Help on Angle of Thrust

[fudge2k85]

Hi guys,

Recently got an F-35 Profile but found out that it didn't really have much power for a pusher. A good bro here suggested I adjust the angle of thrust. However, I don't really know what it's about and am guessing its the angle of which the motor is tilted down.

Can anyone here help me understand what is the relationship of power and angle of thrust, as well as what is a good angle to get the most amount of power out of my motor? Is my concept right to start with, considering that my motor is placed higher than my wing?

Appreciate your help, guys...

[joe yap]

The relationship between thrust angle vs power is absolutely NOTHING! Thrust angle is incoporated to compensate for the gyroscopic effects and pitching moments induced by the differences in thrust line and the drag line of a given airframe.

But to start off, many folks use the word "power" too loosely, thus misleading everybody, while most of the cases, the real power is always available in a given system setup, but not properly utilised. This is more so when some half-boiled experts started go around telling people about the "thrust" developed by a powerplant system equates to the power available. This often leads to folks blindly going around making gadgets to measure the "actual" static thrust of the model or simply hold the model on hand while applying full throttle, and come to a conclusion of telling people whether the model will fly well or not.

Sadly, most did not know that thier model are just lack of "dynamic thrust", or put in simply, not being pushed around fast enough. The most common culprit is just a wrong choice of propellor, which has too low of the pitch. Just experiment with the props with a little higher pitch with make hell lot of difference. If you use a low Kv motor, the difference can be as drastic as between a particular model which can't fly to a superb flying machine.

HOWEVER, do note that simply increasing the prop pitch alone will load up your motor, ESC and batteries, and thus you also need to reduce the diameter of the prop, in order for the current draw to stay within safe limits. The rule of thumb is; if you increase the prop pitch by 1 inch, drop the diameter by 1 inch as well. For example, if I want to speed up my model which drives a 8" x 4" prop, I should use a 7" x 5" prop. This will reduce the model's static thrust a little, but will allow the plane to fly faster.

Always remember, the plane WILL NOT fly faster than it's own propellor's velocity. If you try to do it by diving the model, the propellors will behave like an airbrake instead.

If you have fallen asleep already, JUST CHANGE THE PROP TO A SMALLER ONE BUT WITH HIGHER PITCH! Have fun experimenting, but make sure you know what you are doing!

[fudge2k85]

Wah...... Hmmm.... But is there anyway to calcukate the relevant thrust required?

I understand first time not all the time can work le... But well..... need an estimate.... I have a 1100kV motor with a 700gm model thereabouts...

[Sunstorm]

Dude, for 1100 kv motor you will use a 9x6 inch prop. This is assuming your 1100 kv motor weighs about 55 - 60 gms.

But 9x6 inch to me is kind of big. Normally for profile pushers we do 6 x 4 inch props on 2000 kv and above motors.

[fudge2k85]

Shoot! Wrong combo then... Sianz... I have a 6 x 4 on the 1100.... Not enough!!!

Anyway, in case of future builds, is there like any general rule of thumb? or some kind of formula I can use?

[Sunstorm]

This is the rule of thumb I use

There is a prop size and the pitch -
so e.g. 6 x 4
prop size = 6, pitch = 4

add them together = 10

So 3000 kv = about 9 (use a 4.7 inch prop max)
2000 kv = 10 (use a 6 or 5 inch prop)
1600 kv = 12 (use 8 x 4 inch, or 6 x 5.5 inch)
1100 kv = 15 (use 9 x 6 inch)

Also get yourself a watt meter if you do not have once.

Also google drivecalc, there is an application that helps you estimate amp draw and thrust

good luck

[joe yap]

Fudge,

When a designer selects a powerplant setup for his design, thrust is not the first concern, unless you are talking about 3D planes that hover. The first thing we they do, is to calculate the stall speed of the model, based on the wing section, wing area, model's weight, etc. With that estimation, they aim for a motor/propellor setup that can produce velocity about 2-2.5 times of the stall speed. With this thing taken care, it is more or less certain that the model will fly well.

Static thrust is another story. It just means that how soon the model will accelerate speed corresponding to the throttle setting. As soon as the model starts to move off, the static thrust will decrease drastically. A pylon racer can have almost 0 static thrust, but can fly very well and fast,provided that the launch is good. On the other extreme, a sport model with a wrong choice of prop, can produce tonnes of static thrust, but may not fly at all if the prop velocity is way to low to pull the model beyond its stall speed.

So my verdict, don't bother to measure or calculate static thrust. It doesn't mean much for our models. Calculating the prop velocity is more critical.

[joe yap]

After all these, it may be still too hard for many to swallow. But fret not. We are no rocket scientist and you don't need to measure and calculate all wonderful figures to find out. Most of us are just simply doing trial and error, and it is more important for an average modeller like us to understand the basics to at least know how to do trial and error systematically. Sadly, a lot of folks out there are still doing trial and error blindly, with too many variables and not knowing what to expect.

If you are designing a brand new design, you have to work out a lot of things to make the model flies well. But with a existing design and setup, it is more practical to find out the limits of the systems first before you start your trial and error.

Many folks can reccommed a prop size for a given motor, but due to different coefficients of different prop designs, it is still better and safer to use the approach mentioned below. You'll need a ampmeter to measure the actual current draw though.

Firstly, find out the limits of your power systems. The maximim continous voltage and current draw of the motor, ESC and battery packs are the main figures you will want to keep behind your head. Amongst the 3 components, the lowest one will be the ultimate limiting factor of the entire project. For example, if the battery's max continous current draw is 50 amps, the ESC is 40 amps and the motor is 30 amps, do not exceed 30 amps draw for the entire powerplant system, with whatever prop you select. And whatever situation it is, never exceed the maximum cell count on all the 3 components. This will make sure that they are operating on safe working voltages.

Secondly, not all reccomended prop designs and sizes are the best for your setup, as there could be still some variables that may not yield the samre results. However, use it first as a benchmark, and measure the current draw from low throttle to full. At any point, if the current reading exceeds the maximum draw on either of the 3 components, you have to downsize the prop. If not check the differences in current draw between the actual current draw at full power and maximum allowable current draw. This will tell you how much you can upsize the prop.

Assuming the current draw is near or at the maximum allowable current draw, this should be taken as a rough guide of maximum output power. If you like to find out the figure, simply measure the actual current draw and voltage and mutiply them together. (Power = voltage x current ). Also take note of the size and the design of the propellor.

Once the maximum output power is determined for a given setup, you can manipulate the static thrust and prop velocity by changing the prop size and design, without exceeding the voltage and current draw. If you want to increase the static thrust, you have to reduce the prop velocity, and vice versa.

[joe yap]

Okay, now this may add up your headache, but I must mention this, in order to compliment what I said earlier. If not, there will be a missing big piece of the puzzle.

Finally, I have to say this ; NOT ALL PROPS ARE CREATED EQUAL!!

Folks will recommend you the prop size, but seldom one will tell you what type and that can explain why thing still goes wrong, despite following all the recommendations.

Let's start from APC Sports electric props. This modern designs are meant to be use on most brushless motors spinning upto around 10000+ rpm. Slow flyer props, are originally designed for brushed motor with gearbox unit that spins at very low RPM. To abosrb more power from the motor system at such low rpms, the blades are very broad. They are still good for very low Kv outrunners, but if you spin them with your average brushless motor with high Kv, you'll see a drastic climb in current draw that can easily exceed the limits. Pylon racer props are meant to be driven at very high RPM like 20000+rpm. To prevent overloading, thier blades are very thin and since pylon racers are designed to fly fast, they have relatively higher pitch than spotr props. If you strap one of these on you sport outrunner, you'll see much lower current draw and not much useful work is being produced.

Having said that, APC prop range are pretty well defined and by sticking to the designated type seldom makes you go wrong. Master Airscrew props are another different story. Their range of electric props are originally designed for brushed motors with reduction gearboxes and are designed to run at below 10000rpms. Many unwary modeller strapped one of these onto thier modern brushless motor and complained that the current draw shoots up beyond safe limits. This props look just a little broader than most standard props, and that may give the false impression to many at first. However, these props have very heavy undercamber airfoils, just like a free flight glider, which produce a lot more 'lifts' when they are spun around.

Now, to futher add up the challenge, many prop designs, especially the cheaper ones, are not classified and labelled for thier intented uses. So it is up to the consumers to use them at thier own risk. However, it is not very difficult to compare the blade designs against the calssified ones, unless you are blind.

So the verdict?????????

Still trial and error, but please at least know how to do it correctly. If your props are underperformed but you want to increase the static thrust a little without changing the prop size, go for broader blades. If you are using a very well suited prop but the current draw is just a little on the high side, choose as slimmer prop!

Good day and have fun messing around!!!

[blueangel]

This is the rule of thumb I use

There is a prop size and the pitch -
so e.g. 6 x 4
prop size = 6, pitch = 4

add them together = 10

So 3000 kv = about 9 (use a 4.7 inch prop max)
2000 kv = 10 (use a 6 or 5 inch prop)
1600 kv = 12 (use 8 x 4 inch, or 6 x 5.5 inch)
1100 kv = 15 (use 9 x 6 inch)

Also get yourself a watt meter if you do not have once.

