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-   -   Transitional Pitched Prop??? (http://www.rcuniverse.com/forum/questions-answers-154/3603497-transitional-pitched-prop.html)

crashdummy6 11-30-2005 02:37 PM

Transitional Pitched Prop???
 
Quick question for you guys. I am planning on putting a Fuji32B on a Midwest Texan. When I acquired the kit it had a prop with it. It is an 18X8-10 I think (If I remember exactly), but anyway, I am curious about the 8-10 thing. I know the second number is the pitch. I also assume that 8-10 means it changes pitch along its length. I also think the higher pitch is inboard gradually reducing pitch out to the tips of the prop. Please correct me if I am wrong in any of this. So my question is, what is the purpose of this design. Benefits, gains, drawbacks, etc. By the way, it is a Top Flite powerpoint prop.
Thanks

TexasAirBoss 11-30-2005 03:16 PM

RE: Transitional Pitched Prop???
 
I was thinking the "8" was the pitch inboard and the " 10 " was the pitch near the tips. But I really don't know. I have had similar props. Some people put them on scale planes with large cowls. The idea is to get more thrust outboard to "reach around " the cowl. I don't know why people would use them on an aerobat. Gee, I wasn't much help, was I ?

crashdummy6 11-30-2005 04:15 PM

RE: Transitional Pitched Prop???
 
Well, by looking at it, it really looks more angled closest to the hub and much flatter toward the tips. Your explanation makes sense to me about the cowling. I am just not sure it holds up with what I see on the prop. Off course it could be my eyes. That is why I need a big plane I guess.
Thanks

js3 11-30-2005 06:28 PM

RE: Transitional Pitched Prop???
 


ORIGINAL: crashdummy6

Well, by looking at it, it really looks more angled closest to the hub and much flatter toward the tips. Your explanation makes sense to me about the cowling. I am just not sure it holds up with what I see on the prop. Off course it could be my eyes. That is why I need a big plane I guess.
Thanks
Even a prop with the same pitch designation along the blade will look like this so that doesn't tell you anything.

The reason the pitch is higher toward the tip is that the tips are more efficient and can handle the higher pitch.

crashdummy6 12-01-2005 07:51 AM

RE: Transitional Pitched Prop???
 
I got to thinking about that last night and thought to myself, "Self, that was a stupid thing to say. All props change pitch along their length."
So in actuallity, the is higher on the tips? A 18X8-10 means 8 at the hub and 10 at the tip?
Thanks

js3 12-01-2005 06:39 PM

RE: Transitional Pitched Prop???
 
Another reason the inner portion will be set to a lower pitch is for the sake of hub thickness. At a lower pitch the hub will be thinner allowing more of the crank shaft threads to engage whatever hub or spinner type would be selected.

jetmech05 12-02-2005 07:31 AM

RE: Transitional Pitched Prop???
 
I THINK I'M PROBABLY GONNA BE WASTING SPACE HERE BUT I'LL TRY.
A PROP PRODUCES VERY LITTLE USABLE THRUST NEAR THE HUB ANYWAY. LIKE WISE WITH THE VERY TIPS.
THE MAJORITY OF A PROPS THRUST OR LIFT IF YOU PERFER IS PRODUCED FROM ABOUT 1/2 TO ABOUT 4/5THS DOWN THE LENGTH OF THE PROP BLADE FROM THE HUB. SO I'M GUESSING HERE THAT THE 1ST PITCH NUMBER IS KINDA USELESS.
SEEYA AND HAVE FUN

Ed_Moorman 12-02-2005 08:53 AM

RE: Transitional Pitched Prop???
 
The reason for the changing pitch, and Zinger was the first to market them, was to allow you to turn the higher pitch outer section at a higher rpm. You do lose some thrust by having a lower pitch near the hub, but the lower pitch also lowers the drag, allowing the prop to turn at a higher rpm. The increase in thrust from the higher rpm more than offsets the loss of thrust due to the lower pitch section near the hub.

As I recall, the original prop of this type was the 9x4-6 that was specifically made for the OS .25FSR which didn't rev enough with a 9-6 and over revved with a 9-4. I don't think they made 9-5s in those days, probably early 1980s.

I always liked the Zinger 18x6-10 on my G-38s, Q-42s and ST 3000s.

crashdummy6 12-02-2005 09:02 AM

RE: Transitional Pitched Prop???
 
