Does RPM have an effect on.....???

I flew a 14" two blade on my 56 model for a while and while it didn’t have the hardest corner in the world it didn’t seem to have a great deal of resistance to change when flying the pattern. By this I mean when you wanted to fly a round loop you just get into a loop and the plane would track nicely and you could get three repeatable loops. On the straight leg maneuvers it seemed to have a soft corner but getting correct angles was a breeze. You didn’t have to over control it.

I ran that prop at 8600-8800 RPMs on the ground at launch.

For three years now when I have been on the Two stroke motors I have been flying on a 12" prop, 4blades. It turns fine. It tracks in the loops real nice and the angles are easy to get correct, well most of the time.

But for the past few flying sessions I have been on an Eather three blade that Brad gave me for Christmas. 13" by 3.75. I tweaked the tips flat to kill all acceleration and I am running it about 10200-10300 at launch. It is moving along pretty quick at a 5.75 lap time. It has all kinds of killer line tension but it wont turn corners as well. Duh...the diameter comes into play I know. BUT it wont track in the rounds either worth a darn. It is always trying to straighten out as it flys a round loop. Result is a nasty looking set of three loops with corrections everywhere. This is the first time in a long time I have taken a good look and worked with a larger than 12" prop. A couple of other flyers here tried the larger three blades over the past season with very similar results.

Normally I would just chalk it up to diameter and move on, but the 14" two blade seemed to turn easier with much more repeatable performance. It has an extra inch in diameter and doesn’t seem to have as much effect on the corner and the accuracy as the shorter 13" three blade.

I am wondering if it is the difference in RPM?

I fully understand that the spinning larger prop will have more resistance to change direction but it 13” at a higher rpm is way more resistant than the 14” at a lower rpm.

Any ideas???

LAST EDITED ON Jul-31-03 AT 12:05 PM (CDT) by ()
 
I am not sure if I am answering your question, but the resistance to turning (the gyro effect) is directly proportional to the rpm. It is also proportional to the SQUARE of the mass distribution (the moment of inertia). So as normal, it depends. In general a 13 inch prop will have a larger moment of inertia than the 12" prop.

Which factor wins out would depend on how the mass (and the amount of mass) is distributed along the propeller length.

You could always cut your prop into 1" segments, weight them, and sum up the weight * the square of their position along the prop length. Of course this tends to destroy the value of that nice carbon fiber prop!
Alan

>I am not sure if I am answering your question, but the
>resistance to turning (the gyro effect) is directly
>proportional to the rpm. It is also proportional to the
>SQUARE of the mass distribution (the moment of inertia). So
>as normal, it depends. In general a 13 inch prop will have a
>larger moment of inertia than the 12" prop.

Square of the radius of gyration, but proportional to the moment of inertia. I = mr^r where I= moment of inertia, m=mass, and r=radius of gyration. Radius of gyration is a measure of where the mass is.

>Which factor wins out would depend on how the mass (and the
>amount of mass) is distributed along the propeller length.
>
>You could always cut your prop into 1" segments, weight
>them, and sum up the weight * the square of their position
>along the prop length. Of course this tends to destroy the
>value of that nice carbon fiber prop!

A torsion pendulum is a less-expensive alternative.

Elvis could gyrate his hips something fierce. I assume it was the gyorscopic effect of his swirling hips that kept him upright on stage whenever he was out of this world on goof-balls.

Brett,
Well I meant to say the moment of inertia (I) depends on the integral of mass and r^2, and since the angular momentum =I*w (w is angular velocity and is proportional to rpm), the net effect is scaling linearly as the rpm and prop mas and more complicatedly as the square of the prop radius. The larger this angular momentum, the harder it is to turn the plane.

Igor,

I don't know what a 2 blade vs a three blade prop has to do with angular momentum, since I think the prop is spiining through a relative large angle during the attempted turn, so I would think to first approximation, the number of blades doesn't matter--assuming you are calculating the moment of inertia correctly of all blades around the spinning axis.

>Igor,
>
>I don't know what a 2 blade vs a three blade prop has to do
>with angular momentum, since I think the prop is spiining
>through a relative large angle during the attempted turn, so
>I would think to first approximation, the number of blades
>doesn't matter--assuming you are calculating the moment of
>inertia correctly of all blades around the spinning axis.
>

The 'dc' component is the same regarldess of the number of blades. The induced vibration ('ac')is of course different. I doubt that it makes a lot of practical difference since you are getting maybe 50-60 revolutions during a corner, and the net difference between getting 55.5 versus 55 revs isn't all that significant.

