Pistons & Props

[weirdjim]

OK, folks, I've argued myself into a hole that isn't lending itself to gentle logic. And, I should know this, having had my share of greasy hands.

Here is the question: In a regular old horizontally opposed aircraft engine, for one complete revolution of the propeller how many trips does the piston make up and down the cylinder?

Jim

 

[Greg Bockelman]

One. It takes two full revolutions to go through all four strokes. Intake, compression, power and exhaust.

And I believe that is true for most traditional piston engines, regardless of configuration.

The caveat is that the prop is not gear reduced. IOW, bolted directly to the crankshaft.

 

[Let'sgoflying!]

one up, and one down per rev.

 

[weirdjim]

one up, and one down per rev.

That's the position I argued myself into and out of half a dozen times this morning. Thanks.

Let me extrapolate on this answer a bit. I'm in the process of designing a homebrew vibration sensor for the magazine. While the vibration of the propeller can be sensed at most any angle, the strongest component of piston/rod mass imbalance (in a pancake h.o. engine) is horizontal. If you take the vertical component (from prop imbalance) and subtract it from the horizontal imbalance, you SHOULD have the engine imbalance as the result.

And, both prop AND engine imbalance frequency will be the same...one cycle per revolution, so the AC signal coming from the vertical sensor and signal coming from the horizontal sensor should be out of phase. This is no big deal to account for AND with a little geometry ought to tell me which cylinder is the offender AND where to put the mass on the prop backing plate to balance out the shake.

A good 2-channel o'scope should also show me which of the blades of the prop is the offender AND where to put the weight for the engine imbalance.

Hmmmm....so far so good.

Thanks again.

Jim

 

[weirdjim]

Let me build on my last comment and please don't hesitate to correct my math.

In a 4-cylinder engine, as you stated, it takes two revolutions to give all four cycles of the 4-stroke engine, with two exhaust "pops" per prop revolution. In a 6-cylinder this would be 3 exhaust pops/rev. Yes?

That means that the FREQUENCY of the exhaust pops is either double or triple the frequency of the prop. Did I get that right?

Example, at 2400 RPM (40 RPS) the frequency of the prop vibration is going to be 40 Hz, while a 4-cylinder exhaust is going to be at 80 Hz, and the 6-cylinder is going to be at 120 Hz. Did I do that math right?

Jim

 

[gismo]

Let me build on my last comment and please don't hesitate to correct my math.

In a 4-cylinder engine, as you stated, it takes two revolutions to give all four cycles of the 4-stroke engine, with two exhaust "pops" per prop revolution. In a 6-cylinder this would be 3 exhaust pops/rev. Yes?

That means that the FREQUENCY of the exhaust pops is either double or triple the frequency of the prop. Did I get that right?

Example, at 2400 RPM (40 RPS) the frequency of the prop vibration is going to be 40 Hz, while a 4-cylinder exhaust is going to be at 80 Hz, and the 6-cylinder is going to be at 120 Hz. Did I do that math right?

Jim[/quote]


Yes but don't expect that all of a 4 cylinder 4 stroke engine/prop vibrations will be at 2X the RPS. There are sources at 1/2 (e.g. cam related, and combustion imbalance), 1X (e.g. prop imbalance, crank imbalance, piston imbalance), 2X (e.g.balanced combustion, 2 blade prop thrust imbalance), 3X (3 blade prop thrust imbalance), plus a few other frequencies due to accessories spinning and several harmonics of every fundamental source. In addition I think you'll find that even without a prop a HO engine will have vibration modes outside the horizontal plane due to the "twisting" of the engine case and crankshaft plus the effects of the engine mounts which can translate inertial loads to a different direction than the excitation.

 

[weirdjim]

Yes but don't expect that all of a 4 cylinder 4 stroke engine/prop vibrations will be at 2X the RPS.

I wouldn't expect them ALL to be at 2X; the Fourier series can be expanded up as well as down. And I really can't tame the harmonics as the amplitude of the fundamental will most likely far exceed any sub or super multiple.

As to the amplitude being less than exactly horizontal, that is understood, but the total vibration amplitude will at least be SEMI horizontal, certainly not vertical which is where I plan on sensing the prop vibration. Somehow I can't see the engine twisting itself 90 degrees on its mounts nor the crankshaft twisting more than a fraction of a degree without shattering.

Jim

 

[gismo]

I wouldn't expect them ALL to be at 2X; the Fourier series can be expanded up as well as down. And I really can't tame the harmonics as the amplitude of the fundamental will most likely far exceed any sub or super multiple.

As to the amplitude being less than exactly horizontal, that is understood, but the total vibration amplitude will at least be SEMI horizontal, certainly not vertical which is where I plan on sensing the prop vibration. Somehow I can't see the engine twisting itself 90 degrees on its mounts nor the crankshaft twisting more than a fraction of a degree without shattering.

