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F8F Bearcat - 125 MPH for $250

Old 02-09-2011, 10:41 PM
  #76  
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Originally Posted by NJSwede View Post
But one thing is true in what you're saying. You *CAN* have "negative thrust" at the tip. Most notably, it happens when you deadstick.
OK - - - -
Now to start another issue!

Mathematically, which has more drag on a model in flight. A prop that is not turning? Or a prop that is windmilling? Bet you get a lot of comments on that. Sometime in the next month or two, I'm going to set this up, by putting a 12 inch prop on a motor, then blast it with the prop of my Hacker A60-16M. Then measure drag on that motor. This may not be accurate, not having access to a wind tunnel. But it would be interesting what the mathematics say will happen.
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Old 02-09-2011, 10:57 PM
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I WAS WRONG!
I WAS WRONG!
I WAS WRONG!

You *can* have a negative angle of attack. I had the prop turning the wrong way in my head when I thought about it. SORRY!!!
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Old 02-09-2011, 11:04 PM
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Originally Posted by kyleservicetech View Post
OK - - - -
Now to start another issue!

Mathematically, which has more drag on a model in flight. A prop that is not turning? Or a prop that is windmilling?.
This is always a good one to start a debate. The answer is not simple because it depends on the pitch/diameter ratio, among other things. For a truly freewheeling prop then the info I have shows that for pitch/diameter ratio less than 1/3 fixed has less drag. For P/D ratio greater than 1/3 freewheeling is lower drag

But.. (and it's a big 'but')... That is only if the prop is truly freewheeling. Add some friction and the static prop is strongly favoured.

I've seen this question come up several times before so I kept a graph copied out of a university thesis on exactly this subject. The graph was produced by wind tunnel testing.. see attached.
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Old 02-09-2011, 11:08 PM
  #79  
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Originally Posted by kyleservicetech View Post
OK - - - -
Now to start another issue!

Mathematically, which has more drag on a model in flight. A prop that is not turning? Or a prop that is windmilling?
I have to give you a boring answer: I don't know, but I can give you a crappy answer that probably ignores a lot of significant effects.

Given my limited knowledge, I can only give you the answer from a static point of view: It doesn't matter. The force of the headwind translates into two things: The drag (which is what you care about) and torque trying to turn the prop. If your prop windmills, the energy of the torque is expended by turning your motor (if you're motor is brushed, you actually generate electricity). If the prop is still, the energy is expended trying to turn your model and eventually goes away as heat around the ailerons when you're trying to compensate.

But here the static case clearly isn't enough. You have to look at it dynamically, with vortices and all that jazz. And that math is WAAAAAY beyond what I master...
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Old 02-10-2011, 12:27 AM
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Originally Posted by NJSwede View Post
I WAS WRONG!
I WAS WRONG!
I WAS WRONG!

You *can* have a negative angle of attack. I had the prop turning the wrong way in my head when I thought about it. SORRY!!!
I've often wondered about that. I've got a model with a Hacker A50-16S motor, and 16X12 APC-E prop. It's interesting to put the model in a dive with the transmitter at about 30% throttle. Might be my imagination, but it sure sounds like that prop sounds much different when the aircraft forward speed exceeds the prop pitch's air speed.
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Old 02-10-2011, 12:30 AM
  #81  
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Originally Posted by NJSwede View Post
I have to give you a boring answer: I don't know, but I can give you a crappy answer that probably ignores a lot of significant effects.

Given my limited knowledge, I can only give you the answer from a static point of view: It doesn't matter. The force of the headwind translates into two things: The drag (which is what you care about) and torque trying to turn the prop. If your prop windmills, the energy of the torque is expended by turning your motor (if you're motor is brushed, you actually generate electricity). If the prop is still, the energy is expended trying to turn your model and eventually goes away as heat around the ailerons when you're trying to compensate.

But here the static case clearly isn't enough. You have to look at it dynamically, with vortices and all that jazz. And that math is WAAAAAY beyond what I master...
Interesting!

Looks like depending on circumstances, either the stationary prop, or the windmilling prop could have the lower drag. And, perhaps that could change from one to the other while the model is coming in for a landing???

I do know that with my 1980's vintage 10 foot wingspan electric launched sailplane, having the folding prop windmilling with the power off was similar to having the spoilers full up. That prop caused so much drag I installed a electric brake on my brush type ESC design. Once the prop stopped windmilling, it folded back as expected.

Learned something new today
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Old 02-10-2011, 12:51 AM
  #82  
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Originally Posted by kyleservicetech View Post
Interesting!

Looks like depending on circumstances, either the stationary prop, or the windmilling prop could have the lower drag. And, perhaps that could change from one to the other while the model is coming in for a landing???
The exact solution to exactly how much drag is generated under which circumstances is probably at (or more maybe even beyond) what's possible to calculate today. A lot of the mechanisms around fluid dynamics are still poorly understood. It's often said that the area of fluid dynamics is the only area in Newtonian physics that's not fully understood.

But that's not to say that it's impossible to come up with good-enough approximations on things like airfoils and props... ...which is kind of where this discussion started!
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Old 02-10-2011, 01:24 AM
  #83  
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Originally Posted by kyleservicetech View Post
I've often wondered about that. I've got a model with a Hacker A50-16S motor, and 16X12 APC-E prop. It's interesting to put the model in a dive with the transmitter at about 30% throttle. Might be my imagination, but it sure sounds like that prop sounds much different when the aircraft forward speed exceeds the prop pitch's air speed.
Well, if you think about it, two interesting things happen around that point:
1) When you're starting to have enough negative alpha, the high-pressure and low pressure zones on the blades trade places (think inverted flight). You're now going to have a higher pressure ahead of the prop than behind it (in the direction the plane is traveling). I'm sure that could change the sound!

2) At around the same point, you'll reach an energy equilibrium where the motor doesn't have to add any energy to turn the prop. Soon after that (as yous speed increases), the propeller wants to turn faster than the motor and it turns into a generator. I'm sure that the motor may sound different as it goes through that phase.

So, no, I don't think it's your imagination!

Last edited by NJSwede; 02-10-2011 at 02:36 AM.
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Old 02-10-2011, 06:33 AM
  #84  
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Originally Posted by JetPlaneFlyer View Post
Most (if not all) props will go to zero AoA at the root first. The tip will generally be the last part of the prop to stop producing thrust and so unless you were in a very steep dive it would not actually occur.

Taking my 13x6 Master Airscrew prop as an example. The 1/3 span station would at 10k RPM go to zero AoA at an airspeed of 83.63kmh (tan22 deg x tangential velocity)

The tip would still be operating at a positive AoA of 2 or 3deg at that airspeed.

Simple trig.. Calculus makes my head hurt but hats off to NJSwede for showing us how it's done. He appears to have a mathematical mind similar to the guy I sat next to and cribbed off to get me through my math exams

Steve
I thought the math was a little too simple to show, and intuitively not necessary given that a thought experiment more than suffices, but sure -

If you take that 11x5" scimitar I pictured earlier and spin it at 10K RPM you get a tip speed of about 34.5" circumference * 10,000 = 345,000 in/min = 28,750 ft/min = 5.4 miles/min = about 325 mph.

Moving 5 inches forward per turn yields about 50,000 in/min = 4167 ft/min = 0.789 miles/min = 47 mph

325 adjacent, 47 opposite = 0.144 Inv Tan = 8.2 degrees

So at 10K RPM you'll get negative 5 degrees AoA at the tip (8 - the 3 degree fixed pitch tip, as pictured). Clearly, 10K is above the design speed of this prop. But, since the chord tapers to near 0 anyway, Lift remains near 0 at the tip, and given the taper any negative lift is massively overpowered by the large chord inner portion of the prop.

