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Kickback with lightweight engine components; starter, flywheel and propeller

vj88

Registered User
Anchorage, AK
I have questions regarding increased risk of kickback with lightweight engine components during electrical and hand prop. I'm looking at O-360/370 experimental engines (8 to 8.5:1 compression) fit with the sky dynamics flywheel, whirlwind ground adjustable propeller and either the Skytec NL or XLT starter.

The Skytec NL starter is 9.3 pounds and has a field replaceable shear pin where as the xlt is 6.5 pounds without kickback protection. What starters are others installing and why. The weight difference between the two is large but is the risk of damaging motor a possibility without the kickback protection?

Ive also heard of the kickback during hand prop being quicker and self injury occurring on upon kickback with the lighter weight components.
 
If your mag impulse(s) is working properly and you have proper switching, kick-back shouldn't be an issue.
 
What PerryB said. Kickback happens for a reason. Nothing is 100% perfect but timing issues are important when addressing this. Lightweight components shouldn't have any effect on whether an engine tries to crack your knuckles, only the speed of the prop when it impacts:).

Whether you go mags or electronic, the timing of all components is extremely important.

And make sure the non impulse coupled mag is OFF when starting!

Web
 
I’ll add to the advice re: Get your timing right, and kickbacks shouldn’t be a problem. And, as Web noted, if you’re hand propping, make sure you start only on the impulse equipped mag...usually the left.

i have zero experience with Whirlwind props, but ~ 2500 hours or so with O-360s equipped with composite props. I’ve never had any problem with kickbacks on those props. If a “lightweight” prop kicks back so hard as to go past compression, you need to fix your timing! Those light blades don’t have the INERTIA to kick back that hard unless something is wrong. Actually, lightweight props are less likely to cause damage than metal props because of kickback due to the difference in mass of blades.

One of the first things you notice with composite props is how quick they stop during shutdown, compared to metal props. That works both ways.

But get your timing right and start properly and the prop should make no difference.

MTV
 
If a “lightweight” prop kicks back so hard as to go past compression....
If it kicks back it doesn't "go past compression", unless perhaps the next cylinder. It kicked back because it fired before TDC. Before the max compression was reached. It's that initial kick that will cause any damage, to starter or fingers.

Those light blades don’t have the INERTIA to kick back that hard unless something is wrong. Actually, lightweight props are less likely to cause damage than metal props because of kickback due to the difference in mass of blades
Some confusion there. The fact that they don't have as much rotational inertia is precisely why they can kick back with greater acceleration and thus speed, thereby exacerbating any damage the kickback might cause.

But yeah - ignition timing sure matters - - -
 
I have had kickbacks on perfectly timed engines - particularly the SF-4 and SF -7 mags. I had a hot start kickback on a C-85 with SkyTek, and it sheared the pin. What a giant PITA. Easy to fix, but still . . .

I am having spectacular luck with B&C on two 85s - one of them a Stroker - with dual impulse mags. I cannot say enough good about the B&C.
 

If it kicks back it doesn't "go past compression", unless perhaps the next cylinder. It kicked back because it fired before TDC. Before the max compression was reached. It's that initial kick that will cause any damage, to starter or fingers.


Some confusion there. The fact that they don't have as much rotational inertia is precisely why they can kick back with greater acceleration and thus speed, thereby exacerbating any damage the kickback might cause.

But yeah - ignition timing sure matters - - -

Gordon,

If you say so, but in a lot of hours working that prop/engine combination, I've never had any kickback issue. In fact, due to the low inertia of the blades, the prop barely wiggles at shutdown. Just stops.

I can't say that I've never had kick back issues with the O-360 equipped with a metal prop, though.

MTV
 
Good information. I would be P mag equipped. Anyone have experience between the skytec XLT and NL? The B&C lightweight starter is 10.2 pounds so not in the running for me. Additionally, several engine quotes I have recently received have all included a skytec starter.
 
Come on Stewart, jump back in the game . . . .

What I'm getting at is that a kickback happens because the spark plug fired before TDC, thus pushing the piston back down and turning the crank backwards. Does it happen? Of course! but it happens because the spark occurred 'to early'. This can be caused by mag timing (internal/external), worn impulse couplers, or mis adjusted electronic ignition components. Can we all agree on that?