Of course, a 3-blade isn't constant in torque delivery. It's just somewhat smoother than a 2-blade.

Brett, it really happens only on one or two blade prop. Three and more blades are perfectly balanced and does not show any of those problems.

Allan, number of blades has lot to do with that “resistance”. The gyro moment itself does not make any “resistance”. The “resistance” appears only if you act against the gyro moment by lines or by rudder or whatever acting against the yaw. Only then you have “resistance”. If you have two or one blade prop, where the mass is only on one line, there are moments where whole thing does not work well. The problem is, that the 2-blade prop does not keep constant speed. The speed of rotation changes twice per one rotation and so fraction of the moment, which should appear at 90 degrees, is converted to moment acting against rotation. It is all because of that missing mass at 90 degrees, which stabilizes symmetric flywheel.

Brett wrote that the speed of rotation is too high, well it can be, but the final "resistance" depends also on mass of fuselage, quality of bearing and so. But practically, if I repalace my two blade prop on light model with Os .46 LA with plain bearing (what is configuration allowing this effect) by similar three blade I feel BIG difference. Like sticking lines, sometimes difficult to manage level flight ... that is why I stayed at two blade prop and damaged bearing.

Allan, I think you exchanged rpm and moment of inertia in your equation.

BUT!!! that is all true only if prop has 3 or more blades. Two or one blade props are different, just becuse they do not have any mass in one direction. It is also reason why they "rattle" in sharp corners.

I have a question, and this probably needs a new thread (which I am thinking of starting) but it has to do with the fact that you made the blades flat in pitch at the tip. I do not understand the logic of doing this. If you depitch the tips of the prop to the point that they are not pulling, they are pushing (OK, dragging). Any increase in rpm will still make this prop more efficient, so I don't see it helping on the downard side. However, could this have some effect in the turn in that the portion of the prop on the inside of the turn will be traveling slower and thus have a tendency now to be more efficient, and the part of the prop on the outside of the turn will be traveling faster and become less efficient? I am shooting in the dark, but thinking that not only is the prop less efficient, but it becomes counter productive to aiding in the turns.

Oh, and Matt is using a 13.5 inch three blade prop on the pipw with just a little extra pitch at the tips. Again, opposite theories, but when we tried it both ways, there was a great deal of added thrust and tension with more pitch at the tips (like a quarter inch, not a lot) while maintaining the same lap speeds.

I should have been more specific about the Tips.

Sorry about that.

On this prop it is a straight 3.75 pitch until the last station, 3/8" then it is a 3.4

It started life as a full 3.75 launch was at 9700rpms. 5.75 lap but very race car-ish in the maneuvers.

Take out the tip pitch and the wind seems to have little effect on it. Wind-up and stuff is not an issue. Launch is now 10200-10300.

This past Nats had a real windy week. More so than normal. The few days I was there I was using a prop that was a pitch 3.6 at center, station 4, and decending all the way out to 3.2 at the tip. Just a gradual slope downward. Then I just cram in the nitro and it just wont go any faster but it feels stronger at the handle. I was getting really slow laps this year. My manuever speed was pretty consistant also. One thing I have noticed with the decending pitch is you can turn in the needle and it wont go any faster. Just rev more as it leans but the prop just holds it back. It works really really really well in the wind. Just as Ted suggested in another thread somewhere else. Berringer does the same thing.

For some reason here in Dallas this year it has been really windy all year long. Using a decending pitch helps kill any acceleration you get from the tips when the nose is pointed up. Also we run our motors a bit different than Matt does. We tend toward more compression and 15-20% nitro. Looking for a more steady run with little or no break. On 5% nitro with the tips flattened it would fall out of the sky like you described when you tried a flatter tip pitch. On the lower nitro setup with less compression you are relying on the acceleration when the engine breaks to get you through. If there is no tip pitch then there will be little acceleration and the plane falls on its butt. In a high compression high nitro setup the motor will go longer before it will actually break. It still revs some RPM but it never goes over and breaks all the way. The motor just chugs along on the heavy duty stump pulling 4s and it doesnt need any tip pitch to to move the plane. Now form time to time when I am messing around I get it to break in this high compression setup. You better be ready because when it breaks with all that nitro it is off to the races!

