Skydive Pilots Around??

[FracCapt]

Hey, I can vouge for the black eye flying divers. I swear to god the brake was on, the wheel just happens to spin when a complete as*hole that complains about the last lift w/you steps on it.

I hope you're joking...because if any pilot ever does that to me, he'll have a lot worse than a black eye. That's dangerous, and could kill somebody...or even EVERYBODY in the airplane. I'm all for fun and games...but things like that(and taking the key) are going way too far.

 

[A Squared]

Backlash damage to engine accessory drives and to the engine itself, any time the slipstream is driving the propeller, is also a real factor to consider.

and

Another very important thing which Avbug touched on is the backlash damage that can occur when the engine is being driven by the air.

Guys,

this is a complete misconception. There is no "backlash" in the accessory drive during an idle descent. Before you answer, think it through, in order for there to be backlash, the accessory would have to be driving the crankshaft. The accessory gear on the crank is always driving the accessories.

Just as a side note, that is not "backlash". Backlash is the quality of the gears not meshing perfectly, i.e.: "slop" if you will. It exists in all mechanical drive trains in varying degrees. What is being described is gear tooth contact face reversal.

Follow mw through this:

You pull the plugs out of an engine and you begin turning the prop forward by hand. the magnetos turn, of course. Is the crankshaft driving the magnetos, or are the magnetos driving the crankshaft? Obviously, the crankshaft is driving the magnetos , the gear tooth contact is on the normal tooth faces, not on the reverses faces. Now put the plugs back in, start it up, and run it at 1200 rpm. Is the crankshaft driving the magnetos, or are the magnetos driving crankshaft? Again, obviously the crankshaft is driving the magnetos. Now, push the throttle in all the way, and take off at let's say 2700 RPM. Is the crankshaft driving the magnetos, or are the magnetos driving crankshaft? Again, obviously the crankshaft is driving the magnetos. Now get up to altitude and slow it up and pull the power all the way back, put it in a gradual, slow descent at 1200 RPM. The air is driving hte prop, but, is the crankshaft driving the magnetos, or are the magnetos driving crankshaft? Again, obviously the crankshaft is driving the magnetos. Now, push the nose over into a dive so that the RPM spools up to 2700 RPM (still with the throttle closed) The air is driving the prop, but, is the crankshaft driving the magnetos, or are the magnetos driving crankshaft? Again, obviously the crankshaft is driving the magnetos.

The *only* way to reverse the contact of the gear teeth on the magneto drive is to; a: turn the crankshaft backward, or b: slow the crankshaft so abruptly that the inertia of the spinning magneto causes it to momentarily reverse the gear tooth contact. In scenario b, the crankshaft is actually slowing the magneto, but the deceleration of the crankshaft has to be *very* rapid, much more rapid than you could possible achieve by yanking the throttle to the stop, and even so it would only be momentary. As a side note, this is one of the items you check on a sudden stoppage inspection, You check for damage to the accessory drive gears resulting from the crankshaft slowing more rapidly than the magneto.

Any time that the crankshaft is turning, whether it is being turned by the pistons, the air on the prop, or by your hand on the prop, the crankshaft is driving the magnetos, it's driving the oil pump, it's driving the vacuum pump, and it's driving the generator; not vice versa.

One exception to this, is starting, on many GA recip engines, (continentals mostly) the starter turns the engine through the accessory gears (the generator drive gear on the O-470) So during starting, only, ,the gear face contact is reversed from "normal" as soon as the engine fires and the starter disengages, the contact returns to normal.

I suppose that if you had a engine driven hydraulic pump, and you connected it to an external hydraulic pump (hand operated or powered, your choice), you could turn the crankshaft with an accessory gear by "motoring" the hydraulic pump on the engine, and the gear tooth face contact would be reversed.

Except for starting, and my far-fetched hydraulic pump example, the crankshaft will always be driving the accessory gears, and gear tooth contact will always be normal.