It is an 18X6-10 not 18X8-10 as I had originally said. Not that it changes the question or answers. Just a matter of accuracy.

multiflyer 12-02-2005 06:16 PM

RE: Transitional Pitched Prop???
 
crashdummy6,

Lots of replies but no one got the main idea. Prop pitch is analogous to the gearing for a car. Going faster needs higher gearing or steeper pitch. For a prop, this is simply because forward motion reduces the actual angel of attach that the prop is seeing. So steep pitch for speed, flat pitch for pull, and the most effective pitch depends on the airspeed and rpm combination. Full size planes use an in flight variable pitch to give the best of both worlds, like having bicycle with more than one gearing available. A varying pitch fixed blade prop design is an attempt to have a little of each. The prop won’t speed or climb as efficiently as a single pitch, but will be generally effective over a broader speed range. It’s like using a multi-viscosity motor oil for your car instead of a straight weight. A compromise.

Multiflyer.

crashdummy6 12-02-2005 09:43 PM

RE: Transitional Pitched Prop???
 
Thanks multiflyer.
I was coming around to the thought that maybe the prop is different pitch at different places along its length to take advantage of the effeciency of a given pitch at a given prop speed. Since the outboard tips obviously are moving at a greater airspeed, they may be less angled to prevent creating too much "lift" at the outboard ends thereby reducing stress to the prop as well. So like you said, best of both worlds maybe. I would think that a prop like this would be suited better to a higher or lower rpm. I just have no idea which it is meant for.
Thanks

da Rock 12-02-2005 11:17 PM

RE: Transitional Pitched Prop???
 

Since the outboard tips obviously are moving at a greater airspeed, they may be less angled to prevent creating too much "lift" at the outboard ends
Sorta....

Somebody used to sell a prop pitch gauge. It would show you the "pitch" at each station you chose to measure from the hub out to the tip. I got one in the drawer downstairs, but since shifting over to flying just R/C haven't had any reason to pull it out. whatever...

As the stations on a prop move out toward the tip, each station does see greater airspeed. But the books actually describe what happens and why the pitch flattens out a bit different than the object being the lift generated at each station. Turns out that the airfoil of most props also changes as it goes to the tip. And they say that because of the speed increase toward the tip each section sees a different angle of attack. That idea caught me out too. But it was clearer when they said that a prop unloads in the air compared to when run without forward motion on the ground, because it was seeing less angle of attack due to the forward motion when flying, that the AOA each station sees is a function of it's speed, and what the entire prop sees is the also a function of forward speed and they add up. That made it easier for me to see that speed had something to do with AOA.

They say that the reason a prop has less pitch toward the tip is to balance out the airfoil versus the AOA (since both are changing as you move toward the tip) so that each station of the prop will operate to the optimum. I guess the idea in prop design is to keep each station pulling with an efficiency that results in the prop pulling whatever the pitch rating says. And in keeping any stations from stalling and others from not producing their share of lift.

Ed_Moorman 12-03-2005 07:36 PM

RE: Transitional Pitched Prop???
 
Hey, a prop does not have less pitch near the tip. It has less angle, but the pitch is the same unless it is one of the multi-pitch props with higher pitch at the tip.

The pitch of a prop is how far it would move forward in one revolution given a perfect medium. As you move outward toward the tip, the distance of one revolution is greated so the angle is less to compensate for this. If you use a pitch guage on a single pitch prop, you'll find the pitch is the same even though the angle decreases towards the tip.

multiflyer 12-04-2005 01:07 AM

RE: Transitional Pitched Prop???
 
Ed said it very well: "a prop does not have less pitch near the tip. It has less angle, but the pitch is the same unless it is one of the multi-pitch props." Like 2 bolts with the same number of threads per inch but one has a larger diameter. The threads on the larger diameter bolt will not spiral up the shaft as steeply, but both bolts will screw in the same distance in the same amount of turns.

The big thing to realize about props is that they are just wings that fly in a circle. Just as there is as may different wing designs as there are aircraft designers, there are as many prop blade variations as there are prop makers. It takes much more than just span and chord to define a wing, and similarly it takes much more than just diameter and pitch to describe a prop. Each different maker tweaks the blade at every station to better achieve their intended result. Assuming that one prop maker's pitch number compares to another maker's pitch number is like assuming that one airplane's airfoil compares to another's.

Multiflyer

HighPlains 12-04-2005 02:07 AM

RE: Transitional Pitched Prop???
 
This thread is like describing an elephant.

The aerodynamics of propellers is very complex, and there are lots of considerations to the design and manufacturing of a propeller. I worked for Chris Machin 30 years ago making Rev-Up props while in college, so I had a chance to learn a bit about it from the inside.