Jim

Consider an engine that wants to oscillate left and right only but is mounted on angled rubber isolators located under the engine. When the engine shifts laterally it may be forced to rise some by the isolators. This is an example of the translation I was talking about. Also, each time a cylinder fires there is a torque reaction through the crank to the prop which will cause the engine to twist in it's mount with similar results in terms of vertical movement. All I'm saying is that I think the system is complex enough that you will find vertical movement unrelated to the prop's weight balance. I think you should go ahead and try your experiment to verify all this, just keep an open mind WRT vibration sources. If you've ever see a magnified high speed video or movie of an aircraft engine running you might be surprised how much everything is moving around relative to other parts of the engine. Cylinders move in and out, up and down, the case twists etc. The parts don't move very far but they move pretty fast.

 

[Dan Thomas]

Consider an engine that wants to oscillate left and right only but is mounted on angled rubber isolators located under the engine. When the engine shifts laterally it may be forced to rise some by the isolators. This is an example of the translation I was talking about. Also, each time a cylinder fires there is a torque reaction through the crank to the prop which will cause the engine to twist in it's mount with similar results in terms of vertical movement. All I'm saying is that I think the system is complex enough that you will find vertical movement unrelated to the prop's weight balance. I think you should go ahead and try your experiment to verify all this, just keep an open mind WRT vibration sources. If you've ever see a magnified high speed video or movie of an aircraft engine running you might be surprised how much everything is moving around relative to other parts of the engine. Cylinders move in and out, up and down, the case twists etc. The parts don't move very far but they move pretty fast.

We have a Chadwick-Helmuth Vibrex 2000 prop balancer that uses one transducer, mounted vertically on the top front of the crankcase. It senses only vertical movement, and that's enough to give the machine an idea of prop imbalance. A reflector on the prop and a small LED and receiver mounted with the transducer give the box a position of the imbalance. The whole thing works well enough. There's an option for a second transducer to be mounted at the top rear of the case to determine any imbalance in the aft end of the engine, though it's a bit harder to fix that.

Dan

 

[weirdjim]

We have a Chadwick-Helmuth Vibrex 2000 prop balancer that uses one transducer, mounted vertically on the top front of the crankcase. It senses only vertical movement, and that's enough to give the machine an idea of prop imbalance. A reflector on the prop and a small LED and receiver mounted with the transducer give the box a position of the imbalance. The whole thing works well enough. There's an option for a second transducer to be mounted at the top rear of the case to determine any imbalance in the aft end of the engine, though it's a bit harder to fix that.

Dan

I'm going to go out on a limb here and say that the imbalance of the prop alone will either be on a line with one prop blade or the other. The idea of the reflector and LED/phototransistor is good for a commercial unit, but to try and do a "homebrew" article in the magazine with easy to find parts, that approach is a little sophisticated. Perhaps in the "Rev B:" article.

My hit is going to use those stick-on wheel weights as a temporary "fix". Fire the engine up, get a read on vibration amplitude, stick a small wheel weight onto the backing plate immediately behind one blade, fire up again, see if vibration went up or down, if up, move the weight to the other blade, if down, increase the weight until the vibration goes through a minima, and then back it down to the minima. Take the weight, measure it or put it on a balance beam and find a bolt-nut-washer combination of that same weight and fasten it to the backing plate in the dead center of where that wheel weight was.

Then turn the sensor horizontal. Repeat.

The sensor? Some form of tiny dynamic (closed back) speaker glued to a mounting bracket, or a piezo speaker, or an electret microphone, or a phonograph cartridge. DIY electronics.

Labor intensive? Sure, but so is building the airplane.

Jim

 

[Dan Thomas]

Labor intensive? Sure, but so is building the airplane.

Jim

A few hours building a basic balancer is better than springing $5000 for the Chadwick if you don't have the business income to pay for it.

Dan

 

[N801BH]

This guy explains the dynamics about as good as one can and not get so technical as to glaze over common peoples eyes. I can add alot more but I have a hard time transfering tech stuff to readable data for the regular guy. Also my prototype engine is a 1.43-1 motor so that makes it a bird of a different feather for your concept. See Kitplanes/ Dec issue experimotive article.

As a side note, when you do find the "sweet spot" for your final weight you will get better dampening by split weighting it instead of putting it all in one spot. An easy calculation but a little more work to affix the weights.

That means that the FREQUENCY of the exhaust pops is either double or triple the frequency of the prop. Did I get that right?

Example, at 2400 RPM (40 RPS) the frequency of the prop vibration is going to be 40 Hz, while a 4-cylinder exhaust is going to be at 80 Hz, and the 6-cylinder is going to be at 120 Hz. Did I do that math right?

