Friday, November 24, 2006

AV - VP Accidents & Icing

The accident figures are similar for other VW-powered designs and whatever the actual accident rate, it's too high. It's also largely preventable, in my opinion, at least with regard to engine-related events.

I've seen my share of fouled plugs and fifth-hand Vertex magnetos with fried points. And even attended a couple of Total Idiot tear-downs where we found no evidence of fuel anywhere in the system and a couple with no oil in the engine. (You really should safety-wire the sump plug.)

The real pissers were the cases where we couldn't find anything wrong. There was fuel in the carb, the ignition system provided a spark at the proper timing and all the controls were intact, at least up to impact. In a couple of cases the engine was still in running condition. If that happened once or twice it would fade into the statistical background but when you personally see a dozen or more cases like that it's a strong indication of a fundamental flaw. But one that leaves no obvious clues.

What you do have is the accounts from the surviving pilots; the classic 'loss of power,' or 'the rpm began to drop'. Track that back to the location of the event, dig out the best records you can find for the weather at that particular time & place and guess what you'll generally find? Conditions ideal for the formation of carb and manifold icing.

Any discussion of icing gets you into a matrix of factors but the basics are pretty simple: The vaporization of gasoline is endothermic -- it absorbs heat. It doesn't matter if you're using a carburetor or the latest gee-whiz slide-valve piece of shit, you've got a mini-refrigerator attached to your engine and if the local atmosphere isn't able to provide enough heat to keep the endotherm above the freezing point any water vapor in the air is going to appear as ice.

With a carburetor you can bootstrap yourself into this situation by reducing the throttle. This leaves just a tiny gap between the throat and the throttle plate (usually of brass) and a lip of ice can appear on the down-stream edge of the throttle plate quick like a bunny, even here in sunny southern California. (The oft-repeated claim that slide-valve gas passers don't ice up because they don't have a butterfly valve (ie, throttle plate) is fallacious. Under the right conditions the whole damn slide can ice up.)

That's why we pull on Carb Heat before we begin our let-down.

But you can also get ice in your manifold, even under full-throttle conditions, assuming you have long intake runners that are not provided with any form of supplemental heating. Lycoming routes their runners through the sump. Continental usta say theirs picked up enough heat from the cylinders. Volkswagen vehicles use the exhaust gases from one jug to heat their manifold. And yes, Virginia, your slide-valve Lake, POSA, Aero-Carb or whatever is just as susceptible to manifold icing as a carburetor. So let's forget the 'ice-free' bullshit.

The key point here is that some of the VW engines which suffered the classic 'loss of power' syndrome had no provision of any form for carb heat and all of the others had carb-heat systems incapable of providing enough heat for worse-case conditions.

How much heat is that? About 90*F over ambient, according to the FAA.

Based on experiments I did here at the shop, a carb-heat stove fed by just one cylinder of a 2180 engine couldn't produce a ninety degree rise when passing enough air for the engine. Reduce the throttle, you reduce the amount of air required but you also reduce the amount of heat available. Catch-22. You need the heat from two jugs, not one.

A related part of the problem is that the typical homebuilder's heat exchange isn't very efficient. I know mine wasn't even though I religiously copied the design advocated by the experts of that era - an old screen-door spring wound around the exhaust stack with a muff made out of a couple of tomato cans. (ie, Pietenpol, Leslie Long, et al). Indeed, after a few months out in the weather you generally got more rust out of the thing than hot air.

Pot-scrubbers worked better than door-springs and didn't rust. (ie, those big stainless steel pot-scrubbers; hardware stores usta carry them back in the paint department; you used them for scrubbing walls & woodwork before applying new paint. And for pots too, I suppose.)

Tapping the waste heat from two cylinders is better than using just one. And you can make a better heat exchanger, too. Assuming you know how to weld and are willing to devote a bit of time to it.

I don't have a pat answer for the carb heat problem although I'm convinced that with converted VW's it's one of the few cases where more is better. My current effort in that direction is using a stud gun to weld a bristle of pins to two sections of carbon steel exhaust pipe which will be mounted side by side inside a stainless steel muff and fed by two cylinders. The idea is to produce the best transfer of heat with the least restriction to air flow and I think the idea will work out... eventually :-)

It's worth mentioning that most of my experiments don't work out :-) Not that they're total failures -- you always learn something -- but they are seldom totally successful. What does work is to keep adding what you've learned from the last experiment to your next one. You'll eventually arrive at a system that meets your needs, although it's never as simple & easy as the original concept. Sorta like life, in that respect :-)

I make my heat boxes out of whatever scrap aluminum is available. Riveted construction. Sized for the Tillotson Model X carb (ie, 1-7/8" inlet). Controls are simple Bowden cables. Drawings & photos are included in the HVX files.


PS - In conjunction with this thread I was asked why all this hot poop I'm handing out hasn't appeared in their favorite aviation magazine. The simple answer is because they don't want to pay for it.

After spending years (in some cases) to develop a suitable solution, such the carb-heat thingee, to convey that information to others in an understandable fashion may require dozens of drawings, photos and illustrations supported by thousands of words of text because in technical writing (which is what I do for a living) it isn't the simplistic straight-line path that's important, it's explaining what to do when things go awry; identifying the potential problems some distant reader might encounter and providing workable methods of avoiding such mistakes and in some cases, of recovering from them.

Package all that in camera-ready copy, send it off to an aviation magazine and if they express any interest at all, they may offer a $100 for your months of effort. If they offer anything at all. Some believe you should give them material for their magazine... which they then sell for a tidy profit.

Old news, really. It's been thrashed out time and again on other newsgroups. Mentioned here in passing because of a couple of messages from folks who were not aware of it. -- rsh

No comments: