|
Thrust calculator
1 Attachment(s)
About a year ago me and fellow RCU member Dipstick began work on a more accurate thrust calculator. It would be different from calculators such as thrust HP mainly by actually computing some values that were assumed as constants in thrust HP. These were the coefficient for thrust, The coefficient for power and the program also included an option to calculate air density. After about half a year we got a Visual Basic version that looked professional and gave pretty accurate results
But not yet accurate enough .I have not heard from dipstick in a while (I'm sure he's busy at work.), but I think there might be more aerodynamic and programming guru's out there (I'm pretty good at q-basic, but just learning VB using the VB editor supplied with word), and I think that this program could be greatly improved. I have attached the text version of the source code, and if anybody wants the whole program, I could email it to you as an excel file to which the program is attached. I want you guys to help with this program. Modify it, post your modifications, ideas, comments, anything. O you can just use it as a free thrust calculator and not modify it, cause the results that it gives are pretty decent. I have attached two text files and one screenshot. The screenshot is kinda blurred for some reason.The first text fileone is a version by dipstick that works. The second does exactly the same but is done by me, but seems not to be working. Could you guys tell me why? Thanks |
RE: Thrust calculator
I am not sure if I can help w/ the code, but I have a lot of RPM data and actual thrust measurement. So I can help verify the result. Please email me the Excel file to [email protected] Thanks.
|
RE: Thrust calculator
Thanks. If you have data to sent, my adress can be seen in my user profile.
I already have a lot of prop data, but it is very uncompiled, and its not very detailed so any data will be useful. |
RE: Thrust calculator
I admire your goals --but how will you show accurate data of a given prop --as it moves from stalled condition to max usable speed ?
So far all the charts I have seen are of no value in this respect. Not because they are poorly designesd but because the prop interacts so much with the airframe it is lugging around-- |
RE: Thrust calculator
I admire your goals --but how will you show accurate data of a given prop --as it moves from stalled condition to max usable speed ? I would love some help with the aerodynamics for this, because I am only an amateur dynamicist. Any help would be greatly appreciated |
RE: Thrust calculator
The thrust the prop provides at a given RPM --whilst the prop is not moving FORWARD, is one thing.
How that changes as it moves and gains speed ,is another. I have a ignition on two of my new engines (ROTO)-that will read rpm -before takeoff --then after the plane lands, you can again read the maximum rpm achieved in flight. (from an LED readout /memory storage system of the ignition. This will show how much rpm was gained in flight. Obviously, if the engine/prop was in a smaller lighter plane - the maximum rpm would change more than if in a heavy draggy plane. So for a given airframe -and a given engine - the BEST prop for a selected area of performance, could be selected. Make sense? |
RE: Thrust calculator
I understand now. You're saying that the enigine unwinds at altitude. You can use this program to see how the engine will perform at ground level. For example, lets say you want a speed plane. You look for a prop that will give you enough thrust that it will take off easily, but give you enough speed. If you are looking for 3d plane, then you look at maximum thrust.
Are there any ways to predict how an engine will unwind? I guess It could work if you set the horsepower as constant in the power eqaution, and then see how fast the prop will spin at a given altitude by calculating the density of air The power equation is power= cp*q*n^3*d^5 cp is coefficient of power, which roughly = pitch/diameter/11.25 q is the mass density of air n is revolutions / second d is diameter I think I can add a function to the calculator where if you know what type of prop and how fast will your engine swing, then you can set the power that comes out of this equation as a constant, and see how fast will your engine swing a particular prop and under fifferent alttidudes. I'll work on figuring out soon, and post the results Thanks for the ideas, and thanks for the prop data you guys have been sending |
RE: Thrust calculator
Mitek,
Glad to see you are still working on the prop stuff. Just drop me a line if you need any help. Steve |
RE: Thrust calculator
What one really need is thrust vs speed chart. By assuming measured rpm on zero speed and using that rpm for all airspeed range one can get a pessimistic estimate of thrust vs airspeed. If engine can swing this prop at that rpm in static condition it will swing it at least as fast in flight. At airspeed equal to prop pitch times rpm there will be no thrust.