Now, on an engine on which the prop is driven through a gear reduction, it is entirely possible to reverse the gear tooth contact in the *gear reduction unit* by allowing the prop to drive the engine. This is a matter of concern for geared engines, *but* it is still impossible to get gear contact reversal in the accessory section during idle, regardless of whether the engine is geared or direct drive.

So, are there concerns for a *direct drive* recip engine in an idle descent?

Yes. There is the controversial "shock cooling". There is also "ring flutter" in which the piston rings move excessively up and down in their grooves when the combustion chamber pressures are very low. This may contribute to failure of the ring lands failure in the piston. Also there may be lubrication issues on the "big end" (i.e.: where the connecting rod attaches to the crankshaft journal) There's an interesting column on Avweb which presents one view of this and other issues.
http://www.avweb.com/news/columns/186778-1.html

One thing that is certainly not a concern is accessory drive gear contact reversal. It doesn't happen. It can't

 

[avbug]

It sure does. In fact, when setting the timing on the big engines you drive, as I have done many times, we set it through backlash, putting tension with a wrench and an assistant, not as the magneto sits on the engine. There is slop in there. I know what you're saying, but I disagree, and so does every other engine expert with whom I've ever worked...ones who know what they're doing.

I've read the avweb articles. I take exception to them. I've had discussions regarding them with the author, and I do disagree. It's been discussed here to death, and has been discussed here in the past, to death.

Allowing the prop to drive the engine is harmful; it's harmful on reduction drives, accessory drives (even though they're at the back of the engine), crank journals and bearing surfaces, etc.

Thermal issues are hotly contested, but are also an issue. I can vouch for that from personal observation and experience. And yes, in some cases, ring flutter may (I emphasise "may") be an issue.

The bottom line, and the point followed here, is that allowing the prop to drive the engine is unwise and bad airmanship for a number of reasons.

 

[A Squared]

Avbug,

You may have misunderstood what I said about backlash, I didn't mean to imply that it didn't exist, nor that it wasn't related to the subject at hand. Rather I was commenting on the semantics, in that it is not "backlash" when a gear train reverses tooth contact, but it is backlash that allows it to do that. I agree that backlash exists in any gear train, from aircraft accessory drives to Swiss watches. Even so-called "zero-backlash" drive trains (non-aviation) have backlash, they are merely spring loaded so that the gear contact is always on the same face. So, I don't disagree that there is "slop" in an accessory drive, any accessory drive. I've timed magnetos, (not on the R2800, so I don't know that specific procedure) so I am aware that in general, if it is done incorrectly you could set the timing with the "slop" reversed, on the "wrong side of the backlash", for want of a better term. That of course would result in an improperly timed magneto. I think that on all this, we agree, 100%.

Where we disagree, is that in a windmilling descent, the accessory drive gears reverse tooth contact, or operate on the "wrong side of the backlash". Now, instead of merely contradicting me, explain to me *how* the accessory drive reverses the tooth contact. I mean really, prepare a detailed explanation of how that happens, as if I were completely ignorant of the subject. If you are certain that it happens, it should be no difficult task to explain how it happens in a way that will be completely convincing both to me and anyone else who might disagree with you.

Just to clarify some of the other issues: I was not advocating windmilling descents, or trying to suggest that they a good thing, in fact, I mentioned that there were some reasons why they may harmful. I think you may have assumed that because I was disagreeing with you on one point, I was disagreeing with you on all points. I'm not. Rest assured, the only time the props are driving the engines on the DC-6 when I am operating it, is for a very brief moment in the flare. I posted the link to John Deakin's article as food for thought, not to "prove" something to you. I am aware that you disagree with Mr Deakin on most things. I don't agree with everything he says either. I do however find it usefull to read such things, and think through whether things make sense, and analyze why I do or do not agreee with them. Sometimes you can gain a greater understanding of something by reading something which is wrong, and thinking through *why* it is wrong.