So...

Not only does lift remain approx equally distributed along the radius of a blade at the prop's design speed, but Lift goes negative, developing from tip to root, as you exceed that airspeed/RPM.

As far as the other helicopter blade question - I don't think variable pitch blades are really relevant to the discussion, as they can pitch any which way. Certainly the entire blade will see negative AoA in an inverted hover.
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Old 02-10-2011, 08:04 AM
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Z8,

If the tip is so narrow then how did you accurately measure the blade angle? If the chord of the blade was, say 1/8", then a few thousanths of an inch in inclination of the leading edge would be the difference between 3 degrees and 8 deg pitch. Unless you have a proper propeller pitch gauge I don't see how you could accurately measure it?

I deliberately picked the Master Airscrew because it's blade shape made for easy and accurate measurement (and I still got it wrong first time round!)

In any case the Scimiter prop with it's pointed blade is not a 'typical' prop shape. It's quite possible that the designers of that prop had something different to the norm in mind when they made it, but one thing for sure; having the tips produce 'backward' thrust when the prop is not even at half of it's design pitch speed could not be efficient, so I suspect you have measured wrongly.

By the way.. the fact that most helicopters have variable pitch has nothing to do with why they dont employ twist. You will find variable pitch on many full size aircraft and yet they still have twisted blades the reason for the difference is airplane props are optimised for forward travel through the air and helicopter rotors are optimised for hovering.. Pretty obvious really, why would it be done any other way?
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Old 02-10-2011, 01:08 PM
  #86  
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Originally Posted by JetPlaneFlyer View Post
Z8,

If the tip is so narrow then how did you accurately measure the blade angle? If the chord of the blade was, say 1/8", then a few thousanths of an inch in inclination of the leading edge would be the difference between 3 degrees and 8 deg pitch. Unless you have a proper propeller pitch gauge I don't see how you could accurately measure it?

I deliberately picked the Master Airscrew because it's blade shape made for easy and accurate measurement (and I still got it wrong first time round!)

In any case the Scimiter prop with it's pointed blade is not a 'typical' prop shape. It's quite possible that the designers of that prop had something different to the norm in mind when they made it, but one thing for sure; having the tips produce 'backward' thrust when the prop is not even at half of it's design pitch speed could not be efficient, so I suspect you have measured wrongly.
I disagree that scimitar is atypical. It is relevant to this discussion because its primary purpose is to optimize aerodynamic efficiency by blending lift to zero as the span progresses root to tip. That is not unlike the goal of any elliptical (the most efficient) wing plan form which is to distribute lift so it tapers to 0 progressing root to the tip, but twisted in the case of a prop blade, to account for the change in radial speed. But the goal remains the same:

http://en.wikipedia.org/wiki/Elliptical_wing
"An elliptical wing is a wing planform shape that minimizes induced drag.[1] Elliptical taper shortens the chord near the wingtips in such a way that all parts of the wing experience equivalent downwash, and lift at the wing tips is essentially zero, improving aerodynamic efficiency..."

Note that slo fly props are also elliptical because minimizing induced drag so dominate their design. I'm sure some will think that APC props are efficient too, and they taper, but are not elliptical. One of the first things you learn in aero propulsion/wing design is that a wing shape with a 0.33-0.40 tip/root ratio (the wikipdedia article above says .4 to .5, but I think that is a little off) gives a satisfactory approximation of an elliptical lift distribution (i.e. no appreciable lift at the tip).

By the way.. the fact that most helicopters have variable pitch has nothing to do with why they dont employ twist. You will find variable pitch on many full size aircraft and yet they still have twisted blades the reason for the difference is airplane props are optimised for forward travel through the air and helicopter rotors are optimised for hovering.. Pretty obvious really, why would it be done any other way?
Sure, but it is equally obvious variable pitch designs go from positive to negative AoA and everything in between.
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Old 02-10-2011, 10:19 PM
  #87  
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guys there is somthing that we over looked at least i did.

i reread through the threads as i understood most and not some of the more caculative ones. but could not find anything that pertained to it.

njswede put me onto this thinking as i do agree with him in some respect to this quote.

Originally Posted by NJSwede View Post
57sailplane, the easiest way to think about it is like this:

Think of a point in time and a point on a propeller blade. At any given time, that point will be subject to two forces: The resistance of carving through the air along its rotational path and the air rushing against it because of the movement of the plane. These two forces are perpendicular, i.e. have a 90 degree angle between them. To understand the actual air flow against the propeller, we need to combine the two forces. Mathematically, we call this a "vector addition".

Instead of going into the mathematics (I think we've had enough of that for today), we can think of it like this: Imagine you're in a convertible car with a 20mph crosswind. When the car is stationary, it's going to feel like the wind is coming from the side. However, if you accelerate the car to, say, 60mph, you're going to feel mostly headwind. This is because the speed of the car turns the "apparent wind vector" towards coming from straight ahead of you the faster you go.

Now think about the propeller again. A point near the hub turns very slowly compared to a point toward the tip. That means that the point towards the hub is going to experience mostly headwind (due to the movement of the plane), whereas the point close to the tip is going to experience mostly its movement along its circular path. Therefore, it makes sense to have a propeller that's twisted so that the areas close to the hub have their leading edge pointed more towards the direction of the headwind, since that's the airflow they'll mainly experience. The opposite is obviously true for the areas close to the tip.

Edit: The example with the car got a little confusing. For it to make sense, you have to think of the "headwind" of the propeller as the air it encounters along its rotational path and the "crosswind" as the wind generated by the movement of the car. Then in makes perfect sense! The point is that an airflow is going to "feel" different dependent on your speed relative to it.
after reading this it threw up a red flag. well then if the center of the prop is designed to cut threw the head wind and it twist to the tip to cut threw the rotating wind then this prop can see a wide variety of lift but only at each segement or profile will it see most efiecient lift so at a certain combined rotating and forward speed only one section of the prop would see most efficient lift. while the rest of the prop would not.

this is the circumstances that we would get if we only look at the head wind of the hub and tip or lift from these to values and only this alone. wich i can say by reading that for most part is all we have been looking at.

so why dose a high pich prop see more forward velocity at a given rpm then one with less pitch. if both props can hit that certain speed then there sections with most efficient lift should produce the same forward speed.

well i looked at two apc props of the same diameter with differant pitches and i found that one had noticably higher pitch near the hub and both were very simular at the tip if not the same.

so what is it that makes that dang high pitch prop go faster.. hmmm well i thought about this a long wile and it could not be that inner part of the prop with more angle of attack produces that much more lift. because the rest of the prop is to flat and simulare to the lower pitched prop and would just slow it down.

then i asked the question why dont the just make the prop that has the same pitch all the way along from hub to tip and they do as mentioned before a variable pitch heli prop.

so why then do they make them differant.

a light bulb went off in my head. being the hot rod enthossist that i am i have looked into turbos, and superchargers and what not.

oh and centrifical superchargers and this is were the key lies.

its not just the the most efficient pitch that gives you foward thrust its the dynamics of the centrifical rotation that i and most overlook.

on a centrifical pump there is an inlet at the hub this air is sucked in because the flats of the charger veins that we would accociate with the hub pitch are shoving the air outward because of the cenrifical forsces.

on a centrifical charger you are increasing velocity because the speed of the outer veins are moving at a much higher rate so the air moves from the hub down to the tip because of centrifical force increasing in speed as it reaches the speed of the tip. thus moving more air because it is at a higher velocity.

this is what is hapening on a prop. the more pitch the flatter it is near the hub to the axis of the motor and thus producing more centrifical force pushing that air toward the tip of the prop were it is pushed straight down wind or creating foward thrust for the plane.

it all makes total sence to me now. what the prop is doing is more then being just a wing it is acting as a centrifical pump also.

this is why props are not made like heli props. as a helli prop needs to move lots of air over a wide area at slow speeds. it need a lot of thrust not velocity.

so actually a fuse that eats up a lot of the center section of the prop could still fly fast if the prop had more headwind area from the hub to the edge of the fuse and the front of the fuse was smooth. this could induce even more centrifical velocity and creat a faster flying aircraft. of corse it probably would not fly as fast as one with a small fuse because of over all drag of the fuse haveing more surface area. but it is posible that you could make a plane with the same fuse go faster by what i am saying more forward pitch to the edge of the fuse then twist.

well like i said before these are just my ideas and thoughts i could be wrong.

Last edited by 57sailplane; 02-11-2011 at 05:48 AM.
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Old 02-11-2011, 08:26 AM
  #88  
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57sailplane,

I like your thinking but despite the apparent logic of your thinking the conclusion is incorrect. Ignoring effects of any fuselage volume that may be in the way the air actually contracts toward the hub as it moves through the prop disk, not 'expand' outward toward the tips. It does this because the air is accelerated as it passes through the prop disk and as the volume of air is unchanged then the cross sectional area of the 'streamtube' must be less at the rear, where the air is moving faster, than it is in front of the prop where the air is moving slower.

You can see this illustrated on the most excellent MH-Aerotools site: http://www.mh-aerotools.de/airfoils/index.htm

As for what makes a course pitch prop go faster... A course pitch prop doesn't really make a plane go faster as such. To go faster you need to have a surplus of power, it's having more thrust than drag that makes a plane go faster and to make more thrust at high speed you need more power. if you put a course pitch prop on a plane that hasn't got enough power then it will go slower, not faster, in fact it might not even get off the ground.

What a course pitch prop does, as per the earlier discussions, is optimise the prop to generate thrust at a higher forward speed compared to a fine pitch which is optimised for lower speed.. But to make use of this higher speed optimisation you still need plenty of power.

I think you will find that the angle of the tip on your course pitch props is in fact higher than on the fine pitch ones. It's just that the angles at the tip are much smaller than at the root so small differences are hard or impossible to spot without a prop pitch gauge.
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Old 02-11-2011, 04:24 PM
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Originally Posted by JetPlaneFlyer View Post
57sailplane,

I like your thinking but despite the apparent logic of your thinking the conclusion is incorrect. Ignoring effects of any fuselage volume that may be in the way the air actually contracts toward the hub as it moves through the prop disk, not 'expand' outward toward the tips. It does this because the air is accelerated as it passes through the prop disk and as the volume of air is unchanged then the cross sectional area of the 'streamtube' must be less at the rear, where the air is moving faster, than it is in front of the prop where the air is moving slower.
Just a thought. It's well known that little of a propellers thrust and horsepower requirement comes from the inner diameter of that propeller. So, perhaps the only reason the inner portion of the propeller has a high degree of twist is to reduce drag
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Old 02-11-2011, 04:43 PM
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Originally Posted by kyleservicetech View Post
Just a thought. It's well known that little of a propellers thrust and horsepower requirement comes from the inner diameter of that propeller. So, perhaps the only reason the inner portion of the propeller has a high degree of twist is to reduce drag
That's exactly why. Think about it: The "side motion" (caused by rotation) is very slow close to the hub, but the headwind coming from the movement of the plane is exactly the same. So the majority of the airflow the prop is going to encounter comes at it from the "front" (in the direction the plane is traveling). If the prop had the same pitch close to the hub as it has at the tip, the wind would hit a wide, flat surface, causing a lot of drag. That's why you twist the blade.

Let's take an example: Your 9" prop spins 10000 RPM. That means the tip of the prop travels at a whopping 267 mph. Let's say your plane is doing 20 mph. Now think of a point 5% (or 0.45 inches) out from the hub. Your rotational speed is only a twentieth of that at the tip, so that point is only traveling at 13.35 mph in the rotational direction. But your headwind is still 20 mph! So it's easy to understand that the prop is going to experience mostly headwind. Hence the increased pitch.

Last edited by NJSwede; 02-11-2011 at 04:47 PM. Reason: Got some numbers wrong. Fixed that...
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Old 02-11-2011, 04:44 PM
  #91  
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Originally Posted by kyleservicetech View Post
Just a thought. It's well known that little of a propellers thrust and horsepower requirement comes from the inner diameter of that propeller. So, perhaps the only reason the inner portion of the propeller has a high degree of twist is to reduce drag
Most efficient props - APC, elliptical/slow flight, scimitar - modulate the increase in AoA (twist) and chord (taper) towards the root to precisely equalize lift after accounting for the radial V delta, then use a tip design, or a full elliptical plan form, a taper ratio with an elliptical approximation, or some blend of all of the above to efficiently reduce lift to zero at the tip.

It is also important to note that fixed pitch props are optimized for a certain forward speed and RPM. Also, that they are parts of a larger system so even if optimized as a subsystem in a vacuum, they may produce overall substandard performance. That is why installed static tests, while interesting because an airplane can start, or achieve, zero airspeed in flight, do not reveal much useful or comparable info about how a prop or motor performs. And uninstalled static tests never contain any usable or useful information.

As far as the pump idea - that is a valid way to look at propellers - but props are best described as axial compressors not centrifugal. And yes, like all wings, there is some small component of spanwise flow, particularly with sweep, but that is more of an optimization issue around the edges than a core design issue, especially with a straight wing/blade design.

Last edited by z-8; 02-11-2011 at 05:11 PM.
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Old 02-11-2011, 05:07 PM
  #92  
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Here's what it would look like if the twist of the prop was there just to compensate for the difference in wind vectors depending on the speed at which the blade travels along its rotational path, together with the forward motion of the plane.

Propeller: 9x6. I'm assuming you measure pitch at 70% from the hub (some people seem to be saying it should be 3/4 of the blade, other say it should be 2/3, so I picked something in between).

This means a theoretical max speed of about 40 mph. At that speed, the optimal pitch at various points on the blade would be as follows. Not sure how that relates to a real prop, but this is what it would look like in theory (and if my calculations are correct)

10% 56.05162215
20% 36.60212425
30% 26.34225326
40% 20.37305981
50% 16.54611732
60% 13.90536506
70% 11.98081356
80% 10.51892784
90% 9.372149978
100% 8.449195674

Pitch numbers are in degrees.

If you graphed it, it would look like this:




Now let's all go measure our props to see if I got it right! :-) These are theoretical numbers and prop designers might have other things in mind and tweak the numbers. But the point here is that the changing wind vector depending on where on the prop blade you look at it seems to translate reasonably to what props actually look like.

(I'm working on a model to calculate thrust on an elliptical prop, but that math got so nasty that my brain shut down last night when I was working on it...)


Last edited by NJSwede; 02-11-2011 at 05:09 PM. Reason: Fixed formatting
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Old 02-11-2011, 05:21 PM
  #93  
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"I like your thinking but despite the apparent logic of your thinking the conclusion is incorrect. Ignoring effects of any fuselage volume that may be in the way the air actually contracts toward the hub as it moves through the prop disk, not 'expand' outward toward the tips. It does this because the air is accelerated as it passes through the prop disk and as the volume of air is unchanged then the cross sectional area of the 'streamtube' must be less at the rear, where the air is moving faster, than it is in front of the prop where the air is moving slower." quote jetplaneflyer

yes i agree. but you forget one minner point not all of the prop is seeing this beacause of the to high of pitch of the prop is stalling until it sees enough head wind. so it would not act as you are saying in this area of the prop until it got up to enough forward speed. so there is only a few places for the air to go. the air cant go out the front because it is seeing new air because we have foward movment caused by the prop tips. some of the air dose go out the back because of forward movement and preasure from the stalled prop but i believe that most of it flows away from the axis of the motor because there is another force and that is centrifical. you cant just take away this force it is there and it is acting. or a centrifilcal pump or supercharger would not work and the scrambler at the fair would not be as fun. lol