When I was running the less compression more break on my 61s I had to have alot of tip pitch to get the drive needed to perform the hourglass and overheads. 4 pitch in the middle and 4.5 on the tips and in August down here in 2001 I had it at 5 on the tips!

Engine setup plays a major role in the type of pitch used. I guess that goes without saying.

More than one way to skin a cat for sure.

LAST EDITED ON Jul-31-03 AT 01:56 PM (CDT)
 
>I have a question, and this probably needs a new thread
>(which I am thinking of starting) but it has to do with the
>fact that you made the blades flat in pitch at the tip. I
>do not understand the logic of doing this. If you depitch
>the tips of the prop to the point that they are not pulling,
>they are pushing (OK, dragging). Any increase in rpm will
>still make this prop more efficient, so I don't see it
>helping on the downard side. However, could this have some
>effect in the turn in that the portion of the prop on the
>inside of the turn will be traveling slower and thus have a
>tendency now to be more efficient, and the part of the prop
>on the outside of the turn will be traveling faster and
>become less efficient? I am shooting in the dark, but
>thinking that not only is the prop less efficient, but it
>becomes counter productive to aiding in the turns.

Isnt more pitch counter productive to the turns?

>
>Oh, and Matt is using a 13.5 inch three blade prop on the
>pipw with just a little extra pitch at the tips. Again,
>opposite theories, but when we tried it both ways, there was
>a great deal of added thrust and tension with more pitch at
>the tips (like a quarter inch, not a lot) while maintaining
>the same lap speeds.

Acceleration with high tip pitch. All that we have found down here and other people have said the same thing for years is the meat of the prop sets the speed of the lap and overall speed of the flight. The tip gives it the acceleration. If you can run it so you have no excessive tip pitch then you can really kill all the accelleration. The cool thing is the motors of today are strong enough when setup to do so that they dont have to rely on tip pitch and accelleration to get through a maneuver. You can just kill all accelleration and rely on the motor's brute strength to drag the airplane through the maneuver. This gives you no funnies or unwanted speedup in the wind or maneuver. IMHO it makes it much easier to get real accurate.

When you setup the motor to run high nitro high compression it just gives up less in the maneuvers. Added tip pitch is not needed. Brad and I found the same thing to be true on the 4s. By depitching the tips on the 14" two blades it really flew the maneuvers slow but there was no loss of power or poor flight characteristics like there is when you run low tip pitch on a 4-2-4 setup. the 4s is a pretty strong motor so you can slow the maneuvers and get more accurate but not lose any power in the maneuvers. You arent relying on speed to get you through but more so on the motors ability to really produce some power.

Doug,

That's interesting. I'm using a 56 and a pretty large venturi. This venturi sets the rpm at about 10,100 on the ground before launc. The prop of choice (at the moment) is a Bolly 12x4.25 3 blade. Cleaned up with LE and TE sharp enough that I don't like to flip it with my bare fingers. Turns a pretty good corner (now that I have the empanage sorted out) and does rounds very nice. Good level flight tracking. Lap times on 66 foot lines are right on 5.2 sec. This is about where I like to fly. Hmm... I had a similar problem when using the Eather 12.5 x 4.5 3 blade "4 Stroke" prop. Turned fine, but getting the turn started took a lot of stick pressure. and it was too fast. The rpm with that prop was about 9800 rpm with the same venturi, so there was more load on the engine. I think Brett has it right.

I find this thread very interesting. In order to simplify the resultant conclusions for my aplicability; I ask the following summary questions.

1. If you are running a two cycle high rpm system you should strive for a propeller configuration that has a decreasing lower pitch at the tips to reduce acceleration out of the corners as compared to a 4-2-4 run setup should have more pitch for acceleration on the break as compared to a 2-2-2 runner.

2. As a general rule, simpllfying all esoteric theories, one should strive for a propeller with the largest diameter with proportionally more mass closer to the hub and perhaps a tapered blade and undercambered backside. Three blades would be better than two blades in terms of thrust, all other factors equal.

3. I assume that thinner blade thickness, reduced mass and stiffer blades is a desirable goal to achieve.

Do these generalizations have merit or am I missing something?

Static ground thrust is measurable as well as rpm. Does striving for more static thrust generally lead to a better flying pattern with more control?

