end of the runway unless it is very long. Even if it gets airborne, it'll barely be able to climb.

The other issue is blowing up the engine itself. There is a value called Brake Mean Effective Pressure (BMEP) that is the total peak pressure in the combustion chamber, and this peak pressure, for a given throttle opening, is higher if the engine is turning slower. BMEP is a factor in the onset of detonation.

Most naturally aspirated engines can tolerate high manifold pressure and RPM below normal values (the pressure being limited to atmospheric), but when you add supercharging or turbocharging, the stress levels on the cylinders go way up. So forcing an engine to run with 20 inches of boost, that is, 45" or 50" inches of manifold pressure, plus the engine held down at minimum RPM by the prop, can raise BMEP levels out of spec, causing detonation, and you take the risk of blowing cylinders up. So you end up in the weeds with ruined engine to boot.

Answer 2

A factor not directly related to performance (takeoff roll, climb rate etc.) is the torque exerted on the airplane on the takeoff roll. WW2 fighter planes with their huge engines and propellers are a handful to keep on the centerline even when operated correctly, the increased torque caused by the coarse prop pitch will make the problem worse. One of the leading causes of accidents for the Bf 109 was groundloops on the takeoff roll.

Answer 3

My airplane has a constant-speed prop, and here's where I suspect the problems arise--and as has been suggested a lot of it is related to pilot preoccupation, pilot fatigue, or a failure of a propellor governor.

1. On a hot day in something like a fully-loaded, fully-fueled heavy bomber, you may absolutely need 100% of the airplane's performance to make it off the runway that's available. So operating at 90% or even 80% of peak horsepower could absolutely put you into the trees. It doesn't seem like the prop pitch would cause much of a hit, but to use your gear shift analogy, imagine trying to drive a manual pickup truck uphill, but starting in 3rd or 4th gear. It can't be done. Now imagine a load of bricks in the back. It double-super can't be done. The nature of absolutely global-scale combat operations often involves having to operate on the absolute fringes of the performance envelope in terms of loaded weights, fuel loads, shortness of runways... and urgency to proceed.

2. Imagine I have a large displacement, supercharged engine making the maximum allowable cylinder pressure. Oh--and the prop is creating a very large torque on the crankshaft because the coarse, low-RPM pitch directly resists engine torque. Now I have a huge problem, I could easily over-temp or over-pressure a cylinder to failure before I even get off the runway, My 4 engine, max weight, hot-day B-17 is now a 3 engine, max weight, hot-day B-17 and an accident is unavoidable. In many GA planes, it perfectly acceptable to fly a little over-square, like 24" MP into a prop governed at 2100 RPM. But in the engines of the 1940s, PLUS huge displacements, PLUS supercharging, you can be in a world of trouble if you aren't managing MP in concert with RPM.