So the debate comes down to WHEN it occurres, does it help or hurt to have a heavier rotating mass in the prop and flywheel. Just from my own observation, I don't feel it makes much difference either way. Seems the extra mass might help the piston continue up and past TDC in spite of the early spark, IF it has enough speed. Can enough speed be generated from cranking a starter motor? But it could also be argued that that same rotating mass could enhance damage when moving in the wrong direction. With a light weight setup will the higher rotating speed cause more damage than the heavier, slower stock setup? All I have is observations.

I'm more concerned with eliminating the problem from happening in the first place. Stop the kickback and the rest is just an argument over beer.

Web
 
I have had kickbacks on perfectly timed engines - particularly the SF-4 and SF -7 mags. I had a hot start kickback on a C-85 with SkyTek, and it sheared the pin. What a giant PITA. Easy to fix, but still . . .

I am having spectacular luck with B&C on two 85s - one of them a Stroker - with dual impulse mags. I cannot say enough good about the B&C.
Bob, pleading ignorance here - - Do those engines' mags have impulse couplings? I'm thinking my C75 T-Craft didn't. But it's been a few decades - - - -
 
And make sure the non impulse coupled mag is OFF when starting!

Web
Although I prefer dual ST switches, my plane came equipped with a Bendix switch and I've not yet swapped it out, but I DID rebuild it a couple years ago. When I pulled it out guess what... no R ground shunt. I do NOT have dual impulses. I don't know why it never kicked back, I guess I just got lucky. When a friend was wrapping up rebuilding our old Breezy, I timed it for him. Guess what I discovered, no R ground shunt, and again only one impulse. There must be gnomes that steal these things in the night or something.
 
I have the Sky tec HT starter, with the self-resetting kickback clutch (no shear pin to replace). Never had a problem, kickback or otherwise, and I start with both mags on (toggle switches, not a key switch). It's lighter than the NL. At one time, it was the leading starter. No one seems to want to put it in anymore, and I don't understand why.
 
Because the big guys now own the company?

Gordon - all the Bendix SF series mags are non-impulse. We are using S-4s on all the Cubs, dual impulse. My Decathlon has Slick single impulse, and I really should start it on the Left mag - but I have been starting it on both for 15 years, without kickback. 180 hp Lyc.
 
Although I prefer dual ST switches, my plane came equipped with a Bendix switch and I've not yet swapped it out, but I DID rebuild it a couple years ago. When I pulled it out guess what... no R ground shunt. I do NOT have dual impulses. I don't know why it never kicked back, I guess I just got lucky.
Just curious, what make of mags do you have? Some mags, especially those which have weak magnets will not fire very well at low rpm. Unless they have an impulse they will not fire until the engine is running. If your non-impulse mag has a weak magnet and is on while you are starting, it will not kick back.
 
Long ago I was involved with an Enstrom helicopter. It had a habit of breaking the casting on the starter while kicking back during starting. I attributed it to the lack of flywheel action because it wasn't connected to a propeller. It had a shower of sparks retard spark starting system. Had I known then what I know now, I would have retarded the "starting points" in the mag. The shower of sparks mags have two sets of points. One for starting and the other for running. This observation backs up Gordon's mention of the heavier the prop the easier the engine will turn through compression, resisting any kick back tendencies.
 
I have Slicks. They're the rebuildable ones but I don't know the model # off the top of my head. I've had them overhauled twice over the years and they seem to perform fine on the tester/dyno. The local shop has one, and always runs them for an hour and then re-checks e-gap. They run great on the airplane, but I have a Skytek starter and a light prop so my crank speed is kinda slow. Especially the "over the top" speed as it comes up on compression. I probably wasn't getting enough out of the R mag to make a spark.
 
I was going to guess Slicks. Their magnet doesn't seem to be as strong as Bendix in my experience, therefor they need to turn a bit faster to get a hot enough spark for a strong kickback if they are not timed correctly. So in your case you were saved from the kickback due to the type of mag.
 
My understandingis that the XLT starter is a continuation of the LS/PM starter which I have not had long term reliability out of on O-360 engines but have had great service on the O-320s. The NL has been a great starter on the O-360 in my experience.
 
Interesting approach by E-Mag on dealing with kickback from low mass props: https://emagair.com/tips-and-tricks/
They simply lag by 4 degrees during startup.

