Critical Understanding #4, ANY loss of RPM is Detonation.

Builders:

Every Corvair manual I have ever published has contained very stern warnings never to fly any engine that you even suspect might detonate. That is pretty plain, but still a number of builders damage their engine each year by ignoring that warning.

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Here is the golden rule of detonation: Anytime your engine experiences a loss of rpm, even as small as 25 rpm, it is detonating, and you must instantly stop or you will damage and possibly break the engine. There is no tolerance for detonation. Where a modern car might reduce the timing or go into limp mode, a pure aircraft engine only has one system to protect it: You. If you do not act, the engine will break.

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This is particularly true any time the engine is at wide open throttle. If you are on the take off roll, and the engine rpm sags off even 25 rpm, you must have previously committed your mind to ABORT TAKE OFF. If you get to even 100′ or more, and you have the option of landing straight ahead, even on the over run, ABORT THE FLIGHT, land. Detonation does not get better until one of the factors is removed. If you do nothing, even for a few seconds, the engine will be damaged, in another few seconds it will blow a head gasket, and shortly after that it will potentially stop altogether. Yet every year, we have a number of pilots, mostly who never personally set the timing on their engine, sit there like a bump on a log doing nothing while the power of their engine is decreasing from raw detonation. They are hoping it will go away, the mental mindset of a sheep.

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Most common causes of detonation (all preventable)

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(one) Not setting the timing.

A subject I have addressed before:

YOU MUST SET THE TIMING ON YOUR ENGINE

When to check your timing, Lessons learned Pt#2

Ignition Timing on Corvairs

Ignition timing on Corvairs, Part 2

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(two) Not Using the Correct Fuel:

I have always recommended using 100LL through all of the test hours. 93 octane fuel can be used once the rest of the systems are validated. UNDER INDIVIDUAL CONDITIONS, some Corvairs with lower compression, perfect timing at a reduced setting, and proven cooling, can use fuels lower than 93 Octane, but this has no across the board approval, and it never has. In 2014, we had a guy destroy his plane on the first flight because he first refused to set the timing for months, then reluctantly did it incorrectly, and went on to fuel his plane with 91 octane car gas. He was at an airport with 100LL, but instead, perhaps to save a few dollars, he instead used 91 car gas. Mind you this is a 150 hour pilot in his 30’s who clearly didn’t have much respect for anything I have to share.

Read: Food for thought on Fuels.

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(three) Having a Carb that is running too lean:

If an engine has perfectly set timing and 100LL fuel, it can still be made to detonate by just letting it run too lean.  Traditional aircraft fuels had dual octane ratings like 115/145 “Purple Fuel” the 115 is the octane rating of it running on “Auto Lean” setting and 145 is the octane rating when the same radial and fuel are running on the “Auto Rich” setting. All fuels work like this, so you don’t want some monkey playing around in your carb and you don’t want to use some carb like this:How I became a genius in 6 minutes.

Read this story about aircraft carbs to understand that they automatically run richer at wide open throttle. There is a lot of information in the story: Air / Fuel ratios on Corvair carbs.

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(four) Using the wrong spark plugs:

If there is a single issue I will never understand, it is why some builders compulsively must use spark plugs which we don’t recommend. What they gain out of it I don’t understand, but I do know what they stand to lose. Read: A Tale of Two Spark Plugs……

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(five) Having an engine with incorrect or  deficient cooling:

I have had many people tell me “I am using your cooling system” but when I look at it, their cowling has untrimmed inlets, no inlet rings, and no lip on the underside of the cowl. All of these make the engine run very hot, and hot engines detonate. Our cooling systems work very well, but this means the builders has to actually build our system, not something that looks like our system. Read more here Corvair Cooling, something of a human issue…..

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Your Corvair, if built and installed correctly, need never detonate, not even once. It is a machine, and it will work correctly if set correctly. It doesn’t have a mind of it’s own, it will never turn on you nor will it decide to harm you. There are plenty of builders who have flown hundreds of hours over many years, who have never experienced it once. Conversely, we have had a person, who I directly warned in writing, proceed to destroy his plane on the very first flight, and put himself as well as his illegal passenger in the hospital. Either outcome success or failure, can be replicated, it is completely up to the builder to decide which he will have.

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Note Book Section:

Make line 4.1 in your Hand book a hand written entry, to seal in your mind that you recognize that any loss of RPM is Detonation, and it is never acceptable.

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Make line 4.2 in your Hand book a notation that you have set the timing on the “A” side of the ignition with a timing light, at full static rpm and it is 30 degrees total. (28 degrees for 93 auto fuel)  Note the actual RPM also.

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Make line 4.3 in your Hand book a notation that you have set the timing on the “B” side of the ignition with a timing light, at full static rpm and it is 30 degrees total. (28 degrees for 93 auto fuel)  Note the actual RPM also.

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Make line 4.4 in your Hand Book a notation that the first 40 hours will be flown on 100LL fuel.  As the hours are flown, amend the entry with the actual measured fuel burn on an average hour of flight, the minimum hour and the maximum in an hour.

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If you choose to use 93 fuel after 40 hours, make line 4.5 in your Hand Book a note stating the timing was reduced to 28 degrees.  Note the new full static rpm. Enter a new test period of 5 hours of solo flight to evaluate the compatibility.  Amend the entry with the actual measured fuel burn on an average hour of flight, the minimum hour and the maximum in an hour.

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Make  line 4.6 in your Hand Book read that the only acceptable spark plugs are AC-R44F, or Denso IWF16-5359, IWF20-5359, or IWF22-5359. No other plugs are considered airworthy.

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Make  line 4.7 in your Hand Book read the EGT of the engine after running for 30 seconds at full static RPM.  While running, pull the mixture out slowly, and the RPM MUST RISE, and the EGT MUST DROP. Note the numbers in the Hand Book.

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Make  line 4.8 in your Hand Book a note of measuring the diameter of inlets, noting minimum as 4.75″, and that they have been equipped with inlet rings. After completing the “Two Minute Test”, note the Static RPM and the CHT in this entry also.

 

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-ww.

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