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Propeller Thrust Estimator

Old 01-29-2012, 10:31 AM
  #1  
Umran
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Default Propeller Thrust Estimator

Dear All,
First i would like to thank one of the forumer here who open this thread, http://www.rcuniverse.com/forum/m_10...m.htm#10907244
Because of the problem posed, the calculation of actual thrust of the prop for a given rpm is critical.

No doubt that there are many thrust calculator out there, but trust me, many are way off from actual figure. Currently there are iteration solution suggested by University of Sydney which can be viewed here - http://www-mdp.eng.cam.ac.uk/web/lib...ler/prop1.html

To me that is a very good technique for estimation. I did draft-out the equations and the iterations are difficult to converge. So you'll end up ashtray...

In view of the proposed idea, i came out with a similar iteration technique, however the basic changeable variables holds a different unit value. No worries if one doesn't understand it, the target is to have a very close to life reading of actual THRUST per propeller dimension with certain rpm.

The inputs are all the yellow colored cells in the spreadsheet. The major output are in red. In using this, one has to get the 'SOLVER' plug-in for excel. Activate also macro for the file usage.

Check any of my posts where I have stated static thrust measurements for my engine / prop combinations. All of these figures I have accurately measured with my test setup. My results have proven to be repeatable, and also correlate with what I experience in the air. They all have shown Thrust HP to be optimistic. The degree varies, but seem to get worse as pitch goes down and diameter goes up.
Here are a few examples:

Mejzlik 20x6 @ 8800 RPM - Measures 20 lbs, 4 oz thrust. THP says 37.17 (used APC airfoil)

Zinger 20x6 @ 8300 - Measures 19 lbs, 14 oz. THP says 31.19.

APC 17x6 @ 9400 - Measures 17 lbs, 12 oz. THP says 22.14.

APC 13x4W @ 11,800 - Measures 8 lbs, 12 oz. TP says 11.93.

You get the picture


Above was posted by Aerosplat in this thread http://www.rcuniverse.com/forum/m_608127/tm.htm We have to extend our gratitude to his work because i managed to simulate the result obtain by him physically via my spreadsheet by a very close margin of within 5 to 7%.

There are few variables that maybe different from actual production propellers. In my spreadsheet i utilized 'CLARK Y' airfoil data both for Cl and Cd. Both parameters have been manipulated to produce a function in terms of actual angle of attack. I do not actually know what type of airfoil being used to produce Mejzlik, Zinger or APC nor did i actually know the hub diameter, the root chord and the tip chord. And the worst unknown is the actual profile of the each blade when you view from front; on this juncture most prop blade will have a slight curvature at the root, grow a bit and taper down at the tip. In the spreadsheet however I modeled the blade to be a straight taper from root to tip.

The equations being derived as such a way that, this spreadsheet is usable even when the forward velocity is zero. You'll have actual static thrust reading. When you add in some forward speed, you'll see a reduction in thrust. You may continually adding speed until it reaches the 'pitch speed', because of the nature of the airfoil, even at pitch speed, it'll still produce a small amount of thrust.

The button "CALCULATE" is for the SOLVER program to work-out the quantity required for 2 variables in order to satisfy a few equations. At times it may not converge, this can be viewed via the green solver cells which is not 'zeroed'. Most of the times this may be caused by illogical value being inserted such as more forward speed than the pitch speed itself.

The inputs are,

Prop Diameter (inch)
Hub Diameter (inch)
Pitch (inch)
Forward Velocity (inches/sec)
Root Chord (inch)
Tip Chord (inch)
Revolutions Per Minute (rpm)
Number of Blades
Altitude of airport (ft) - this is to determine the average air density.


And for example, i uploaded the file with example data for mejzlik 20 x6 spinning at 8800 rpm. The result can be seen on it...

The solver may take a few seconds to work-out the solutions, so be patience a bit...Especially when one is using a relatively slow computer.

One other point, this is my work for my thesis, use it as you please, but please take note the original developer of this.

I hope many will find this useful, and if there are any question, i'll be glad to answer them. And for those who are actually testing propellers, your outputs are most welcome in this thread.