"As for what makes a course pitch prop go faster... A course pitch prop doesn't really make a plane go faster as such. To go faster you need to have a surplus of power, it's having more thrust than drag that makes a plane go faster and to make more thrust at high speed you need more power. if you put a course pitch prop on a plane that hasn't got enough power then it will go slower, not faster, in fact it might not even get off the ground." quote jetplaneflyer


i agree you need enough thrust to overcome the drag of the plane but its actually the velocity of the thrust that gets you there. if it were just thrust then a bigger diameter prop that produce the same amount of thrust would go faster. but we know it wont it may even go slower but producing more thrust over a bigger area. so it definatly the velocity of the air that makes you go faster.

if it were simple and there were no pumping forces as i describe and it worked as you are saying then also the opisite would be true if the inner part of the prop was seeing the effects as you say at high foward speed then the prop tips would no longer see proper angle of attack and act as drag. they would not stall but be pushed threw the air like a windmilling prop trying to push in the opposite direction thus the airplane would only go as fast as the outer pitch of the prop wich we both no has more area then the inner.

man it realy makes sence to me now.

ps. this was into responce of jetplaneflyer and was by no means a pick. i just could not get the words onto the computer fast enought lol..
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Old 02-11-2011, 05:42 PM
  #94  
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after thinking this all through i really do think that since a plane with a big fuse such as the one you have on this thread z-8 could see some improvments in speed with a totally differant design of prop then the ones available now. a prop that has the high pitch runing out to near the edge of the fuse then having twist. this would give you more pump action and more velocity because as the prop stands now its just stalling anyway in this area and is trying to force air back into the fuse. it would be better it it were directly forced more outward to were the prop can do some work.

maybe and maybe not but i would tend to think so. lol
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Old 02-11-2011, 05:55 PM
  #95  
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Originally Posted by 57sailplane View Post
after thinking this all through i really do think that since a plane with a big fuse such as the one you have on this thread z-8 could see some improvments in speed with a totally differant design of prop then the ones available now. a prop that has the high pitch runing out to near the edge of the fuse then having twist. this would give you more pump action and more velocity because as the prop stands now its just stalling anyway in this area and is trying to force air back into the fuse. it would be better it it were directly forced more outward to were the prop can do some work.

maybe and maybe not but i would tend to think so. lol
You mean a prop that starts with a high pith near the hub and continues like that all the way out to the tip? Or even one that has higher pitch towards the tip? The problem with that is that you'd get such an extreme angle of attack that you'd generate a ton of drag. There's a good reason that propellers are designed the way they are and for the most part look very similar. It's simply the best way of designing them. I read somewhere that the prop the Wright brothers designed is only 5% less efficient than props commercially available today. And that's a 100 year old design! When a design survives that long, it typically means someone got it right...
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Old 02-11-2011, 06:08 PM
  #96  
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"You mean a prop that starts with a high pith near the hub and continues like that all the way out to the tip? Or even one that has higher pitch towards the tip?" quote from njswede

no not one with the high pitch to the tip.. one that has the high pitch from the hub out to the edge of the fuse then twist more drasticly to the tip. i feel that the twist could be shortened up and made more drastic because it would see more air from the pumping forces from the inner part of the prop.

this of corse would only be benificial to this size of fuse but i would think you could make a prop like this that would cover an aray of large fuses. there would definalty be some point were this prop would out perform the regular prop depending on the size of fuse.

