Question of the day

[pho_phizzat]

yeah i like the challenge


edit: do you work in the oilfield in Oklahoma?

 

[oilwell1415]

Nope, I'm not a roughneck. Oil isn't nearly the business here it used to be.

 

[D's01Snake]

do you sell tornado insurance?

 

[cobra_matt]

Close enough. The landing gear is mounted on the inboard engine nacelles. By applying brake to one wheel and adding power to the engine outside it you can make the opposite side of the plane move baskwards. Alternating side to side results in the plane backing up.

I was searching around on google trying to find the answer to no avail. So I had to get my thinking cap out and while looking at the picture you posted, I noticed that the inboard engines were off. Then the :idea: came on and I figured the only way to back it up is if you do a bobcat kind of thing where one side is being powered while the other is stationary. Didn't think about applying the brakes though.

 

[James Snover]

That ... is freaking amazing! I thought I knew a thing or two about aviation, but I never came across this tidbit.

Yes, more aviation questions!

Here is one for you, oilwell1415: all P&W radials could run clockwise or counter-clockwise equally well, but it required one very specific adjustment. What was that adjustment?

Jim Snover

Close enough. The landing gear is mounted on the inboard engine nacelles. By applying brake to one wheel and adding power to the engine outside it you can make the opposite side of the plane move baskwards. Alternating side to side results in the plane backing up.

 

[oilwell1415]

do you sell tornado insurance?

lol, no.

I was searching around on google trying to find the answer to no avail. So I had to get my thinking cap out and while looking at the picture you posted, I noticed that the inboard engines were off. Then the :idea: came on and I figured the only way to back it up is if you do a bobcat kind of thing where one side is being powered while the other is stationary. Didn't think about applying the brakes though.

The inboard engines not running was just a coincidence, but I can see how that would be a clue. With them at idle and the props flat they wouldn't make any useful thrust.

 

[ThinkTwice]

shoot, and I thought it had something to do with the plane sitting on some sort of conveyor!

 

[cobra_matt]

The inboard engines not running was just a coincidence, but I can see how that would be a clue. With them at idle and the props flat they wouldn't make any useful thrust.

Yeah, I know. But giving me the visual of some engines running and some not is what put the lightbulb on for me.

shoot, and I thought it had something to do with the plane sitting on some sort of conveyor!

:lol::lol:

 

[zerocool]

shoot, and I thought it had something to do with the plane sitting on some sort of conveyor!

Please god don't start that again.

 

[oilwell1415]

That ... is freaking amazing! I thought I knew a thing or two about aviation, but I never came across this tidbit.

Yes, more aviation questions!

You mean Jim Snover learned something from me? I should get a medal or something!

Here is one for you, oilwell1415: all P&W radials could run clockwise or counter-clockwise equally well, but it required one very specific adjustment. What was that adjustment?

Jim Snover

This has the makings of a trick question. The engines could be made to run backwards by simply altering the timing of the cam rings. However, that simple act alone would also have supercharger (if equipped, but you said "all") spinning backwards and not working at full efficiency. The one change that could be made to spin the propellor just as well in the opposite direction was to change the planetary gear for opposite rotation, which really isn't an engine adjustment. This was the method used on production aircraft that needed counterrotating props to avoid excessive torque at low speeds. So I guess I have a question in response to your question: Are we talking about spinning the engine in the opposite direction, or just the propellor?

On a side note, why do all radial engines have odd numbers of cylinders on each row? This one is fairly easy, but I don't want to use all the good stuff up.

 

[James Snover]

Ooops. I messed up the question. You are right about the cam rings, I forgot about the superchargers spinning the wrong way.

Ok: the inherent geometry of a four stroke engine demands that radials have odd-numbered cylinders because otherwise you would get all cylinders firing on one revolution, intake on one, exhaust on another, all with massive vibration and crank loading problems, super-chargers surging, turbos getting hit with a huge periodic amount of exhaust, etc. An odd number on each row means every-other cylinder fires/intakes/exhausts/compresses which allows the engine to run better and last longer.