And here is the link to download this file http://www.mae.my/download/ThrustEstimator.xlsm

Thanks
Old 01-29-2012, 01:53 PM
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ggraham500
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Default RE: Propeller Thrust Estimator

1. Where/how do you estimate "Forward Velocity (inches/sec)"?

2. Tip chord would depend upon the tip end shape, right? So how so you take those various measurements?
Old 01-30-2012, 02:53 AM
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Umran
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Default RE: Propeller Thrust Estimator

The actual forward speed can be measured via a radar gun or some other measurement tool... just for info 1 mile/hr = 17.6 inches/sec

The question about the tip chord, that is a good one....

Kindly refer the attached diagram, the center drawing represent most of the production prop profile, to actually simulate it, the sectional chord can be measured individually in a fashion shown on the bottom picture. However in my spreadsheet, like i said before, i modeled it as a straight taper as shown in top picture. The root chord is the largest chord while the tip chord is the last available parallel chord a long the airflow direction.

If you choose to measure each individual sectional chord, you may modify the cells F5 to Y5 to accommodate your dimensions. In fact for a propeller designer this will be a very good tool to simulate the best possible profile of a prop where the thrust produced at max while keeping the total negative torque down.

Let us know your findings... Thanks.
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Old 01-30-2012, 07:47 AM
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pe reivers
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Default RE: Propeller Thrust Estimator

Umran,

Can you export the file in XLS format? My software cannot handle XLSM.

For your information: The THP calculator is known to be extremely optimistic. When I use my own calculator, and set the fuselage drag to 3, I get:
22.33 lbs
18.99 lbs
15.97 lbs and
9.9 lbs

edit:
updated thrust calc data, calculated using factor 3 for fuselage drag

Old 01-30-2012, 03:10 PM
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Default RE: Propeller Thrust Estimator

XLSM file extension is an Excel Macro-Enabled Workbook file. Therefore you may need to enable the macro to be run on this particular spreadsheet.

If you have 2003 or below excel, you may download the compatibility pack via this link, http://www.microsoft.com/download/en...aylang=en&id=3

The macro in this file is a set of instructions which I wrote for executing the SOLVER function to iterate each individual element (20 of them) variables for induced velocities in axial and radial directions.

In simple term, it is not a simple closed form calculation i.e. executing a direct function such as y = 3x, where you just plug in x value, you’ll get y value. No it is not the case here…

The solution require a few guessed values, calculate and check the result against pre-set conditions, if okay – stop, if not okay – re-insert a new values, re-calculate, re-check and so on and so forth until all the values inserted satisfy all the set conditions. Off course these loop processes can be done manually, but it’ll take ages to get it done. Therefore we leave it to the computer to do this task as it never complain as long as it has the electrical power to run… hehehehe… This is where the SOLVER plug-in for excel is required. And that is why also I cannot upload XLS file format alone, because it won’t work!

You may load the solver plug-in via these instructions from Microsoft, http://office.microsoft.com/en-us/ex...001127725.aspx

I may write the codes using matlab, but not everyone have access to it, but with excel I think great many people will have it in their PC.

With regards to the drag, we deal only with the induced drag on the propeller blades that is trying to slow down the rotation of the propeller itself. If you are referring to the post made by Aerosplat, all he did was run the prop at an rpm, measured the static thrust via either a spring scale or a calibrated weight. The output here will not depends on aircraft drag what so ever. And that is what our goal here, to simulate actual thrust available when you run a propeller.

The final aircraft movement forward however will depend not only on the available thrust but the aircraft aerodynamic drag as well. And determination of aircraft drag is whole new subject altogether. Maybe if time permit, I’ll deal with it…

But on the surface, let us say your aircraft weighs 12lb and if the available thrust is 14lb, when the aircraft is statically vertical, it may hover forever because the available thrust is more than the weight that is pulling the airplane down. In this scenario, the aerodynamic drag of the airplane doesn’t play any role in pulling down the airplane.