To really estimate how much engine RPM increase with speed you would need a power curve of engine- hp vs rpm. For sure such curves are rare for model engines, may be one can construct them by measuring max rpm of given engine with different diameter props. May be you can make some options- constant rpm thrust and constant engine power thrust vs airspeed. |
RE: Thrust calculator
Thanks for the suggestion
I have contacted dipstick again, and he's done a program like that, one which incorporates the torque curve and dynamic thrust. I have asked him to send me a copy of it, and I'll test it out,. I'll keep you guys posted |
RE: Thrust calculator
I don't know how accurate you can get this, as there are variables that are going to be ever changing. As Dick Hanson remarked, an engine doesn't unwind as well on a dragy airframe as it does a light, clean airframe. Added into this is the humidity conditions. We have all flown our planes at times when the high humidity not only affected acceleration of the engine, but also how agile the plane felt. I assume that even if the engine were not affected by the humidity, the ability to pull the airframe changes as humidity fluctuates. By no means am I putting your efforts down, I greatly appreciate your efforts to get closer than what Thrust HP is. When I flew smaller planes, I used to think it could be used as a benchmark, but now that I have a few larger planes, I have not found it to be useful at all.
Are you able to incorporate relative humidity and altitude into the equation? Is there any way to clasify each aircraft, not just by sqaure inches, but by the thickness of the wing and the planes weight? Just food for thought...:eek: Andy |
RE: Thrust calculator
Adding altitude is easy. All you have to take into account is the change in air density. Fortunately, this change is quite linear for the first few thousand meters of the atmosphere. As most of us will be flying well below 5000 meters (15000 feet) this simple equation is quite effective:
density = 1.22 * 0.9 ^ (altitude / 1000) where: density is in kg/m^3 altitude is in meters (feet * 0.3048) BTW, I also have some knowledge of VisualBASIC programming (I have VisualBASIC 6.0) and a small understanding of how propeller aerodynamics, so if there's anything I can help with, please don't hesitate to ask. My e-mail is: [email protected] |
RE: Thrust calculator
originally from Flyer Freq
As Dick Hanson remarked, an engine doesn't unwind as well on a dragy airframe as it does a light, clean airframe I'm pretty sure I have humidity included in the equation which calculates the q, but I'm not sure. I'm sure I have altitude in there. If you want a little bit more clarification, look at the picture in my first post. Maybe dewpoint has something to do with humidity. I'll have to ask Dipstick Adding altitude is easy. All you have to take into account is the change in air density. Fortunately, this change is quite linear for the first few thousand meters of the atmosphere. As most of us will be flying well below 5000 meters (15000 feet) this simple equation is quite effective: density = 1.22 * 0.9 ^ (altitude / 1000) where: density is in kg/m^3 altitude is in meters (feet * 0.3048) Thanks for all the input |
RE: Thrust calculator
As I understand, which is most likely not true as it is based on old textbooks, the engine unwinds not because of altitude (altitude is really constant in RC flying- few hundred meters doesnt change much, effect of weather is bigger) but because of speed. If speed is faster than pitch of propeller times rpm, you basically have a windmill, switch off the power and you still have some rpm. I guess the increase in RPM has to do with induced drag of the blade (shame on me to bring this drag here). If static, the angle of atack of blade is high, CL is high and you have higher induced drag and therefore lower rpm, or even worse, the blade is stalled- then you have high drag anyway. As plane moves faster the angle of attack reduces and therefore CL reduces too, Induced drag goes smaller and you get higher rpm with same power input from engine. Until at some speed angle of attack of the blade is zero and when going even faster (in a dive) prop will produce negative thrust- a windmill. May be bad for your engine.
Check out this site- there are few words about propellers http://www.mh-aerotools.de/airfoils/index.htm good luck! |
RE: Thrust calculator
Gents ...
To keep all your " comparisons " of thrust Accurate you must correct evey air dencity figure not only for relative humidity & temp, but allso your barometric pressure, all performance figures for "air movement"..ie thrust is normally quoted @ ISL conditions ...good luck..;).. |
RE: Thrust calculator
I have an analogy that, while not perfect, makes things a little clearer in regard to humidity, draggy air frames and engines unwinding. Lets take a car in perfect shape, the air filters are clean, the carb is tuned for peak efficiency and the tires are properly inflated. When you start to accelerate, the engine is overcoming the desire of the mass to remain at rest. Because the tires are properly inflated, there is very little rolling resistance. As the car picks up speed, the inertia of the mass assists the engine in maintaining speed. The engine now can efficiently accelerate even further. Once up to speed, the engines load is greatly reduced since inertia has greatly overcome drag. At this point, greater RPM can be generated because the engines load is low.
Now lets take the same car. The air filter is dirty and someone let most of the air out of the tires. The engine now has a harder time accelerating. Even though inertia continues to help maintain speed, the added friction of the low tire pressure causes a greater load on the mass/engine and causes it to decelerate more easily, thus, the engine has to use more of its energy to maintain speed. This, coupled with the dirty air filter, decreases the engines ability to rev once up to speed. Now, tie this into our planes. If an engine is mounted on a clean airframe, humidity is low so the engine breathes well, the plane is going to accelerate easily. As it accelerates, inertia steps in to help the engine maintain speed. Inertia could be related to the "windmill effect" spoken of earlier. The engine is able to easily rev up further because less of its horse power is being used to maintain speed. Same engine/prop, but now we add high humidity(in first analogy, dirty air fliter) and double the thickness of the wing. The high humidity would have adversely affected the engine and airframe on the first plane, but now we have the engine on,... lets say a fun fly frame instead of a quickie 500 frame. As the plane accelerates, once again, inertia steps in to help, but the added friction/turbulance of the thick wing causes the engine to have to carry a larger percentage of the load. Once up to speed, the engine still can't unwind as much, because too much of its power is being used to maintain speed. I admit, it is not a perfect analogy, and there are more factors present but I think it points us in the right direction. To get a more accurate thrust calculator, some factor for plane weight , wing thickness and overall cleaness of the design has to be added to the equation. Personally, I think it could be carried too far for our models, but it would be neat, since we are talking about it:), for all this to be incorporated. Feel free to jump in and dissect my analogy, I can only learn from it...;) Andy |
RE: Thrust calculator
Right ideas - and -to add -
you simply cannot calculate for best propeller will do unless you know WHAT it is supposed to do. Try this: the job for a propeller, is to convert the energy output of an engine (or motor) into best use for a given task. The rpm at a stationary test tells you one thing - -power absorbed for that prop when the motor is stationary. once that prop and motor are in motion ,everything changes - given NO resistance to overcome ,the prop and engine (motor) will eventually reach a speed where it can go no faster. the motor may limit this and/or the prop may be the ultimate limiter. Real world: there are resistances and the sum of resistances may literally be infinite .. If you can come up with a FINITE resistance/load whatever you want to call it- - then you can determine how effective that prop is for that load,when driven by THAT engine (motor). So this is why I look at all current "thrust charts" , as simply a lot of numbers . Much like many of the "calculations" on c/g etc., unless you know the entire situation/application - the calculation is just --busy work numbers . I have a ignition that tells me what rpm is reached in flight - simply a tattletale device. Now I know, what rpm the engine is turning for a finite test -be it level flight -vertical climb ability - full speed vertical dive -whatever I choose. If I then read that number -I will know what the engine turned during that escapade. So - if the engine develops max HP at say 7000 rpm and the hp of that engine drops sharply from there on up -I will try to keep the prop turning at 7000 for the job selected. Maximum torque may be another number -but really good IC engines have max torque close to max hp. turbines/ electric motors are also used now - so you have to know best outputs for those and somehow read their shaft speeds - |
RE: Thrust calculator
All this thinking is giving me a headache!:D Great topic!