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler

>I find this thread very interesting. In order to simplify
>the resultant conclusions for my aplicability; I ask the
>following summary questions.
>
>1. If you are running a two cycle high rpm system you
>should strive for a propeller configuration that has a
>decreasing lower pitch at the tips to reduce acceleration
>out of the corners as compared to a 4-2-4 run setup should
>have more pitch for acceleration on the break as compared
>to a 2-2-2 runner.
>
>2. As a general rule, simpllfying all esoteric theories, one
>should strive for a propeller with the largest diameter with
>proportionally more mass closer to the hub and perhaps a
>tapered blade and undercambered backside. Three blades
>would be better than two blades in terms of thrust, all
>other factors equal.

I would say, with no malice, that these are not really generally true. The diameter and pitch, mass, etc, all go into too many things to make any sort of generalizations like these. 1, for sure, is not really true at all - I have had very poor results with anything other than straight helical distributions on TP motors, probably because the effect is very pronounced.

>3. I assume that thinner blade thickness, reduced mass and
>stiffer blades is a desirable goal to achieve.
>
>Do these generalizations have merit or am I missing
>something?
>
>Static ground thrust is measurable as well as rpm. Does
>striving for more static thrust generally lead to a better
>flying pattern with more control?
>

Not necessarily, and in general not at all. My PA61/12.5-3.9 3-blade pulls like a tractor on the ground. The 40VF/11.5-3.5 Bolly s-blade pulls vastly less. But in the air, the performance, all factors considered, isn't a whole lot better or worse.

As another example, you seem to like your LA25 airplane. But the prop is rather heavy, rather thick, rather small diameter compared to what the engine can handle, and not particularly stiff. A 9-4 APC seems to work a lot better than a 10-4 or a 10-3 on a 20fp, not because of any aerodynamic advantage, but because the engine prefers less load.

All of these can be true in some cases (and in fact, constitute "common knowledge" in some venues), and not at all true in others. It's just way too complicated to simplify to the degree shown here. I would be very, very cautious about making these sorts of generalizations.

LAST EDITED ON Jul-31-03 AT 06:50 PM (CDT)
 
Bob,

As Brett said you cant really generalize anything in this game. He has no luck with decending pitch yet me and some others around here have real good luck with it.

I dont know many people who do use decending pitch on their props. But it is something to think about on the real Windy days. It works around here and worked well for me at the nats this past year. My straight pitch props were all way to fast in the winds and the decender was much more consistant in the maneuvers.

Since there isnt one setup that works better than all others there will never be a general best setup and prop scenario that is best for all. Not to mention how the location can play havoc on the setup as well.

Your number 1 genralization is to general. You cant make prop generalizations without specifying a setup. There are lots of different setups so the props could be all over the place.

Your number 2 is one I used to follow to a degree...get the biggest prop I can handle and go for it. Well since I went away from that theory I have had good results. You have to find the prop this is going to best match how you plan to setup the motor. It goes hand in hand. Sometimes that will be the one you described and other times it wont be.

Your number 3 I would say is correct. Thin as possible before strength is comprimised. If strength is given up for thinness then you may as well use a wood prop.

There is alot to it but then sometimes when you are the field and you have been at it for a long time and run the same setups for year after year it just seems so simple.

>Bob,
>
>As Brett said you cant really generalize anything in this
>game. He has no luck with decending pitch yet me and some
>others around here have real good luck with it.
>

Well, that's a lot more than I said - in fact, I have had very little success with either wash-in or wash-out on TP engines. Wash-in tends to make the airplane leap out of corners, and washed-out makes it slow in the corners, among other things. The big problem is that .1" of pitch is too small to reliably measure at the tips, but more than you need under normal circumstances.

I have always found that (on tuned pipe engines) anything that washing the pitch in or out did for you, just changing the pitch in the same direction but keeping the distribution helical did better. And also, didn't cause it to do something funny in the corners and overheads. For example a 12.5-3.9 with 4.2" at the tips might work pretty well, but has a pretty significant burst in the corners. A 12.5-4 straight helical might work the same in the rest of the pattern, but without the leaps.

The technique seemed to be much more predictable and beneficial on 4-2 break motors like the ST46. My baseline ST-46 prop is a cleaned-up Rev-Up 12-6 with a base pitch of 5.8", washing in to 6.3 at the tip. The mechanism is, I think, compensatory. The added corner pull tends to make up for the engine bogging down under load. or at least that's one aspect. It works too well on TP motors that don't bog down nearly as much because of the regulating action of the pipe, and greater momentum. The engine-feedback effects are also much reduced on the piped motor

My limited observation is that 4-strokes tend to act a lot like TP motors in regards to varying pitch distributions, but might not be too "touchy" to control reliably. Every significantly washed-out *rigid* prop I have seen did pretty much as it did on TP motors. Of course, the original Berrringer prop has the same sort of distribution at rest but was intentionally flexible, which is an entirely different ball of wax whether it works as described or intended or not.