its definatly a loosing battle for the large fuse and i do agree that props have there reason for design. but i wonder how much effort has been put into what i am talking about.

very fun conversation. even if i am way off base i have learned somthing.
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Old 02-11-2011, 06:18 PM
  #97  
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Originally Posted by 57sailplane View Post
no not one with the high pitch to the tip.. one that has the high pitch from the hub out to the edge of the fuse then twist more drasticly to the tip. i feel that the twist could be shortened up and made more drastic because it would see more air from the pumping forces from the inner part of the prop.
Ahaaa! Maybe you could. The point is that with most props (most notably props with a square or only slightly tapered trapezoid planform) you're going to have something like 80-90% of the thrust on the outer 50% anyway. And the other point is that there's only so much you can do in terms of messing with the pitch. If you mess too much with the pitch curve I showed a few posts back, drag inevitably will occur.

But I guess you could say "to heck with it" and design a prop that has zero lift and is optimized for minimal drag only where it's obstructed by the cowl or engine and go for maximum lift outside the cowl. Maybe there are props that do that. I honestly don't know.
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Old 02-11-2011, 06:28 PM
  #98  
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Originally Posted by 57sailplane View Post
""the outer pitch of the prop wich we both no has more area then the inner.
The outer part of the circle having more area than the inner part has already been accounted for with the increase in radial velocity. More V sweeps more area. So to consider it again, is double accounting. I think that is where a lot of people get confused.

Also, the area swept, or the velocity, is not the same as lift produced, or V would be the only term in the Lift equation. In fact, if you vary both chord and Cl, like efficient props do, you can directly offset the affect of increasing radial velocity (squared).

That is why props do not create more lift toward the tip, same as any tapered wing with constant AoA (the prop pitch must twist to keep AoA constant). What you have is equal or decreasing lift (or even negative lift) as you progress outward from root to tip - matched to the simple physics affecting the decreasing ability to carry force by any lengthening cantilever.
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Old 02-11-2011, 07:06 PM
  #99  
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Originally Posted by z-8 View Post
That is why props do not create more lift toward the tip, same as any tapered wing with constant AoA (the prop pitch must twist to keep AoA constant).
Are you saying that's true for ALL props?

What about this one?


(HobbyZone Super Cub 9x6)

...or this one...


(Full size EP-3E Orion)

...or this one...



(Unknown)

They all look pretty rectangular (albeit with a rounded tip) to me.
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Old 02-11-2011, 07:07 PM
  #100  
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sorry for the double acounting z-8.

i have and idea i am not sure if it will work but it would be very interesting to try and i just might.

you see the 118mph in my avitar that was developed by pulling a gws fw-190 or a brick through the air. it took a 9x9 prop on the right motor to get it there. now this plane has an even biger fuse then the one that z-8 has not taking away from z-8 plane i am just saying its an even worse fuse to make go fast.

the speed was repeated over and over 3 out of the 5 passes were the same 118 with 2 being 115 and 116mph.

since what i am saying with my idea would could creat more pumping action and also more drag brought out by this conversation. i think there may be an alternative to having it all done on just one prop.

my idea is this leave the 9x9 alone then take another prop of what size i dont no as i will have to experiment and cut off the tips near the edge of the fuse and mount this in front of the other prop. i would gather using a prop of at least the same pitch or more would be needed and am not sure of what dia yet because props of larger dia. are thicker near the hub giving me even more pumping action out to the tip and yes more drag on the motor so an even smaller in dia. main prop may need to be used like a 8x8 or a cut off 8x9. of course i must take the speed of these individual props into acount first.

so it might not be the same as the prop already made this way but its definatly going to be fun trying it.

you guys have created a monster now i have a use for my theory and a way to prove or disprove it or mayby neither as there could definatly be more to it then this simple test. but hay it will definatly be fun and now i have an excues to do those high speed low fly bys.

let me know what you think as i am definatly going to try this.
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