There are a lot of different pitch profiles out there. The most simplistic is just using a constant pitch for the entire length of the blade. And since most modeler don't own a pitch gage, but do own tach's, there has been a trend to fudge a bit on the actual pitch. For instance, on the large wood props that you were asking about, it take a very thick piece of wood to hold the same pitch down to the hub if the blade width is to remain about the same. And of course, the hub of the prop needs plenty of meat just to hold together. So one way around this problem is to lower the pitch at the hub. This works to our advantage anyway, because the air at the fuselage moves slower than the free stream air further away, so de-pitching at the hub unloads the prop when the aircraft is in flight.

Now a lot of props are also washed out towards the tips as well. This is a bit like wash-out in a wing, moving the lift inboard on the wing or prop to give an elliptical lift distribution for lower drag. With the lower drag, then you end up with more rpm. This works well with most sport models where the pitch is much lower than the diameter, and they fly over a larger range of speeds.

For racing models where the pitch and diameter are about the same, most are increasing pitch as they approach the tip, and the only area that is lower pitch is closer to the hub for the reason in the first paragraph.

Regardless of what brand of props you like, it takes experimenting to find the one that performs best with your engine and airframe. One size does not fit all, and the recommendation that comes with the engine may not work well at all. Some very creative marketing out there with heavy loads (high pitch and diameter) as well as hp ratings that you would only see with very small light props. Brand X must be better than Brand Y because they use a bigger prop. Experiment, buy a dozen sizes and fly them all.

revupmgr 05-23-2006 01:44 PM

RE: Transitional Pitched Prop???
 
All right Guys...I Am the former 10 year Manager @ Rev up Props...till the owner died & I moved on...ask me any questions about....Rev -up or Zinger....(exec...talked back & forth with me during my tenure @ rev-up)...anything about..size...type...etc...Don't ask where 2 get them...all sold out...find a vendor...many were told we were closing up shop & ordered Big...some vendors like tower try 2 tell u they don't have any but their...show planes have rev-up on them....u also might look into military vendors....our biggest client was US Gov...& Britain...for their spy r/c-planes...my email is
vikings_chiefs@yahoo.com........My name is Dan Dekat-39 yrs. old...

da Rock 05-24-2006 07:33 AM

RE: Transitional Pitched Prop???
 

Hey, a prop does not have less pitch near the tip.
Actually some do and some don't. And some have different profiles and almost all have different chords. There is no single description that fits all props. And some don't have what was designed into them because the "plastic" was pulled from the mold too soon or not soon enough or the cutters were dull or poorly placed.

One of the "nicest" pitch gauges was sold for a few years with a scale that was "wrong". The answer was that there really is no definition of pitch that accurately describes each station along the prop. Pitch is mostly what the engineer thinks the prop (the whole) prop should do in the air. And they can say it's measurable with an AOA gauge, but that's just hot air.

Truth is, since every station along a prop is locked to every other station rigidly, and every molecule of the prop is moving forward at exactly the same speed, and pitch is a description of forward motion, then no matter what AOA of any single part of the prop every part of the prop has absolutely the same pitch.

for example...... the prop's hub has the same pitch as the prop's tip. It doesn't have the same AOA. It doesn't have the same zero lift angle. It doesn't have the same airfoil. It doesn't have the same camber. And it's L/D is a lot different.

"Pitch" is a good word that is most often a description of the whole and also an ok description of how each station looks. Beyond that, it's confusing to try and use it to describe each station of the prop unless you're just casually suggesting how angled the flat bottom of that station looks to you.

BiplaneLem 06-07-2006 06:56 PM

RE: Transitional Pitched Prop???
 
Ok.... getting way too technical here. I have used and still do use a 18 6-10 prop on a Quadra 35 motor in a H-9 100 " Super Cub. After trying several different props I tried the 18-6-10 prop and found it was the best for the airframe/motor combo. What I found in flight was that the plane had the relative speed found with the 10 pitch prop at high throttle and the relative power of the 6 pitch prop at low throttle. Why? I have no F******* clue..... but I do know what the plane does. I also have a GP Super Chipmunk with a 6-10 pitch prop. When I tried the 6 pitch it landed nicely but did not have the speed it should. The 8 pitch prop made it hard to land... ie too fast.... but still did not have the speed. The 10 pitch Prop gave me the speed I wanted but had to stall the plane about 6 inches from the ground to get it to land. ( I since got my flaps working better...LOL) The moral of all this ranting is that the 6-10 pitch prop alows me the speed at full throttle and the power and slow speed at low throttle. Regardless of the engineering and the all the technical mumbo jumbo... the 18 x 6-10 prop works well for me in the two situations I mentioned. try the prop and see what you think verses other props. then use what works best for you.


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