I might also point out that what you *measure* as pitch can sometimes have very little to do with what it actually is. An Eather 12-3.75 U/C that's 3.75 at every point to a gauge, really acts like it is signifcantly washed-in, because the zero-lift line is greatly above the blade chord line. To get similar performance with a flat-back prop, you would have to add a large amount of *measured* pitch at the tips. Or, if you wanted the zero-lift line to be truly helical, you'd have to wash the tips out severly. perhaps even to the point that it *measures* negative.

Among other things, that's why a Eather U/C prop can measure 3.75" and have to spin 9900 on the ground, and a Bolly that measures the same might have to be 11000 to get the same speed. That has nothing to do with which is *better*, but does explain why they might be different.

But it's all just my observation. I know enough to know I don't know enough to pooh-pooh anything out-of-hand where props are concerned. I do know that most of the "rules" about props are greatly oversimplified at best, and excessive reliance on rigid rules can make you overlook or not try a lot of things that might work better.

> I might also point out that what you *measure* as pitch
>can sometimes have very little to do with what it actually
>is. An Eather 12-3.75 U/C that's 3.75 at every point to a
>gauge, really acts like it is signifcantly washed-in,
>because the zero-lift line is greatly above the blade chord
>line. To get similar performance with a flat-back prop, you
>would have to add a large amount of *measured* pitch at the
>tips. Or, if you wanted the zero-lift line to be truly
>helical, you'd have to wash the tips out severly. perhaps
>even to the point that it *measures* negative.
>
> Among other things, that's why a Eather U/C prop can
>measure 3.75" and have to spin 9900 on the ground, and a
>Bolly that measures the same might have to be 11000 to get
>the same speed. That has nothing to do with which is
>*better*, but does explain why they might be different.

Man, I am all ears. Please, could you do some more?

I have always been confused about whay this occurs. I thought it was just uncambering. Bolly's always seem to need more overall pitch. They also seem to need more tip pitch. I assumed this was due to blade width.

This would explain the Gordy vs. Paul pitch difference....

>
>> I might also point out that what you *measure* as pitch
>>can sometimes have very little to do with what it actually
>>is. An Eather 12-3.75 U/C that's 3.75 at every point to a
>>gauge, really acts like it is signifcantly washed-in,
>>because the zero-lift line is greatly above the blade chord
>>line. To get similar performance with a flat-back prop, you
>>would have to add a large amount of *measured* pitch at the
>>tips. Or, if you wanted the zero-lift line to be truly
>>helical, you'd have to wash the tips out severly. perhaps
>>even to the point that it *measures* negative.
>>
>> Among other things, that's why a Eather U/C prop can
>>measure 3.75" and have to spin 9900 on the ground, and a
>>Bolly that measures the same might have to be 11000 to get
>>the same speed. That has nothing to do with which is
>>*better*, but does explain why they might be different.
>
>Man, I am all ears. Please, could you do some more?
>

Sure- it is the result of the airfoil. I have parts of a large article on the topic, but the basic idea is explained here:

http://www.supercoolprops.eftel.com/ARTICLES/article_4.htm

by Joe Supercool himself. There are a lot of nuggets on this site, so I consider it worth a look.

LAST EDITED ON Aug-01-03 AT 10:56 AM (CDT)
 
I agree with Doug and Brett here, but based on experience, not math. I don't know jack about prop physics. There are certain things I've learned by trial and error, much as Brett noted that he'd tried props with wash in or wash out and noted the outcome. I've taken setups (engine, prop, tank) from one plane and put it into a very similar plane (weight, area, etc). What worked wonderfully in one plane is a dog in the other. I decide on an engine for a plane and when the plane is ready to fly, I traipse out to the flying field with every prop I own that I believe has a chance of being effective. After trying each in succession, I keep the ones that were better and put away the ones that were ineffective. Then start doing some thinking about why one prop might be working better than another. Then it's make adjustments based on my experience and the flight trials and try again. Eventually, I come up with something that works pretty well in certain conditions. Keep at it with props that work in other conditions, blah, blah, blah.