Jim Snover

You mean Jim Snover learned something from me? I should get a medal or something!



This has the makings of a trick question. The engines could be made to run backwards by simply altering the timing of the cam rings. However, that simple act alone would also have supercharger (if equipped, but you said "all") spinning backwards and not working at full efficiency. The one change that could be made to spin the propellor just as well in the opposite direction was to change the planetary gear for opposite rotation, which really isn't an engine adjustment. This was the method used on production aircraft that needed counterrotating props to avoid excessive torque at low speeds. So I guess I have a question in response to your question: Are we talking about spinning the engine in the opposite direction, or just the propellor?

On a side note, why do all radial engines have odd numbers of cylinders on each row? This one is fairly easy, but I don't want to use all the good stuff up.

 

[oilwell1415]

Ooops. I messed up the question. You are right about the cam rings, I forgot about the superchargers spinning the wrong way.

The same could be done with a modern automotive engine by installing a reverse rotation cam, but it would actually run better. Why is that?

Ok: the inherent geometry of a four stroke engine demands that radials have odd-numbered cylinders because otherwise you would get all cylinders firing on one revolution, intake on one, exhaust on another, all with massive vibration and crank loading problems, super-chargers surging, turbos getting hit with a huge periodic amount of exhaust, etc. An odd number on each row means every-other cylinder fires/intakes/exhausts/compresses which allows the engine to run better and last longer.

Jim Snover

Dead on. It would run like a Harley and shake the airframe to pieces with an even number of cylinders.

 

[James Snover]

I don't know, but I'll take a shot at it: Assuming you mean a V8, reverse rotation would better mimic a flat-8? Provide 180-degree exhaust and intake pulses?

I'm dying to know!

Jim Snover

The same could be done with a modern automotive engine by installing a reverse rotation cam, but it would actually run better. Why is that?

 

[oilwell1415]

I don't know, but I'll take a shot at it: Assuming you mean a V8, reverse rotation would better mimic a flat-8? Provide 180-degree exhaust and intake pulses?

I'm dying to know!

Jim Snover

The same could be done with a modern automotive engine by installing a reverse rotation cam, but it would actually run better. Why is that?

I mean almost any modern automotive engine regardless of make, model, or configuration. It was discovered long ago that a lot of engine noise was caused by the piston rocking around in the bore as it moved up and down. To fix this, the piston pin was offset to the side of the bore that the the crank pin approache from on the compression stroke. The offset is usually .030"-.060". When offset, it creates a smaller rod angle during the compression and exhaust strokes, and a greater rod angle during the power and intake strokes. This makes it quieter, but also costs some power. Some builders that don't care about the noise or slightly reduced engine life have installed the pistons backwards to gain a little extra power. The same results could be had by simply spinning the engine backwards.

 

[James Snover]

I knew about the pin offset and why it is there. I never imagined it could make much of any real difference in output. How much are we talking about in terms of hp?

Jim Snover

I don't know, but I'll take a shot at it: Assuming you mean a V8, reverse rotation would better mimic a flat-8? Provide 180-degree exhaust and intake pulses?

I'm dying to know!

Jim Snover



I mean almost any modern automotive engine regardless of make, model, or configuration. It was discovered long ago that a lot of engine noise was caused by the piston rocking around in the bore as it moved up and down. To fix this, the piston pin was offset to the side of the bore that the the crank pin approache from on the compression stroke. The offset is usually .030"-.060". When offset, it creates a smaller rod angle during the compression and exhaust strokes, and a greater rod angle during the power and intake strokes. This makes it quieter, but also costs some power. Some builders that don't care about the noise or slightly reduced engine life have installed the pistons backwards to gain a little extra power. The same results could be had by simply spinning the engine backwards.

 

[oilwell1415]

One test I saw on a carb'd 302 was about 20 hp on a ~400 hp engine, but it makes a helluva racket and cylinder wear is much higher.