The DC-3:
And the B-17:
I've written about the DC-3 before, so you can go read this post, which has a link to another one. In surfing around the Internet for this post, I came across this site. It has a lot of stuff on it and a boatload of photos. But it also has a few head-scratchers, like this page on how to fly a DC-3, which has this headscratcher of a statement:
The distance the pistons travel in the cylinders (stroke) is related to the power used. The piston rings, over time, wear a ridge into the cylinder wall at the top and bottom of the stroke, If 42" is used for takeoff, the bottom ridge will develop further up the piston wall than in an engine that has seen rated power for Takeoff. If the time comes when full rated power is needed (short runway, engine failure, etc.) and a power setting greater than 42" is used, the piston is now forced past the ridge in the cylinder created by many hours of using de-rated power. Forcing the piston rings past this ridge can cause them to break.I'm not an engine mechanic, but that strikes me as being dead wrong. The crankshaft is a rigid machined chunk of solid steel that spins in a set of bearings. The connecting rods are also steel and they do not stretch as power is applied.
Can anyone think of why someone would say something like that, unless it was just bullshit to placate a nosey-parker on why they were not using maximum power on take-off?
5 comments:
ha! elastic piston rods - whocudanowd ?? !!
The bearings have slop, and the inertia of the piston makes it want to go a little higher at top of stroke and lower at bottom of stroke at high rpm's compared to at low rpm's (the faster the piston is moving, the more energy it contains), but the "it'll break" thingy seems dubious to me. The worst I've seen is a bit of oil usage at high RPM's on engines that haven't been conditioned to run at high RPM's, mostly because the oil rings were too weak to handle the ridges at top and bottom of stroke.
- Badtux the Dubious Penguin
My father, who was of the appropriate era for these planes, also thought this. While he didn't think the pistons would break, he did tell me that you wouldn't be able to get full power from your engine after these ridges had developed.
I don't think much of the slop hypothesis. First, the slop in the piston travel is very small -- if it weren't, the wrist pins would be pounded to pieces. Second, the crank/rod/piston chain is under compression throughout the compression and power strokes, and carbon buildup occurs during the power stroke.
Also, "many hours at reduced power" is meaningless. ALL engines spend many hours at reduced power. Conduct an entire flight at rated hp and you'd better land at a place where there's an engine shop. By his reasoning, any power setting other than cruise power should break the engine.
He's talking out of his ass.
I can see why someone would make the statement, but that engine would probably have to have an awful lot of hours on it at low load to create enough wear that the rings would run into the unworn section of the cylinder from piston and rod growth, upon the application of a lot more throttle and RPM. Even then the amount they would hit should be pretty small. The radial engine is reasonably well balanced so other than growth from the increased load and I'm assuming RPM I agree that this is unlikely.
On this note I will say that there were a lot of old wives tales about what was going on in engines 75 years ago. My grandfather was a factory trained Packard mechanic and owned a piston ring company in the early part of the last century and some of the engine stuff he told me had been proven misguided 40-50 yrs ago. Everyone was learning from a base of very limited knowledge, including engineers.
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