Another aspect of Thrust HP was its prediction of air speed. Since air frame design is a function of engine RPM and RPM is a function of air frame speed, how do you crack that egg??? Isn't that the old,"what came first, the chicken or the egg?" And, what about prop design, how did they ever come up with RPM numbers based on different manufacturers? ...and then throw that into the equation with how slippery the airframe is. In a way it is like trying to design a crystal ball. Again, how far should we go, and have we already gone too far. I would settle for being within 8 oz of thrust for the birds I fly, or even a pound. I recall that one of the props I chose to fly was supposed to develop 24 lbs. of thrust at the chosen RPM, according to Thrust HP, but for those who had the equipment to measure true static thrust, they were getting around 17 lbs at that RPM. That is just too far off. Andy |
RE: Thrust calculator
The "maximum speed" that ThrustHP gives is not based on the airframe. Since there's no airframe specified, how can it calculate the maximum speed?? That speed is the "zero thrust" speed of the propeller. This means, that at this speed, the forward motion of the aircraft will cause the propeller blade to have a zero relative angle of attack, aka zero pitch. This is also the reason why an engine unloads with flight, since the AoA (relative, the absolute reaians the same, as long as the prop is rigid...) of the blade reduces as the speed goes up. Reduced AoA means less lift and consequently, less drag, so the prop "eats" less power to rotate at a given RPM. Since the engine power doesn't change, it revs up. This is explained by simple vector physics.
But there's a serious flaw with the ThrustHP calculation, and that is that it assumes that the prop will generate 0 lift, at 0 AoA. This is only true for symmetrical (non-cambered) airfoils, what no prop (at least that I know of) uses. The kind of airfoil that most props use is cambered ones, resembling tha famous Clark-Y. And this kind of airfoil generates 0 lift at a negative AoA, so the speed given by ThrustHP is lower than it should be. And, as each brand of prop uses a different airfoil, it's very difficult to have accurate results using generic data. |
RE: Thrust calculator
For actual use- forget the calculations-- they don't and won't work-
You guys who actually fly models -havealready found this to be what happens. It is a cut and try thing. But if you love to play with numbers -by all means - have at it - |
RE: Thrust calculator
I think the thrust calculator is really useful thing. They even went to space with non-cup engineering using only computational methods and not a single wind tunnel test of SpaceShip One. Propeller must be a joke when compared to that.
EagleOne- every airfoil has a zero lift angle of attack. It is zero for symmetrical and negative for positively cambered profile. You can define absolute angle of attack so that zero lift occurs at zero angle. But even prop with positive camber will work as windmill at high enough speed. Pitch times rpm may be not the most accurate way to estimate zero thrust speed, but as first approximation it should be fine. Some props are not constant pitch props- pitch at tip is different from the middle and near the hub. For reason that such prop loses some top performance but have wider usable speed range. For example, if you have high pitch prop you may not be able to take off- when static, blade will be stalled at high rpm and no enough thrust at lower rpm to get moving, but decrease the pitch at some part of blade and voila- you are in the air. I guess the zero thrust speed of propeller at given rpm (which comes from the limit of engine) is always higher than maximal speed of aeroplane in level flight (you may dive to gain speed). It may not be interesting to know this speed but for sure it does exist for every propeller at given rpm value. The maximum speed of whole aeroplane in level flight is where thrust vs speed curve of prop-engine intersects the drag vs speed curve of airoplane. Thrust=drag. There is no eggs here. Better not to solve it analytically, graphic method is good enough. |
RE: Thrust calculator
Go ahead solve it with calculations
I double dare you. Why ? It simply can't be done. any given engine propeller combo will respond differently in different types airframes. Leave the airframe out of the formula and you get info- but to what does it apply? You can get close - but I can get just as close closing one eye - and guessing- based on comparitive actual data. |
RE: Thrust calculator
Hi,