Point is, in the end, I have a selection of 2 or 3 props that I know work in a certain range of conditions, in a certain way, for that plane. But as I've learned, the only thing that's tranferable is the knowledge gained. Different plane, different props; even with the same engine and engine setup. As Brett notes, there are just too many variables to make credible predictions or generalizations.

Randy,

I have doen this very thing many times. Even from one Bear to another and the setups just dont go from plane to plane to well.

I couldn't agree more with your entire statement.

>I agree with Doug and Brett here, but based on experience,
>not math. I don't know jack about prop physics. There are
>certain things I've learned by trial and error, much as
>Brett noted that he'd tried props with wash in or wash out
>and noted the outcome. I've taken setups (engine, prop,
>tank) from one plane and put it into a very similar plane
>(weight, area, etc). What worked wonderfully in one plane is
>a dog in the other. I decide on an engine for a plane and
>when the plane is ready to fly, I traipse out to the flying
>field with every prop I own that I believe has a chance of
>being effective. After trying each in succession, I keep the
>ones that were better and put away the ones that were
>ineffective. Then start doing some thinking about why one
>prop might be working better than another. Then it's make
>adjustments based on my experience and the flight trials and
>try again. Eventually, I come up with something that works
>pretty well in certain conditions. Keep at it with props
>that work in other conditions, blah, blah, blah.
>
>Point is, in the end, I have a selection of 2 or 3 props
>that I know work in a certain range of conditions, in a
>certain way, for that plane. But as I've learned, the only
>thing that's tranferable is the knowledge gained. Different
>plane, different props; even with the same engine and engine
>setup. As Brett notes, there are just too many variables to
>make credible predictions or generalizations.

It's really the scientific method at it's most pure. You try stuff based on some notion or underlying theory. It either works or doesn't. Either way, the results alter your underlying thory, leading to other ideas on what to try next.

Most of my efforts in the engine and prop area are directed not only towards getting something that works, but to also understand the underlying principles so that it's less random trial and error the next time. It's a guided experiment, not just randomly trying things. We (stunt fliers) have just scratched the surface of these topics.

And how about humidity, does this have an effect??? I'm in Florida, where it's almost always above 80%. Can't help but picture an outboard boat motor, in the water moves the boat great, out of the water, even on wheels, that little engine/prop can scream and you are not going anywhere...so...as we are playing with little airplanes and very minor prop modifications like these, can things like humidity (ambient temp that day, etc) skew our results into a false belief? At minimum the humidity will affect the combustion efficiency of our 2 cycles.

Just curious.
EricV

Ah man, how do I get a 3 blade prop to run on my Fox 35 at the speeds mentioned? Do I flatten the tips? I'm just confused. Definately gonna haveta hang with some stunt grunts other than playing with myself out here in KC. My Brodak mustang doesn't feel good going over the top at anything over a 5.5. I have been flying loops and such at 6 second laps that really impress me, my Banshee would be on the pavement, but I can't imagine going over the top at 6.2, wow.

When I get the Vectors going, and I'm planning on LA46's for powerplants, should I look for a 3 blade propellor or 2? I ask only to plan on landing gear setup and I'll need to look for a 3 blade spinner and such.

Then I have to become a propellor mechanic too?

Whew!

>And how about humidity, does this have an effect??? I'm in
>Florida, where it's almost always above 80%. Can't help but
>picture an outboard boat motor, in the water moves the boat
>great, out of the water, even on wheels, that little
>engine/prop can scream and you are not going
>anywhere...so...as we are playing with little airplanes and
>very minor prop modifications like these, can things like
>humidity (ambient temp that day, etc) skew our results into
>a false belief? At minimum the humidity will affect the
>combustion efficiency of our 2 cycles.
>
>Just curious.
>

Oh, sure. The air density is significantly effected by the water content - more water = less dense, since a water molecule is is lighter than an oxygen or nitrogen molecule. An excellent paper on the topic, along with a very useful air properties computer program, can be found at:

http://geocities.com/CapeCanaveral/Galaxy/4707/Aeronotes/pwscol.htm

This is just one of a very large number of pure gold nuggets at the site. There are some broken links, but if you look at the directories themselves, it's pretty to figure out what they should be.

Brett,

Well, I hope I learned something along the way. Certainly, I know what props to put in the box to try with a new plane and have some notion as to why one might work better over another. I think most of us that have been around awhile have developed certain theories as to why one approach will work over another. There is an amount of "art" to our decisions and a lot of knowing what didn't work before.

It could be the airflow over the tail is different (covering more of it) with the larger prop. If you want to see proof look at NACA Wartime Report WR-I-761 "Effect of propeller operation on the pitching moments of single engine monoplanes" at the NACA site; WWW.naca.larc.nasa.gov
Andrew Raney

Hi Gentlemen,

Great stuff. I have a Laser 200 Saito .72 740 sq in, 84 oz.
Large mass fuse the front end is 6" wide . I now am using a .312
venturi with a Top Flite Old Paddle blade prop 8800-9000 rpm.
When I drilled out the venturi it increased the ground rpm to
9000 WHOLE NEW AIRPLANE! Tracks well turns some what soft but
easily now. At lower rpm it was a lot of work to fly the plane .
Not now. I have yet to have an expert fly the plane but it sure
seems like it flys well regardless of the weight and size.
My question is :

Will the 4 stroke and mere mass of the Fuse help this plane
even with a 6 pitch prop in the wind? I have yet to fly in a whole
lot of wind and am concerned that a lower pitch may not haul the
Large heavy plane? I have heard that Large bulky fusealages help
to create drag thus decrease wind up? I love the Laser and am seriously considering a Sukhoi Su-26m about the same size .
any comments . Thanks Walter Hicks

LAST EDITED ON Aug-02-03 AT 09:20 AM (CDT)
 
Doug,
Lets pick the low hanging fruit first.
The Eather 4 blade is 0.42 oz. heavier than the 3 blade.
Could it be simply that your a/c cg was different with the two props?
That's a fair amount of nose weight to change. Tracking in manuevers would possibly be changed.
Frank

Frank, I tested wide blade BE 12"x3.5" UCT 2bl and BE 12"x3.5" UCT 3bl with narrower blades. The weight is only little different, in air or ground RPM without measurable difference, needle not touched. The 3 blade was FAR worse than 2 blade. Model was trimmed for that two blade. The difference was exactly that gyro effect - once the nose got the direction, it was difficult to change it, so it did problems even to make perfect level flight, but also in loops trying to elswhere, but not concentric and round ... surprisingly did not make troubles in corners.

Well ... do not take me wrong, I do not say 3bl is wrong, it can be that if I would trim my model for 3bl I will trow away that 2bl prop ... I jus say IT IS DIFFERENT

>Doug,
>Lets pick the low hanging fruit first.
>The Eather 4 blade is 0.42 oz. heavier than the 3 blade.
>Could it be simply that your a/c cg was different with the
>two props?
>That's a fair amount of nose weight to change. Tracking in
>manuevers would possibly be changed.
>Frank

Ahh yes, but the whole reason I stuck to the 14" two blade Eather over all the other multi blade props was the weight. It was the lightest of the bunch.

Much more precession than a Bolly 13-5.0 three blade at amost 1000 RPM more. Much more resistance to turning also. The Bolly is about .5 oz heavier.

I just weighed the two props.

One is 12"X3.6" 4 blade Eather. 42grams.
The other is 13"X3.75" 3 blade Eather 38grams.

4 grams in the nose didnt make the difference I saw.

In the first post I made I mentioned the difference between the 2 blade and 3 blade.

Well the weights of the above 13" prop and the 14" two blade is 38 grams. They are the same.

The 14" two blade is a hoss.

Ron B.
F4Fguy

Better late than never,I guess.

The original question,as I recall,was;does RPM have an effect?
The answer is(as in the the title of that old arab quiz show)You Bet Your Ass.

I finally got around to digging out some old stuff I did years ago.I hope the scans are legible.The first three sheets are calcs I did using measured polars for a number of props of different size,weight,and configurations,and different line lengths,speeds,and corner radii.

The last sheet is an experimental method of determining polar moment of inertia which is somewhat easier to set up than a bifilar pendulum.

Doug,

Paul Walker wrote on his trimming booklet that more diameter and more pitch affects the planes turning ability (for worst).

This means, I assume, that a smaller diameter, less pitch and more revs would make it turn better.

About 2 or 3 blade props, Bob Hunt once said that his Saturn with a 3 blade would give a better line tension, but if the wind was bad, he would use a 2 blade. He would loose the feeling that "the plane was on tracks on the hemisphere", but it turned better.