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Propeller Thrust Calculator

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Old 03-08-2003, 02:46 AM
  #1
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Default Propeller Thrust Calculator

What is your Propeller Thrust and Wing Loading Calculator ?
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Old 03-10-2003, 05:05 PM
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Default Propeller Thrust Calculator

Are you referring to the computer based Thrust Calculator program downloaded from: http://www.bmaps.net/ ?

Go to "Goodies". While an interesting program and fun to use it is overly optimistic when you get into larger prop sizes.
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Old 03-20-2003, 09:25 PM
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Default Propeller Thrust Calculator

Get "ThrustHP" off the rcfaq website.
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Old 03-27-2003, 11:07 AM
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Default Propeller Thrust Calculator

The above calculators are the same..get it from either place
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Old 03-31-2003, 05:39 AM
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Default Propeller Thrust Calculator

I find that Thrust HP's calculated static thrust is, like said above, "overly optimistic", even on small props. On a plane-engine setup that Thrust HP says I will have 1.5 thrust-to-weight ratio, I can barely hover at full throttle. I feel that ThrustHP's static thrust calculation is too high by 30~40%
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Old 03-31-2003, 11:26 PM
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Default Also,

ThrustHP does not use prop pitch as a factor in it's calculation... only diameter.

I guarantee that a 12x6 turning 10K is going to put out more static thrust than a 12x4 turning 10K rpm.

It's a nice toy, but I don't know how much I'd trust it to be accurate.
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Old 04-03-2003, 09:54 AM
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Default Propeller Thrust Calculator

You know, I think ThrustHP is a good program. Remember, static thrust is different from thrust while the prop is "unloaded" in the air. The thing to note is how much horse power is required to turn a given prop at a selected RPM. I have a Magnum .91 4-stroke in a Super Kraft Cap 232 Sport, and I've tried a variety of props. A 12x8 Scimitar will turn at 10,000 RPM and require 1.2 horse, and give me 6.2 pounds of thrust. The model weighs in at 6.5 pounds It won't pull it strait up but close. A 13x8 shows 8.5 pounds at the same RPM. It will pull it strait up, but not too fast. My point is, it's a good place to start when trying to find a prop for you engine.

Remember to see what your engine is rated at power wise, and note the power requirements to turn a given prop to a selected RPM. Then ask yourself if the engine can pull that many RPM.
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Old 04-03-2003, 05:37 PM
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Default Propeller Thrust Calculator

ThrustHP is nice, but does me no good at the field. I made a small spreadsheet which I uploaded to my Palm Pilot so I can get insight into different props at the field.

You can play with Excel's Solver if you like to see what RPM you should be at for a given RPM; in it's current layout, you can see if you are under-propped based upon the horsepower of the engine and the prop used. You can also check pitch speed as well as total Watts used.

http://www.flindt.us/planes/prop_performance.xls
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Old 04-07-2003, 11:58 PM
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Default Propeller Thrust Calculator

Ok, I just downloaded the Excel file and checked out.
14x8 & 14x10 at 9700 rpm
showed same thrust values.
Thrust formula does not include the value of pitch, why is that?

Running a YS 91AC with 14x10 @ 9700 rpm has no difference from running a 91FX with 14x8 @ 9700 rpm?
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Old 04-08-2003, 12:08 AM
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Default Propeller Thrust Calculator

The thrust calculation has a CL coefficient of lift. Propeller pitch has an effect on this CL term, as well as airfoil shape, and chord width. None of these factors exhibit a linear relationship with the coefficient of lift, that's why basic calculation such as ThrustHP doesn't bother including them. To obtain an accurate relationship between coefficient of lift and airfoil, chord width, and pitch, experimental analysis needs to be conducted to form an elaborate database. Only then can we better "approximate" the thrust result of a given prop & rpm.
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Old 04-08-2003, 12:53 PM
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Default Propeller Thrust Calculator

Quote:
Running a YS 91AC with 14x10 @ 9700 rpm has no difference from running a 91FX with 14x8 @ 9700 rpm?
I used the same formulas as Thrust HP for Thrust, and it only considers diameter and RPM:
Thrust = 2.83E-12 x RPM^2 x D^4

MotoCalc seems to do a good job at approximating thrust values...180 oz of thrust and 194, respectively for the two props mentioned at 9700. I'll have to explore how it is done there and once I come to an answer, update the spreadsheet.

That said, your change in pitch did have a big change in pitch speed, from 73 mph with the 14X8 to 91 mph with the 14X10; a 25% increase.
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Old 04-08-2003, 01:30 PM
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Default Propeller Thrust Calculator

ThrustHP's help file documents the forumulas it uses, their source, and comments on pitch.