I've found a little free program that calculates speed (mph), horse power, load, CF and Static Thrust (lb), based on the prop data input such as: prop type, diameter, pitch, number of blades and rpm. I would like to hear your opinion about this program's usefulness. To download and install it free, click on following link: http://www.bmaps.net/software/thrusthpv20d.zip Below is the link to the site where I've found the program: http://www.bmaps.net/ |
RE: Thrust calculator
ORIGINAL: antter I think the thrust calculator is really useful thing. They even went to space with non-cup engineering using only computational methods and not a single wind tunnel test of SpaceShip One. Propeller must be a joke when compared to that. EagleOne- every airfoil has a zero lift angle of attack. It is zero for symmetrical and negative for positively cambered profile. You can define absolute angle of attack so that zero lift occurs at zero angle. But even prop with positive camber will work as windmill at high enough speed. Pitch times rpm may be not the most accurate way to estimate zero thrust speed, but as first approximation it should be fine. Some props are not constant pitch props- pitch at tip is different from the middle and near the hub. For reason that such prop loses some top performance but have wider usable speed range. For example, if you have high pitch prop you may not be able to take off- when static, blade will be stalled at high rpm and no enough thrust at lower rpm to get moving, but decrease the pitch at some part of blade and voila- you are in the air. I guess the zero thrust speed of propeller at given rpm (which comes from the limit of engine) is always higher than maximal speed of aeroplane in level flight (you may dive to gain speed). It may not be interesting to know this speed but for sure it does exist for every propeller at given rpm value. The maximum speed of whole aeroplane in level flight is where thrust vs speed curve of prop-engine intersects the drag vs speed curve of airoplane. Thrust=drag. There is no eggs here. Better not to solve it analytically, graphic method is good enough. And yes, above that speed, every prop will windmill. Doesn't matter if that speed is 1000 mph, at 1001 mph the prop will be windmilling (spelling??). ThrustHP gives wrong results for everything. It's very interesting as a learning point, but not for designing your planes over the data it outputs. Now, since all props have a variable pitch bladewise speaking (pitch at blade root is higher than at blade tip), the only way to accurately calculate the blade behaviour (thrust and drag) is through splitting the blade spanwise in a number of points (let's use 100 for an example) and calculate the data for each point. Treat the blade as a wing to simplify matters. But for this, much more than the basic diameter, pitch, RPM and forward speed inputs is needed. And even though, as dick hanson said, you still will get an error margin, because predictions are never what the real thing is. But will be much more accurate than the method ThrustHP uses. adam_one - That's exactly the program I am talking about. Good for playing around, but don't take the results seriously, as they are very erroneous. If they actually match a real-world situation, that's coincidence. |
RE: Thrust calculator
EagleOne,
I dont really get what you mean here- geometric angle is just an angle of chord, the chord is kind of arbitrary line on airfoil- it is defined in terms of geometry but has little to do with aerodynamics of airfoil. I dont see how this is going to change the calculations? Every airfoil has zero lift angle and angle relative to that should go for calculations. If manufacturer measures the prop in terms of geometric angles (as propably is), then real pitch is a bit higher than geometric, whats the big deal? Just find absolute angle based on airfoil and pitch. This must be least problem in there. By the way- Clark Y polars are plotted not against chord angle but against flat bottom line angle of attack. At least originally. I perfectly agree with you in second part. That is exactly how you should treat the propeller- cut the blade into pieces and treat separately, then you miss cross-flow from one part to other (similar to spanwise flow in the wing) and are in trouble. May be if you do this thorough test for one prop in family (say APC or master screw) then you can correlate results for similar scaled up or down props? |
| All times are GMT -8. The time now is 09:30 PM. |
Copyright © 2026 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.