When you are flying straight and level, the speed of the airplane is limited to the speed of air being dump rearwards. The different of speed between the air being dump to the back and the forward speed of the airplane is the actual thrust available. If at that moment there are no accelerations (i.e. your airplane is flying at constant speed) then those available thrust are equivalent to the airplane drag. As you accelerate the airplane, the different in speed drew closer and closer resulting in your available thrust drew lower and lower until it reach an equilibrium state once again.
Old 01-31-2012, 09:51 AM
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pe reivers
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Default RE: Propeller Thrust Estimator

Umran, thanks for the links. I will see what my office3000 system is capable of. (I do have Macros enabled)

But on the surface, let us say your aircraft weighs 12lb and if the available thrust is 14lb, when the aircraft is statically vertical, it may hover forever because the available thrust is more than the weight that is pulling the airplane down. In this scenario, the aerodynamic drag of the airplane doesn’t play any role in pulling down the airplane.
Unless you measured the thrust with a fish scale tied to the plane, allow me to disagree here. Tie a tiny chute behind your plane and see if you can still taxi out! Plane drag and propeller thrust are connected to the moving plane system, and operate in opposite directions. Because the plane fuselage is right in the middle of the prop wash vortex, the specific drag plays a major role in thrust available to measuring devices, unless these devices (and everything behind the prop disk) are shielded from the prop blast.
That is where calculated and measured thrust can diverge bigtime.

Now I will first see if I can check out the fruit of your grey cells.
Old 01-31-2012, 11:04 AM
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Antique
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Default RE: Propeller Thrust Estimator

In actual testing with a fish scale Pe's chart for thrust and speed is as close as anything we have seen...ThrustHP chart is just ridiculous...
Old 01-31-2012, 11:23 AM
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Default RE: Propeller Thrust Estimator

Antique and Pe:
Please help me here!
In putting my prop dia and pitch
23/8 compared to 22/10 for a G 62
In putt 6500rpm
Prop const at 1.35

I have red numbers popping up on the 23/8

Green numbers on the 22/10

Also telling me power compared to engine #2 is 98%
Compared to engine #1 is 102.1%

Assuming engine #2 with the 22/10 is the better prop choice.

PS:
Using Pe Reivers Prop Chart.

Thanks for all the info you guys have and will supply.
Old 01-31-2012, 12:55 PM
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pe reivers
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Default RE: Propeller Thrust Estimator

red numbers means you spin the prop in the noise region. Yanks like that, Europeans get banned for that.
To define the better prop, look at the thrust charts for engine 1 and engine 2. Often the small pitched prop will be better at taxi speeds, but as soon as airborne, higher pitches cut the cake.

Use the chart as a tool, not as an end in itself. [email protected], juggle the numbers in engine 2 until you find comparable horsepower at 6500 rpm. This prevents you from getting overoptimistic dreams, and narrows down the choices of props you have.
Old 01-31-2012, 01:28 PM
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pe reivers
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Default RE: Propeller Thrust Estimator

Umran, the links work. I already had solver installed.
A very nice piece of work.

stupid remarks deleted:

I will have to check how your data can be used to check my own work, especially the in flight thrust graphs.

Old 02-01-2012, 11:26 AM
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Umran
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Default RE: Propeller Thrust Estimator

First, congrates for the success in installing the required add-in for excel to operate the solver function.

About the aircraft drag, it has nothing to do with actual thrust available from the engine - this holds true. Every action there is an equal and opposite reactions. Just think for while, if we have a thrust of say 14lb, if we need to mount this engine on a table, at a minimum we need to have the engine mountings able to resist 14lb of force pulling it apart.

Now for the same engine, if we were to mount it on an airplane, the pulling force received by the firewall will be at least 14lb also. Now if somehow we manage to deflect the total mass airflow forward of the prop disk, then what we are actually doing is reversing the thrust. Under this condition, the engine mount will receive pushing force instead of pulling!

Most of the time, the nose of the airplane does deflect the airflow slightly outwards, however for any typical aircraft the deflection amount doesn't cause the airflow to reverse in direction, therefore the mass amount of air still being dump towards the rear of the prop disk. Looking at this, we know that in order to balance the momentum of the airflow, the airplane firewall will 'feel' the pulling force...

So be it on a test bench or on an airplane, the nett thrust produced by a propeller(same pitch + same rpm + same ambient condition) remain constant unless we can deflect it until the direction of the airflow moves towards the front of the prop disk.

Okay back to this file. With ref to "2. Tip chord would depend upon the tip end shape, right? So how so you take those various measurements?" a very good question posted by ggraham500, i figure it is good if user only need to insert the prop diameter, pitch and rpm without having to worry about the root chord and tip chord.

So the solution is, based on a typical propeller that i have, i took the plan form coordinates, graphed it out. Equate those coordinates on a 4th order polynomials (curve fitting process), and end up with a written function of chord dimension at a given radius. Using this function, each element average chord can be determined with ease therefore more precise than initially modeled straight taper from root to tip.

The setback... as the plan form area of the simulated prop matches the real thing, the solution will over-predict the thrust produced say around 7 to 10%. The reason for this is simple, in real life, there is a point on the propeller blade that does not produce any thrust at all. The point is the tip of the blade itself. The higher pressure region at lower part of the tip will try to balance itself out to the top surface of the tip. This balancing act causing the tip to loose its thrust. We call this phenomena "tip losses". And because of this also, a shrouded prop will be better in terms of its efficiency.

To overcome this over prediction, Prandtl did suggest tip loss correction factor by modelling it as a vortex element. At this moment however, i leave it out first. Okay enough said, for those who would like to have this modified spreadsheet kindly download here, http://www.mae.my/download/ThrustEstimator2.xlsm

Thanks
Old 02-01-2012, 01:15 PM
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pe reivers
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Default RE: Propeller Thrust Estimator

Umran,
Do a simple experiment with fish scales please. The force on the firewall is indeed constant, but that is not the case of the force between whole plane and rest of world.
Fprop=Ffirewall
Fplane=Ffirewall-Fdrag OR
Fplane=Fprop-Fdrag  This is what fish scales register.

Any shielding of the prop vortex blast must NOT be connected to the engine firewall system. In that case you can get rid of the drag factor.

Old 02-02-2012, 06:13 AM
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Umran
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Default RE: Propeller Thrust Estimator

Dear Pe,
We'll discuss further on that.

Yesterday's file that i uploaded, by changing the cell for inputs i missed out the pitch speed calculation... sorry... anyway it has been corrected.

Prop Size Rpm Thrust (kg) Thrust Estimate (kg) Diff (%)
11 x 7 13000 3.08 3.49 11.747851
11 x 7 12000 2.72 2.98 8.724832215
11 x 8 12500 3.12 3.50 10.85714286
11 x 8 11400 2.74 2.91 5.841924399
12 x 6 12400 3.65 3.84 4.947916667
12 x 6 11300 3.25 3.19 -1.880877743
12 x 8 10800 3.3 3.52 6.25
12 x 8 10500 2.99 3.33 10.21021021
13 x 6 11200 3.64 4.10 11.2195122
13 x 6 10600 3.32 3.67 9.536784741

Average Variance (%) 7.745529654 on over prediction.


To progress further, above table was included in the latest file (same 2nd link as before). It shows the different between calculated and actual measurement done by ROSSI engine. No i'm not promoting rossi or any other manufacturer, but the way the measurement being taken and the way the data being presented seems sound and good. There are a few things which were unknown to me, they were the plan form of those props that they used and the average air density during the testing. This may account for another 1 or 2% variance.

Looking at this, for an average RC hobbyist who wanted to have a close estimate of required thrust for estimating take-off distant, hovering or just for fun of it may be able to use this file. For a serious propeller designer/developer, this tool will be their friend. By changing the dimension, pitch and profile, one may predict the actual behavior of their designed propeller.

Attached also the actual file produced by them (rossi).

rgds
umran
Attached Files
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Ec87627.pdf (187.9 KB, 95 views)
Old 02-02-2012, 08:54 AM
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Default RE: Propeller Thrust Estimator

Hi!
Come on guys!
Those static trust figures are worth nothing!
Just bolt a prop on and fly the plane and see what works best in the air instead ...sooo simple! why complicte things that don't have to be complicated???
Old 02-02-2012, 09:02 AM
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Default RE: Propeller Thrust Estimator

Look at the attachment and So if the experts say their plane with a 6 pitch prop goes over 100 mph @ less than 10 or 12,000 rpm

are Rossi engineers wrong ??
Old 02-02-2012, 11:04 AM
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Default RE: Propeller Thrust Estimator


ORIGINAL: jaka

Hi!
Come on guys!
Those static trust figures are worth nothing!
Just bolt a prop on and fly the plane and see what works best in the air instead ...sooo simple! why complicte things that don't have to be complicated???
in some aspects this is true,

BUT when you are buying $30-$40 and maybe even $100 dollar props you would like to have some calcs to help you along.
Old 02-03-2012, 02:56 AM
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pe reivers
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Default RE: Propeller Thrust Estimator

That is exactly what the calculators are for. + if you have a good one, make a decent speed estimate of your plane.
My calculator for example calculated the Spitfire prototype top speed only 4 mph too low. The sundowner using prop and rpm data provided by Ralph was spot on in top speed calculations (checked by radar gun).
I think this also answers JAKA's argument as to the uselessness of static thrust only. Indeed, a good calculator must go one step beyond that.

Whilst my calculator is based on quite general prop formulae and on NACA publications plus some educated guesses plus field report feedback, Umran's work may turn out to be very valuable for us model pilots, either by providing scientific verification of observations, or by providing extra calculation confidence base. As a mechanical engineer, this is beyond my scope of education, so I hope for some synergetic results.
Old 02-03-2012, 11:24 AM
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Umran
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Default RE: Propeller Thrust Estimator


ORIGINAL: jaka

Hi!
Come on guys!
Those static trust figures are worth nothing!
Just bolt a prop on and fly the plane and see what works best in the air instead ...sooo simple! why complicte things that don't have to be complicated???
Dear jaka,
Radio Control hobby is vast. Those involved in this hobby come from all walks of life. Some are millionaires and some are just making enough for living. For those who can afford there will be no problem at all and your statement suites them just fine. However for those who can't, we definitely do not want them to be discouraged. For this group of hobbyist, every penny that they spend better make it worth!

We may argue by saying, "if you are just making enough to live, why must you get involve in this rather expensive hobby?". The answer is very simple, this highly technical hobby have produced and will continually produce many fine engineers who develop, design and maintaining those things that flies today. You maybe seated in one of them during your travel and perhaps i myself maybe involved in ensuring the safety of that particular flight, possibility is there and you'll never know.

Myself was exposed on this hobby since my childhood days, but back then in 1978, an average family can't afford to buy a model airplane for their son. Because of the deep curiosity i continuously pester my father to buy one. Lucky me, i finally got my hands on that green plastic spitfire with cox 0.049 engine control line model. That my friend kick start everything.

I believe, many young students out there who get involve in this hobby will one day develop more than what we can today.


rgds,
umran
Old 02-03-2012, 11:55 AM
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Umran
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Default RE: Propeller Thrust Estimator

Dear Pe,
What i can offer is, since you have large variety of prop brand selection, you feed me the sectional chord lengths. For example,

Prop Brand Base Dimension HUB diameter sta 1 sta 2 sta 3 .................................................. .............. sta 20
Master AS 23 x 8 .05 .04 .035 .033 .................................................. .............. .018
APC 14 x 10 .... .... .... ...... ......
APC-W 13 x 4 ..... .... .... ...... .................................................. .............. ......
........
.......
.......



For each brand, only 1 diameter is enough, for an example APC prop, you have only 14in diameter, this data is enough because plan form design of a brand rarely differ as the diameter grow or shrink. But a different model of does i.e. APC normal and APC-W are different, then say you have APC-W 13 inch, then collect those data as well.

After getting this data, i will draft out the required polynomial function depicting the chord length against the radius. By having this function, as the prop diameter grow or shrink the planform for the simulated prop will maintain as the ratio of originally measured chords. This way each prop brand calculation will be unique. Once that done, you may compile it in your sheet.

In the above table example, the base dimension is in inch however for the rest is in meter....
Old 02-03-2012, 12:33 PM
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Umran
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Default RE: Propeller Thrust Estimator


ORIGINAL: Antique

Look at the attachment and So if the experts say their plane with a 6 pitch prop goes over 100 mph @ less than 10 or 12,000 rpm

are Rossi engineers wrong ??
Dear Antique,
I believe your are referring to this statement below from the file produced by ROSSI,

"The maximum possible speed (in miles per hour) of a propeller-driven airplane equals the
propeller pitch in inches times the propeller RPM in thousands.
12 - 6 prop @ 12,000 rpm : 6 X 12 = 72 mph maximum possible speed
12 - 8 prop @ 12,000 rpm : 10 X 12 = 120 mph maximum possible speed"


Okay let us see it together....