Also google drivecalc, there is an application that helps you estimate amp draw and thrust

good luck

Interesting rule.. how do you derive that 3000 kv = 9 , or 1100 kv = 15 ?

Not technically correct but a good guide while deciding a prop to use i guess ?

[mave]

Hi Joe,

after not flying for awhile, I kind of forgotten the motor, prop size and pitch thing. This is a good refresher course. I have a naggy question that I have been bothering me, can you help me with this? I took the following spec off a scorpion website.

Specifications

Stator Diameter ............................ 40.0 mm (1.575 in)
Stator Thickness ........................... 35.0 mm (1.378 in)
No. of Stator Arms ................................................ 12
No. of Magnet Poles ................................................ 8
Motor Wind ............................................. 4 Turn Delta
Motor Wire ..................... 27-Strand 0.25mm (30 AWG)
Motor Kv ............................................. 800 RPM / Volt
No-Load Current (Io) ................. 2.50 Amps @ 8.4 volts
Motor Resistance (Rm) ............................. 0.010 Ohms
Max Continuous Current .............................. 100 Amps
Max Continuous Power .............................. 4200 Watts


From the spec it says that the max continous power is 4200watts. If a person wants to tap into the full potential of the motor, what is the best way to achieve it? (given that the motor can only accept 12s Max)


also, I also heard before that the motor efficiency tends to drop after certain power range. is there a mathematical way to calculate this eficient range?


Thank you in advance and sorry if I OT.

After all these, it may be still too hard for many to swallow. But fret not. We are no rocket scientist and you don't need to measure and calculate all wonderful figures to find out. Most of us are just simply doing trial and error, and it is more important for an average modeller like us to understand the basics to at least know how to do trial and error systematically. Sadly, a lot of folks out there are still doing trial and error blindly, with too many variables and not knowing what to expect.

If you are designing a brand new design, you have to work out a lot of things to make the model flies well. But with a existing design and setup, it is more practical to find out the limits of the systems first before you start your trial and error.

Many folks can reccommed a prop size for a given motor, but due to different coefficients of different prop designs, it is still better and safer to use the approach mentioned below. You'll need a ampmeter to measure the actual current draw though.

Firstly, find out the limits of your power systems. The maximim continous voltage and current draw of the motor, ESC and battery packs are the main figures you will want to keep behind your head. Amongst the 3 components, the lowest one will be the ultimate limiting factor of the entire project. For example, if the battery's max continous current draw is 50 amps, the ESC is 40 amps and the motor is 30 amps, do not exceed 30 amps draw for the entire powerplant system, with whatever prop you select. And whatever situation it is, never exceed the maximum cell count on all the 3 components. This will make sure that they are operating on safe working voltages.

Secondly, not all reccomended prop designs and sizes are the best for your setup, as there could be still some variables that may not yield the samre results. However, use it first as a benchmark, and measure the current draw from low throttle to full. At any point, if the current reading exceeds the maximum draw on either of the 3 components, you have to downsize the prop. If not check the differences in current draw between the actual current draw at full power and maximum allowable current draw. This will tell you how much you can upsize the prop.

Assuming the current draw is near or at the maximum allowable current draw, this should be taken as a rough guide of maximum output power. If you like to find out the figure, simply measure the actual current draw and voltage and mutiply them together. (Power = voltage x current ). Also take note of the size and the design of the propellor.

Once the maximum output power is determined for a given setup, you can manipulate the static thrust and prop velocity by changing the prop size and design, without exceeding the voltage and current draw. If you want to increase the static thrust, you have to reduce the prop velocity, and vice versa.

[Sunstorm]

Interesting rule.. how do you derive that 3000 kv = 9 , or 1100 kv = 15 ?

Not technically correct but a good guide while deciding a prop to use i guess ?

You are right its not technically correct but hey some guesstimation is better than none, that's why there is software like Drivecalc, Motocalc and Ezcalc to help you estimate your amp draw to prop.

I use drivecalc personally.

[joe yap]

Hi Joe,

after not flying for awhile, I kind of forgotten the motor, prop size and pitch thing. This is a good refresher course. I have a naggy question that I have been bothering me, can you help me with this? I took the following spec off a scorpion website.