Below quoted from their P model installation manual.

Units with firmware V40 (and after) have an automatic 4 degree
starting lag to make certain (start) firing occurs well after TDC.


 Prior to firmware V40, start firing occurs where the ignition is timed.
These units can implement a starting lag by CLOCKING the engine 2-3 degrees AFTER TDC. This will shift startup firing, and will also shift the operating range (in the less aggressive direction).


Background: Low-mass props can decelerate rapidly as the starter motor pulls thru each compression stroke (TDC being the top of each compression stroke). If the prop slows enough, it’s effectively become stationary when it reaches TDC. In these conditions, combustion can send the prop backwards. Delaying startup firing is a simple hedge against this risk.
 
Nothing new. Make sure you know if your engine needs 20° or 25° BTC timing and time Pmags at TC. If you time a 20° motor like a normal 25° Lycoming? Bad things happen.
 
Wilco.
I do like that these mags allow for the change up in timing between start-up and run phases.
 
Nothing new. Make sure you know if your engine needs 20° or 25° BTC timing and time Pmags at TC. If you time a 20° motor like a normal 25° Lycoming? Bad things happen.

A bit off-topic, but I've never understood why these airplane engines vary so much on the ignition timing.
As I recall, it was 28 BTDC on my old O-200/O-300's, 25 on my old O-320, and 22 on my current O-470.
It even varies between different versions of the same basic engine--
for example the O-470 engines as used in the C180: A @ 26 degrees; J @ 20; K, L, R, & S @ 22; U @ 24.
 
Advanced timing can cause increase in CHT's and detonation. Some engines handle it better than others compression ration also comes into play. Very common problem for the new experimental guys with high HP engines and restricted cowlings having CHT'S at 400 or greater in cruise with advance timing curve, usually easy to do with electronic mags. Pretty easy to solve by backing down the timing.
DENNY
 
A bit off-topic, but I've never understood why these airplane engines vary so much on the ignition timing.
As I recall, it was 28 BTDC on my old O-200/O-300's, 25 on my old O-320, and 22 on my current O-470.
It even varies between different versions of the same basic engine--
for example the O-470 engines as used in the C180: A @ 26 degrees; J @ 20; K, L, R, & S @ 22; U @ 24.

Timing controls when peak cylinder pressure occurs relative to crank angle. Here’s a link that provides a pretty good explanation. This pretty much summarizes what I was told during a very friendly and long conversation with the head guy at Emag. We talked about my condemned 400, the new 390, and compared them to parallel valve 360s and 375s. Different cylinders, different pressures, different timing specs.

Pmags have the ability to run on two pre-set curves. A is 34° max advance, B has 39° max advance. Mr Emag told me I should remove my A/B curve switches and stick with A curve for my engine in my Cub. If memory serves the B curve is better for LOP. I’ve run my 390 at 20° and 15° timing and can’t recognize any difference, so I use 20° per the engine data tag. Translation- my Pmags are timed at 5° after TC to compensate for Pmag default 25° start-up advance.

https://www.sciencedirect.com/topics/engineering/ignition-timing
 
Wilco.
I do like that these mags allow for the change up in timing between start-up and run phases.

They are doing, electronically, what impulse couplings do, mechanically. When impulse couplings catch on the pin, the drive gear continues to rotate, bringing the spark closer to TDC.

Web
 
Pmags (and other EI that I know of) adjust timing depending on engine load (MP). Pmags have two pre-set curves or you can add a computer port and manipulate the values to whatever you want (EICAS)
 
They are doing, electronically, what impulse couplings do, mechanically. When impulse couplings catch on the pin, the drive gear continues to rotate, bringing the spark closer to TDC.

Web
The impulse couplings work for start up only. Serving two purposes. One to retard the spark for starting and the other to spin the mag at a higher speed to generate the spark. The P-mags work on start up and shut down, utilizing manifold pressure and rpm for it's decision making, while electronically producing a hot spark. Thus eliminating kick back during shutdown making for a smooth shut down. This is particularly noticeable on the 0-360 engines which normally have a rough shaky shut down with mags, particularly when shut down from an idle rpm.
 
Just pointing out that the lag on startup produces the same effect as the lag produced by the impulse coupling.

As to all the other features, the LASAR system was doing that years ago.

Web
 
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