LSP972

My Feedback: (13)

What will affect the load on an engine the greatest: Diameter, pitch, or mass? I realize chord is a factor as well, but lump that in with pitch for now.

Assume all other parameters (engine, rpm, airplane, air density, phase of the moon, etc.) are identical.

Here's the reason for asking:

I'm trying to determine the best prop for a YS 63 on a SIG Somethin' Extra. I've only flown it so far with an APC 13x6 Sport prop. It makes a pylon racer out of the model; I want to give it a wider slow-speed envelope without loading the engine down. I have the following to try:

1. Top Flite 14x5 wood
2. DynaThrust 13x6 nylon
3. Master Airscrew 14x6 wood
4. Top Flite 14x6 Power Point wood

Thinking about trying APC 13x4W or 14x4W.

Have at it...

Steve

Rudeboy

An inch increase in diameter give you much more load than an inch increase in pitch. Mass only has an effect when going from idle to full throttle and back. Your engine will take longer both ways with a heavier prop. Also, due to the inertia of a heavy prop, your airframe will be subjected to more intense vibrations.

If your plane is to fast, go one inch up in dia. and one inch down in pitch. So I'd say a 14-5 or a 14-6. Take her out for a spin and see how she does.
Finding the right prop for a plane-engine combination can take a few tries, but it is well worth the time.

LSP972

My Feedback: (13)

Anyone else with ideas?

shupack

rude boy hit the nail on the head, except for

quote-Also, due to the inertia of a heavy prop, your airframe will be subjected to more intense vibrations.-Quote

from an engineer's standpoint i think that's wrong. the heavier prop will act to absorb engine vibrations, not amplify them. a lighter prop won't amplify them either, but will absorb less. unless you're prop is out of balance, then no matter what the mass is, it'll shake the hell out of your plane.

Deanoo

What about a 3 bladed prop? Would that too help reduce engine vibration? What are the advantages of using a 3 bladed prop?

Steve Collins

My Feedback: (8)

I am currently using a Graupner 12.5 X 7 3 blade prop with my YS .63 on a Great Planes Corsair. It was too much prop running on 15% nitro. I am now using 30% heli fuel and the engine is now quite happy with this prop.

I don't notice any significant engine vibration on any of the props I have used on this engine. All of the props have been APCs.

Hobbsy

My Feedback: (102)

Steve, it's been my experience that you can get a slower smoother idle with a three blade, but both my YSs will idle at 2,000 so that is not an issue here. The combination of a three blade with less pitch and the YS slow idle would give you some of the braking effect you are looking for. If your plane is already faster than you like, then the reduced pitch and very slight loss of effeciency of the three blade would be welcomed.

downunder-RCU

I know that there's an actual advantage with a 3 blade prop and IF I remember correctly it's that in a very hard turn up or down with a 2 blade prop then there's a difference in angle of attack between either blade. Effectively this means that during this hard turn the blades have different pitches and so develop different thrust on either side which tends to make the plane yaw. This problem apparently doesn't happen with a 3 blade prop because of the spacing of the blades.

LSP972

My Feedback: (13)

Thanks for all the replies. I'm hoping the 14x5 or 14x4W will do the trick; if not, I'll be looking at three bladers.

Hobbsy, mine holds a steady 1960 with the 13x6 APC Sport. So idle is not a problem; I just want to slow the thing down a bit, but am concerned about loading the YS up and causing it to overheat.

Next, I'll start tweaking the regulator screw to keep the top end rich while preserving the idle.

I know; tread carefully there...<G>

Steve

Rudeboy

How will a heavier prop "absorb" vibrations?

I am by no means an engineer but I like to think about things so this is how I see things:
A heavy prop is harder to accelerate, therefore, during the combustion stroke, the reaction on the torque generated by the engine is that the engine (thus also the airframe) will try to move in the opposite direction more than it would with a light prop.
Have you ever mounted an engine on swing rubbers? All mine are, and I can clearly see the difference in engine movement between a light and a heavy prop. Particularly at and just above idle rpm's you see the difference. Also the resonance frequency of the entire system changes with the use of lighter or heavier props.

shupack

the heavier prop should absorb vibrations just due to the increased mass...

you're right about the acceleration, especialy with single-cylinder engines, since there is power applied to the prop once per revolution (or every 2 for a 4 stroke) the combustion accelerates the prop for about 1/4 of a turn, then the prop decelerates the rest of the way round untill the next combustion, so the prop is accelerating and decelerating constantly. a heavier prop will act as a flywheel and smooth out the power pulses, but the larger prop will also put more load on the engine, making acceration harder, and the deceleration more profound, which gives you more engine-movement in the rubber mounts. the rubber mounts serve to smooth the pulses by allowing the engine to move counter to the prop (violently) instead of jerking the prop up to speed, as the rubber de-compresses the engine returns to a normal postion, but since the crank has moved on by then, that power isn't put back into the prop speed, so you effectively lose HP at the prop through the soft mounts. this engine movement is completely different from vibrations due to being out of balance, it is a pure torque reaction. at higher rpms, the engine can't move back and forth completely between each combustion, because of the dampning action of the rubbers, so it apears to vibrate from the power-pulses.
I believe the only way to tell the difference between balance induced vibration and power pulseing would be to remove the plug, and spin the engine with an attached electric motor (a starter would dampen the vibrations).

Geistware

My Feedback: (16)

One inch of diameter loads the propeller more than one inch of pitch.