12 x 6 prop, 6 inch pitch - Theoretically 1 revolution will move the airplane 6 inches.

For 12,000 revolution per minute, it has move 12,000 x 6 = 72,000 inches per 1 minute.

In 1 hour, 72,000 x 60 minute = 4,320,000 inches per 1 hour.

We know 1 mile = 63,360 inch. Therefore 4,320,000/63,360 = 68.18 mph.

The second statement 12 x 8 prop - with the same calculation = 90.91 mph. But I believe they want to meant 12 x 10 prop, if so with the same calculation, 113 mph.


mmmm.... close but not exact. But in real life, if the airplane is not in diving mode, you will never achieve this speed. If however one can, wow... that guy will be rich because he can design an airplane with ZERO drag!!! hehehehe....
Old 02-04-2012, 06:43 AM
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pe reivers
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Default RE: Propeller Thrust Estimator

Umran,
I cannot give you these values. In my calculations I use a prop load factor where all variables are accounted for. The load factor of  some propellers were derived from dyno tests. From these known propellers the others were guestimated and corrected if usable field results became available.

On pitch speed. I do believe, and field measurements confirm it, that a plane can fly at pitch speed. Why?
1) our prop pitch is measured using the lower airfoil tangent line. Not the true foil chord line. So prop pitch is larger than stated on the propeller.
2) The prop airfoil has camber. That means that the zero lift angle of the foil is negative. How much negative depends on foil camber.

combine 1) and 2) , and it will be clear that at pitch speed the propeller still produces a considerable thrust.
Old 02-04-2012, 08:06 AM
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Default RE: Propeller Thrust Estimator

My problem is not with pitch speed, but with people who think their pride and joy can fly FASTER than pitch speed...
Old 02-05-2012, 07:03 AM
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pe reivers
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Default RE: Propeller Thrust Estimator

That too.
Old 02-05-2012, 07:07 AM
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Default RE: Propeller Thrust Estimator


ORIGINAL: Umran

Dear Pe,
What i can offer is, since you have large variety of prop brand selection, you feed me the sectional chord lengths. For example,

APC 14 x 10 .... .... .... ...... ......
APC-W 13 x 4 ..... .... .... ...... .................................................. .............. ......
........
.......
.......



For each brand, only 1 diameter is enough, for an example APC prop, you have only 14in diameter, this data is enough because plan form design of a brand rarely differ as the diameter grow or shrink. But a different model of does i.e. APC normal and APC-W are different, then say you have APC-W 13 inch, then collect those data as well.

snip

In the above table example, the base dimension is in inch however for the rest is in meter....
Umran,
APC is a very bad reference platform, because they change the basic propeller design with evry different diameter size. In my spreadsheet I would have to use a dedicated prop constant table for APC alone! Lacking reliable reference data, it is impossible for me to produce it.

Old 02-05-2012, 10:20 AM
  #25  
AeroFinn
 
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Default RE: Propeller Thrust Estimator


ORIGINAL: pe reivers

Umran,

Can you export the file in XLS format? My software cannot handle XLSM.

For your information: The THP calculator is known to be extremely optimistic. When I use my own calculator, and set the fuselage drag to 3, I get:
19.47 lbs
18.3lbs
16.95lbs and
9.4 lbs
It seems your test rig has slightly lower drag values, so if set to 2.8 my sheet is spot on, except for APC. That is due to APC having very different designs for different prop sizes, so each prop size would need it's own constants.
Hi Pe and others

Thank you for great attempt to create a usefull thrust calculator

What am I doing wrong? Is there some input values I'v missed?

When I try to use your calculator:

Mejzlik 20 X 6 (prop Constant 1,18; RPM 8800; fuselage drag estimator 3; altitude air pressure calculator 100 meters; local air temperature 20deg Celsius)

I get 22,19lbs not 19,47 lbs (ref. what you seem to get) ??

BTW for a Great Planes Yak 54 25%: Is the drag estimator 3 pretty much correct?

Danke viel best regards,

Artto


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