Specifications

Stator Diameter ............................ 40.0 mm (1.575 in)
Stator Thickness ........................... 35.0 mm (1.378 in)
No. of Stator Arms ................................................ 12
No. of Magnet Poles ................................................ 8
Motor Wind ............................................. 4 Turn Delta
Motor Wire ..................... 27-Strand 0.25mm (30 AWG)
Motor Kv ............................................. 800 RPM / Volt
No-Load Current (Io) ................. 2.50 Amps @ 8.4 volts
Motor Resistance (Rm) ............................. 0.010 Ohms
Max Continuous Current .............................. 100 Amps
Max Continuous Power .............................. 4200 Watts


From the spec it says that the max continous power is 4200watts. If a person wants to tap into the full potential of the motor, what is the best way to achieve it? (given that the motor can only accept 12s Max)


also, I also heard before that the motor efficiency tends to drop after certain power range. is there a mathematical way to calculate this eficient range?


Thank you in advance and sorry if I OT.

Charles,

For a fully charged 12S Lipo pack, we are looking at about 50.4Volts. In order not to exceed 4200watts, the current draw should be limited to 4200/50.4= 83.33amps. Of course, that is theorectical input power and the actual figures will change a little. When the system the running, the actual voltage will probably drop a little. On the other hand, you can try to draw 100 amps by using input voltage of 42volts (10S).

However, it is always better to use maximum input voltage than input current in order to reduce resistance in the circuit. As the resistance goes up, the overall electrical effeciency will be reduced. With the 2 examples mentioned above, the actual output power of the 10S will be lesser than what you'll get from the 12S setup.

With the targeted current draw and number of cells determined, next thing is to select a gearbox that bring down the rpm to practical range. The only thing that can spin at 40000+rpm is the pinion gear itself. I don't know what gearbox is available in the market for such motor, but I think gearboxes between 6:1 to 10:1 reduction ratio will bring the rpm to some practical range for large props. Find one that will run at or near the targetted voltage and current and yet can produce sufficient prop velocity with sufficient pitch. From there, change the prop with different diameter and pitch for the best performance as necessary.

As for the motor efficiencies, the only reason for motor efficiencies dropping drastically beyond a certain power range I can think of, is when the motor itself is overloaded. This is common when folks strapped in propellors or fan units designed for brushed motor at lower rpms. When the motor is overloaded and starting to stall, the increment of rpm and torque will be much reduced with the advance of throttle stick position, but the current draw shoots up drastically. The motor will start to cook and left for a period of time, something is going to get burnt.

I think the best way to find this out is to compare the RPM vs voltage chart and current vs voltage chart. By dividing the rpm with the current draw at each particular increment of the input voltage, you can tell the efficiency immediately. However, each individual motor design will have its own chart. You have to obtain them or run your own test to find out.

[renold]

Just to add on, thrust is related to prop velocity. Thrust is just means force that able to propel the plane in forward direction less opposing viscous friction (air). Mathematicaly, force (thrust) is defined as: change in momentum. That is F = MV2 - MV1; where M is the mass of air washed, V1 is the air velocity before prop and V2 is after prop. Increase prop pitch, increase air speed for a same rotating speed.

Why static thrust is not the only factor to decide whether the plan would flies well. Because static meaning stationary without considerating the viscous friction. The nett thrust while plane air in air is reduced by opposing air friction which the quantity depending on aerodynamic profle of your plane, drag. This value increases to the square of plane velocity. Meaning the higher viscous friction with higher air speed.

[joe yap]

Thanks, Renold. That's true.

However, in the real world there is another point we often forget to take into considerations, which often mislead us to start measuring static thrust.

A prop with low pitch will probably work well in static condition, whereby measuring the static thrust, may have some meaning. But if a high pitch prop is made to run at static condition, the actual measurement of the static thrust could be even much lower than the previous experiment. This phenonmenon somehow seems to oppose to what the statement of "Thrust is related to prop velocity". Now in fact, it seems that the opposite is true.

The actual fact is that if the blade pitch is set high enough, the blades could be stalled when made to spin at static conditions. But if the propellor and its motor is made to move forward while spinning, the additional ram air forward of the prop can unstall the blades and the system will suddenly spring to life and accelerate much faster and reaching speeds beyond what the first setup can achieve.

For that reason, it further explains the futility on measuring static thrust, unless you know exactly what you want to achieve.

For a fullsize aircraft, pilots and engineers do not like the idea of having to give that extra push for the plane's propellers to get on thier step, as far as possible. As such, planes powered by engines driving fixed prop are often designed for relatively slower speed, like the tigermoth biplanes. For better performance like most WW2 warbirds, engineers started to use variable pitch prop to overcome the blade stalls.

For our models? Most of us are already happy with tossing them into the air to give the extra push to unstall the prop blades in the first place. So why do we still need to measure static thrust in the first place?