Hobbsy

My Feedback: (102)

Shup, I posted on here somewhere that a single cylinder Continental gocart racing engine had a flywheel on the cam shaft to prevent tortional impulses caused by the valve springs from thrashing and in turn destroying the timing gears. Many people mistakenly think that big Saitos vibrate a lot but are mistaking tortional impulses caused by the engines light weight and strong power pulses for vibration. For example, I put my Old high compression Saito 150 on a Sullivan DynaMount and as you stated above, at speed it moved very little but at idle and just above the rocker arm cover end of the engine moved a lot, too much to suit me. My Saito 100 moves very little and my YS .91FZ hardly at all. The prop driver end of all three does not move at all, at least not that you can see. I could go on here because I love to try and check things out but this is long enough.

downunder-RCU

Just to follow on with what shupack said, there can be a surprising change in prop speed due to the torque variations over each cycle. But a picture is worth a thousand words. I have a graph at http://members.coolcats.net.au/~scep...stant_rpm.html (courtesy of Brett Buck) which shows the calculated RPM at all points of rotation. The graph of course is specific to that particular engine and it's method of operation and the prop used was a 12x5 wood which would give a greater variation in revs than a heavier prop. The link at the top of the graph goes to another similar graph showing the calculated torque (both positive and negative).

shupack

wow, almost 400 rpm change in crank speed over one revolution, that's violent. it says over the top of the graph "four stoking" does that mean it's running so rich it's firing on every other stroke? that's what the graph seems to show. i think the only way to get rid of torqe impulses would be a mult-cylinder engine, now if they were just a little cheaper......
I remember reading somewhere that a single of a certain displacement will put out more power than a twin of the same displacement, i can't see why that would be true, can anyone shed some light?

Flypaper 2

A twin has more surface area friction drag than a single of the same displacement. Also more volumetrically efficient.
I wonder why no one has tried a floating counterbalance as used on Wright Cyclone since 1939, to smooth out torsional impulses. This is not a dynamic balancer as we know it.

Boomstriker

The more weight that is spinning, the more the torquing affect to the plane.
Engines with heavy flywheel/fan magnetos, heavy props, spinners, hubs and heavy nuts have large rotating mass.
Remember the old WWI radials that spun the case and all the jugs with the prop, and the crank was stationary?!!
All the energy to spin this weight up to speed is transfered to the plane with every movement of the throttle.
If this weight is kept to a minimum, you will have excellent throttle response and less torquing.

Kirk

big max 1935

My Feedback: (9)

I have read all these threads & have come to the conclusion that there are a lot of people smarter than me! Me, I would have dumbed my way into using a 4 pitch prop with a 13" or 14" diameter & give it a go. That will keep the revs up with a good bit of pull & slower speed. Just don`t let it fall out of the sky!>>>>>
>>>>>>>>>>big max 1935>>>>>>>>>>>>>>>>>>>

downunder-RCU

The problem with the old rotary radials wasn't the torque reaction needed to get them up to speed but the fact that their huge gyroscopic precession forces made the planes almost impossible to bank and turn in one direction (depending on which way the engine revolved). Large rotating masses just give a slower throttle response.

shupack

flypaper, i've never heard of the floating counterbalance you're talking about, how's it work?

Flypaper 2

Not very good at explaining things but I'll give it a go. There is a tab on the crank where the counterweight would normally go. the weight has a slot in it that fits loosly over the tab. two bolts or pins hold the weight to the tab. Now drill the holes in the tab about 3 times the diameter of the bolts, or pins if you will. So now the weight floats in the enlarged bolt holes and allowed to move sideways around the circumference. As the crank accelerates on the firing stroke the weight trails behind the crank slowing its acceleration down. when the crank reaches the 90' point the crank starts to slow down causing the weight to move forward, by its inertia, forcing the crank to accelerate. With the right weight and motion the weight will smooth out the torsional vibration to 'O'. Works at all RPM. At high RPM centrifugal force allows the weight to move only a small amount. At low RPM it moves over a larger radius because of the lower centrifugal force. This is where it differs from an automotive dynamic balancer. Would like to copy the article out of the engine manual but I don't know how copyright works after 54 yrs. I have an old Q35 Iwill try it on and let you know wether it's worth it or not. This has nothing to do with engine parts balance, only rotational vibration from acceleration,deceleration, as in shaking itself to pieces at idle.

BasinBum

My Feedback: (2)

Steve,

I have a SE with a YS 63 and the APC 13X6 is the best prop I've had on it. If you come up with something better let me know! I use to fly it with a 46fx and 11X5 APC and it was quick and twitchy. Now with the four stoke I've gone to the recomended throws (which I thought were insane when I had the two stroke on it).

RD

Steve Campbell2

RD,

My first SE I built from the kit, with a .46 SF for power. It flew well, but I agree; this design begs for a four-stroke.

I've tried a half-dozen different props; a Top Flite 14x5 wood has been the best, but of course I broke it on a sloppy landing last week-end. I've got a Dyna-Thrust 13x6 (nylon, wide-chord blades) on it now, and it does well. But I ordered an APC 14x4W to try.

The APC 13x6 Sport (skinny blades) did fly it fine, but too fast for my tastes.

I'm hoping this 14x4W is the ticket. For sure, the APC 16x4W is THE prop (IMO) for a sport/3D plane with a YS 91. I've tried that one on two different models (UCD and Kangke CAP 232 Sport) and it is simply awesome.

If the APC 14X4W doesn't work out, I'll buy a passle of Top Flite 14x5s...

Steve