The following (to end of this post) is cut and paste from ThrustHP's help file.
----------------------------------------------------------------------
Static Thrust Information

Formulas from AMA mag Oct 86

Load = Prop Diameter^4 * Pitch

Speed = Pitch * rpm * 0.000947

Horse Power = Load * rpm^3 / 1.4 * 10^17

Static Thrust = 0.00000000000283 * rpm^2 * Prop Diameter^4 * Air Density/29.92 * CF value


Note:

Regarding thrust to pitch variables, PRACTICAL test revealed very little if any change in thrust due to pitch variation at the same RPM. This I think is partly due to any increase in thrust being negated by blade stalling and a more turbulent influx area with increased pitch. This obviously only applies to static conditions, it's a whole new ball game under dynamic i.e. flying variables. The program is not designed to calculate thrust under dynamic conditions so it may not be of use for Ducted Fans.
------------------------------------------------------------------------
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Old 04-14-2003, 03:42 AM
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Default Propeller Thrust Calculator

Remember, a propeller has a non-constant angle-of-attack of the airfoil section when you go from root to tip. I was searching on the web for lift vs AOA experimental data, and someone found that 5 degrees to 25 degrees gives the optimal lift-over-drag ratio for a typical airfoil. So while increasing propeller pitch can increase lift toward the tip, the section toward the root will approach stall and generate very little lift; hence the lift generated by the entire prop is not too much different.

After a little calculation, I estimated that if diameter is 3~4 times the pitch, then most of the prop section will be in the optimal lift range. So props like 12.25x3.75, 16x4, 18x6, 24x8, 30x10 fall in that range.

Also remember that stall angle is dependent on velocity. A high pitch root prop section may stall at 6000 rpm, but at 10000 rpm it may be lifting.

All in all, it's not possible yet to precisely predict static thrust of a given prop via calculation, because there are too many unknow variables. The best method with today's technology is through experimental data and provide some sort of correlation plot for other users.
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Old 04-14-2003, 03:56 AM
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Default Thrust Calculator

Seanychen,

I see you are estimating that ThrustHP is 30% or so on the optomistic side.

It seems a lot of people think the program is very optomistic. Do any of you experienced engine gurus and ThrustHP users have a different estimate of how optomistic? Anybody ever test specific static thrust with a scale and compare it with ThrustHP?
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Old 04-14-2003, 10:10 AM
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Default Re: Thrust Calculator

Quote:
Originally posted by fatflyer41
Seanychen,

I see you are estimating that ThrustHP is 30% or so on the optomistic side.

It seems a lot of people think the program is very optomistic. Do any of you experienced engine gurus and ThrustHP users have a different estimate of how optomistic? Anybody ever test specific static thrust with a scale and compare it with ThrustHP?
The reason why I said it's about 30% over optimistic is from real-life correlation. I had a 6 lb GP Extra 40 ARF w/ Saito 72 turning 13x4W @ 11000, and it will hover at near full throttle. ThrustHP says I should have 10.37 lb static thrust, which is 1.7 thrust-to-weight ratio. But it feels like I have a 1.2 thrust-to-weight ratio.

Another example. My 7.5 lb UCD w/ Saito 100 swinging 15x4W @ 10300 rpm, according to ThrustHP has 2.1+ thrust-to-weight ratio. It flies like 1.5 thrust-to-weight, because its vertical acceleration looks like 0.5G, w/ drag accounted for.

Aerosplat did some experimental analysis, using Saito 100 swinging 15x4W @ 10300 rpm, pull like 11.5 lb. of vertical tow weight.
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Old 04-17-2003, 01:27 AM
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Default Real vs. T-HP

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
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Old 04-17-2003, 02:14 AM
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Default Thank You Aerosplat!

Thanks for compiling the figures from some of your tests. It seems like ThrustHP is off, in some cases, more than I expected. I guess I'll have to build my own little thrust stand so I can accurately compare brands as well as diameter & pitch.
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Old 04-17-2003, 02:15 AM
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Default Propeller Thrust Calculator

It's encouraging to know that APC 17x6@9400 yields 17 lb of real thrust. That's what my Enya R1.55 does exactly. So I can put that into a 12 lb. plane and know that I will have close to 1.5 thrust-to-weight ratio.
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Old 04-17-2003, 02:17 AM
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Default Propeller Thrust Calculator

what would somebody recommend for a saito 150 four stroke. i have a sukhoi su 31 69" span wieghs about 10lbs. i have a 16X6 running about 9000 rpm static now. is there better than this??? does anyone have some advise its welcome> shane
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Old 04-17-2003, 02:20 AM
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Default Propeller Thrust Calculator

Quote:
Originally posted by crzy4mot
what would somebody recommend for a saito 150 four stroke. i have a sukhoi su 31 69" span wieghs about 10lbs. i have a 16X6 running about 9000 rpm static now. is there better than this??? does anyone have some advise its welcome> shane
What brand is the 16x6? Zinger 16x6@9000 is very different from APC 16x6@9000.