Jim

Yes but don't expect that all of a 4 cylinder 4 stroke engine/prop vibrations will be at 2X the RPS. There are sources at 1/2 (e.g. cam related, and combustion imbalance), 1X (e.g. prop imbalance, crank imbalance, piston imbalance), 2X (e.g.balanced combustion, 2 blade prop thrust imbalance), 3X (3 blade prop thrust imbalance), plus a few other frequencies due to accessories spinning and several harmonics of every fundamental source. In addition I think you'll find that even without a prop a HO engine will have vibration modes outside the horizontal plane due to the "twisting" of the engine case and crankshaft plus the effects of the engine mounts which can translate inertial loads to a different direction than the excitation.[/quote]

 

[gismo]

Yes but don't expect that all of a 4 cylinder 4 stroke engine/prop vibrations will be at 2X the RPS. There are sources at 1/2 (e.g. cam related, and combustion imbalance), 1X (e.g. prop imbalance, crank imbalance, piston imbalance), 2X (e.g.balanced combustion, 2 blade prop thrust imbalance), 3X (3 blade prop thrust imbalance), plus a few other frequencies due to accessories spinning and several harmonics of every fundamental source. In addition I think you'll find that even without a prop a HO engine will have vibration modes outside the horizontal plane due to the "twisting" of the engine case and crankshaft plus the effects of the engine mounts which can translate inertial loads to a different direction than the excitation.

Ben, somehow your quoting got hosed. The words I quoted from your post are what I wrote and the part your post attibuted to me was written by Jim. No big deal but it's a little confusing.

Also, do you have a link to the article you were referring to? I'd be interested in reading it.

 

[N801BH]

Ben, somehow your quoting got hosed. The words I quoted from your post are what I wrote and the part your post attibuted to me was written by Jim. No big deal but it's a little confusing.

Also, do you have a link to the article you were referring to? I'd be interested in reading it.

Sorry about the quote thing. I must have configured the type incorrectly.. To guys who subscribe to Kitplanes the article is partly on the front cover and details are on a few pages further back. To the others who don't just PM me for a way to view it ,http://www.kitplanes.com/issues/26_12/exploring/9026-1.phtml

without having to pay the 7.50 PPV.

 

[gismo]

Sorry about the quote thing. I must have configured the type incorrectly.. To guys who subscribe to Kitplanes the article is partly on the front cover and details are on a few pages further back. To the others who don't just PM me for a way to view it ,http://www.kitplanes.com/issues/26_12/exploring/9026-1.phtml

without having to pay the 7.50 PPV.

Interesting project Ben, sounds like a lot of fun. I especially liked the description of your first attempt at a full power takeoff at low DA, sounded like an underware damaging moment.

I do have one concern, specifically about your lack of carb heat. Carb ice doesn't normally form as a result of the conditions your described, it's far more likely to occur when the temp is around 70F and the humidity is very high. What you apparently tested for is "impact ice", something that normally shows up on the air filter not the carb throat. A fairly simple way to check for the potential of carb ice is to rig a way to read the temp of the throttle butterfly, or better yet the fuel nozzle behind it. As long as that remains above freezing when the air warmer than a few degrees below freezing you won't have an issue with carb ice which forms as a result of the adiabatic cooling in the high velocity area near the throttle plate.

 

[Dan Thomas]

As long as that remains above freezing when the air warmer than a few degrees below freezing you won't have an issue with carb ice which forms as a result of the adiabatic cooling in the high velocity area near the throttle plate.

Or the evaporative cooling of the fuel entering the airstream. The temp drop due to pressure drop is as much as 40°F. With the evaporative cooling added, it can run to 70°F, so carb ice is a threat up to around 100°F. It can also occur at temps to minus 5°F or so, since water can exist as supercooled droplets down to that temperature.


Dan

 

[N801BH]

Interesting project Ben, sounds like a lot of fun. I especially liked the description of your first attempt at a full power takeoff at low DA, sounded like an underware damaging moment.

I do have one concern, specifically about your lack of carb heat. Carb ice doesn't normally form as a result of the conditions your described, it's far more likely to occur when the temp is around 70F and the humidity is very high. What you apparently tested for is "impact ice", something that normally shows up on the air filter not the carb throat. A fairly simple way to check for the potential of carb ice is to rig a way to read the temp of the throttle butterfly, or better yet the fuel nozzle behind it. As long as that remains above freezing when the air warmer than a few degrees below freezing you won't have an issue with carb ice which forms as a result of the adiabatic cooling in the high velocity area near the throttle plate.

Thanks for the concern on carb icing. The aluminum engine transfers heat so well the carb never gets cool enough for ice to grow in the venturi. With that in mind that is why I did the 33 f and freezing drizzle low passes as that should have duplicated your concept of 70f days, warm air in the cowling heading down the carb, that is moisture laden and a big pressure drop though the throttle plates. Impact ice is not an issue in my mind, Remember, the Holley sits on top of the motor and heat rises.:smile: . Not like a regular aircraft carb suspended below the motor in a cool area .I have had it flying for more then 300 hours now, in all sorts of weather conditions. ie. -35f up to 92f in clear skies, with visible moisture streaming off the windshield, snow squalls and everything in between. Like the article said, it might build ice but it is going to take one heck of a weird atmosphere condition to do it. There is a reason the EXPERIMENTAL sticker is posted on the side. <GG>.

Ps, glad you liked the article. I thought the author did a great job. I never met him before or since but I will buy him an "adult beverage' next time I see him.

Tailwinds.