Saito 150 should swing APC 16x8 or 17x6 @ close to 9000 rpm.
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Old 04-17-2003, 02:38 AM
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it is a master airscew k series (four stroke prop) 16X6
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Old 04-30-2003, 04:32 PM
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Default Propeller Thrust Calculator

Remember, a propeller has a non-constant angle-of-attack of the airfoil section when you go from root to tip. I was searching on the web for lift vs AOA experimental data, and someone found that 5 degrees to 25 degrees gives the optimal lift-over-drag ratio for a typical airfoil. So while increasing propeller pitch can increase lift toward the tip, the section toward the root will approach stall and generate very little lift; hence the lift generated by the entire prop is not too much different.

Seanychen -

I would just like to comment about this piece of technical info....lift is the product to airspeed and AOA. You prop has a higher AOA at the root because its airspeed is slower than the tip. The net force (thrust) is the same at the root of the prop as it is at the tip. The tip is not near a stall because it does not and will not exceed its CAOA (critical AOA), which is the only time an airfoil will stall.
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Old 04-30-2003, 07:04 PM
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Default Propeller Thrust Calculator

Quote:
Originally posted by fireflier
Remember, a propeller has a non-constant angle-of-attack of the airfoil section when you go from root to tip. I was searching on the web for lift vs AOA experimental data, and someone found that 5 degrees to 25 degrees gives the optimal lift-over-drag ratio for a typical airfoil. So while increasing propeller pitch can increase lift toward the tip, the section toward the root will approach stall and generate very little lift; hence the lift generated by the entire prop is not too much different.

Seanychen -

I would just like to comment about this piece of technical info....lift is the product to airspeed and AOA. You prop has a higher AOA at the root because its airspeed is slower than the tip. The net force (thrust) is the same at the root of the prop as it is at the tip. The tip is not near a stall because it does not and will not exceed its CAOA (critical AOA), which is the only time an airfoil will stall.
The lift is product of angle of attack and airspeed... unless it stalls. An airfoil stalls when AOA is too high and air speed is too low. Airfoil at the root inherently has lower air speed and higher angle of attack. So to reduce root stalling, one can reduce the pitch hence reducing the root angle of attack. Therefore, at modest hovering rpm, airfoil at the root will not stall.
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Old 04-30-2003, 08:42 PM
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Default Propeller Thrust Calculator

OK, first of all, the AOA on the root of the prop is higher than the tip, not lower. This is, as I said before, to compensate for the reduced airspeed at the root, to maintain a constant amount of lift throughout the entire prop blade. Second of all, a stall is not dependant on airspeed at all. It is important to remember that the word "stall" is completely independant on how much lift is being created. A stall will always occur at the same AOA for a given airfoil. This is called the "critical angle of attack", or CAOA. At the CAOA, the coeffecient of lift (CL) is at its max. Anything beyond the CAOA, the the CL decreases. At this point, an airfoil is stalled. You are correct, however, to say that the CL generated by an airfoil is reduced as airspeed is decreased, unless the AOA is increased to maintain the same CL. If you were to look at a CL vs. AOA graph, you will see that an airfoil is stalled only at the AOA's higher than the CAOA. If the graph you are looking at is of a symmetrical airfoil, you will see that CL is zero when AOA is zero, no matter what the airspeed is, but this is not a stall. This is merely insufficient lift. CL is less than the weight of the aircraft, and the aircraft will not fly, but is not stalled.
By the way, I am a flight instructor by profession. If you have any other questions, or I have confused you an any way, feel free to ask, and I'll see if I can clear up anything. Also, you can consult "Flight Theory For Pilots" by Jeppesen, and you can see all of the math to goes into this if you are interested.
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Old 04-30-2003, 09:20 PM
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Default Propeller Thrust Calculator

Quote:
Originally posted by fireflier
OK, first of all, the AOA on the root of the prop is higher than the tip, not lower.
I thought this is what I said: "Airfoil at the root inherently has lower air speed and higher angle of attack "

As far as CAOA independent of airspeed, I guess I stand corrected. I mistook "insufficient lift" with "stall". An airplane wing "stalls" when it falls below certain airspeed, or so they say. It really just "reduces" lift and cause airplane to drop.

I am a novice when it comes to aerodynamics. I work as a mechanical engineer in a non-aero related industry. My resource comes from only aerospace course when I was getting my MSME, and some self study. I am guessing that an airfoil stalls when air separated at LE fails to converge at TE through laminar flow. A typical airfoil w/ 90 degree AOA will definitely stall upon any air speed. I am wondering for a symmetrical airfoil at 45 degrees AOA, does it generate any lift at a low airspeed, say 5 mph?

Some technical education would be enlightening!
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