Pitchspeed calculation
Or: How fast does my plane really fly?
We all know the things saying on the boxes like "up to 200km/h!" etc. I need to get you down now, 99% of those informations are not even worth the ink with which they are printed. i can understand the manfucaturer, because these are really good marketing arguments, and the wording "up to" does not say that it is really that fast out of the box. These informations unfortunately lead to the fact, that most peoplay are not able to estimate anymore a correct speed or distance. A positive thing is the upcoming telemtry and more and more people are seeing that a 4m glider isnÄt anymore that big when he is 400m away and also that a foam Hotliner can not really reach 200km/h.
Not even if you attach a stone to it an drop it from a real plane.
Wanna have a little explain of what I'm talking about? Try to estimate how fast ma TA05 is driving in the video:
Not even if you attach a stone to it an drop it from a real plane.
Wanna have a little explain of what I'm talking about? Try to estimate how fast ma TA05 is driving in the video:
Yes, I know, estimating speeds in a video is always hard, but give it a try. What would you say? 50km/h? 70km/h? 100km/h? 120km/h? The following line is written white on white, so you need to mark it to be able to read it for the solution:
My TA05 was meassured multiple times with around 80km/h.
So, how accurate have you been? If you have been close, I'm happy, if not, you maybe know now why it is not unimportant to know the pitch speed of your plane to estimate how fast it is flying.
My TA05 was meassured multiple times with around 80km/h.
So, how accurate have you been? If you have been close, I'm happy, if not, you maybe know now why it is not unimportant to know the pitch speed of your plane to estimate how fast it is flying.
The meaning:
What does "pitchspeed" means? The pitchspeed is a theorethical value which means how fast does the air which gets through the propeller is accellerated. The propeller is seen as screw which screws through the air, according to his rotation, he is doing a distance in corrospondending time. Crucial is only the pitch of the propeller and the roations per minute. What we know from that, is the theorethical, absolute maximum possible speed for our model which it will never reach.
Note: Pitchspeed is the speed, that the propeller can achive theorethical straight forward.
You are going now to ask what we want with that value, we want to know the flow velocity whioch is done by the propeller, this is the value that our plane can never achive. Yes, you are fully right. But as many times in life, this is an intressting phenomen: The efficency of a propeller is not 100%, that's impossible, that's why we are substract 10% of the pitch speed. So the flow velocitiy is 90% of the pitch speed? No, not exactley. As you may also realized, the rotations per minute are increasing during flight slightly. Intresstingly also about 10%. So from the 90%, we will add again 10%. of course, mathematicaly this will not be 100%, but simplified we can say, that the flow velocity in real life will be around the theorethical pitch speed.
Note: The pitchspeed equals the flow velocity.
But, how fast is my model going? Unfortunately, we still can't say that. but in normal conditions, it is between 60% and 90% of the pitchspeed. A pylonracer which has nearly no drag will reach maybe 90%, a fat plane with a lot of drag will maybe only archive about 60%. But you know one thing definitely: Faster then the pitchspeed you've calculated, you model won't go never ever! (of course except glider which archive higher speeds while gliding without engine power).
Note: Pitchspeed is the speed, that the propeller can achive theorethical straight forward.
You are going now to ask what we want with that value, we want to know the flow velocity whioch is done by the propeller, this is the value that our plane can never achive. Yes, you are fully right. But as many times in life, this is an intressting phenomen: The efficency of a propeller is not 100%, that's impossible, that's why we are substract 10% of the pitch speed. So the flow velocitiy is 90% of the pitch speed? No, not exactley. As you may also realized, the rotations per minute are increasing during flight slightly. Intresstingly also about 10%. So from the 90%, we will add again 10%. of course, mathematicaly this will not be 100%, but simplified we can say, that the flow velocity in real life will be around the theorethical pitch speed.
Note: The pitchspeed equals the flow velocity.
But, how fast is my model going? Unfortunately, we still can't say that. but in normal conditions, it is between 60% and 90% of the pitchspeed. A pylonracer which has nearly no drag will reach maybe 90%, a fat plane with a lot of drag will maybe only archive about 60%. But you know one thing definitely: Faster then the pitchspeed you've calculated, you model won't go never ever! (of course except glider which archive higher speeds while gliding without engine power).
Calculation if you know the rpm's on the ground:
You know the rpm of your combustion engine or electric engine on the ground because you've meassured it with a rev counter? Good, then you can use that formula:
rpm= Meassured rpm on the ground
P= pitch of the propeller in inch (e.g. with a 10x6 prop this would be 6)
P= pitch of the propeller in inch (e.g. with a 10x6 prop this would be 6)
The outcome will be the pitchspeed in km/h, e.g. you've counted 13.000 rpm and you are using a 10x6 propeller, then the pitchspeed would be 119km/h. This model will never ever fly faster then 118km/h, maybe between 90km/h and 100km/h.
Calculation if you don't know the rpm's on the ground:
You don't have a rev counter or you just don't know the rpm's? Also no problem, you should already know the data of your electric engine. Then you can use this formula:
S= Cell count
V= Voltage of one cell under load (LiPo=3,7V, LiIo=3,6V, LiFe=3,3V NiMh=1V)
KV= kV value fo your engine
P= Pitch of the prop
V= Voltage of one cell under load (LiPo=3,7V, LiIo=3,6V, LiFe=3,3V NiMh=1V)
KV= kV value fo your engine
P= Pitch of the prop
The outcome will also be the pitchspeed in km/h. Short example: Engine with 3000kV, 4S LiPo, 4,7 prop equals 318km/h. So the model will never ever be faster then 318km/h. At this speeds, the efficency also become smaller, so the maximum speed will be something around 250km/h.
But how fast it goes really?
Of course we want to know the real speed fo your models. There are many different options to get that, started from light beam meassurements up to GPS meassurements. if you don't have GPS and a telemetry system, the meassuring with the doppler effect is the most precise and also one of the easiest.
never ehard about the doppler effect? Of course you did, but you haven't known it. look at a Formula 1 race in television and concentrate on the sound of the car when it passes the screen. Even if you haven't known it yet, you realized that the car sounds different when it is going straigth to you and when it is going straight away from you. This effect is called the doppler effect, and with the difference from the sound, the speed of the object can be calculated.
The first possibility is a quite good app which can be used on the smartphone:
www.rcspeedo.info/
It is worth the few bucks, you have nothing to do then turn it on and make a straight fly over it.
RC Speedo is automated very much, so unfortunately, there are a few models which can't be meassured with the app because they are too silent etc...
if that happens, you should get an audio recorder for your smartphone (I'm using tape-a-talk) and use that with a very high sample method and a not comprimated format. 44100kHz and wav. seems to be very good.
Now you are going to record the whole flight including the low straight passes and when you are at home, you are going to cut out only the passes. With that data, you are going to use wave-os-scope and then you know the achived speeds.
Seems easy, or? Once you know how fast your model is really flying, it will be easier for you to estimate the speed of other models and you will have a smile in your face when you hear some estimates from other guys.
never ehard about the doppler effect? Of course you did, but you haven't known it. look at a Formula 1 race in television and concentrate on the sound of the car when it passes the screen. Even if you haven't known it yet, you realized that the car sounds different when it is going straigth to you and when it is going straight away from you. This effect is called the doppler effect, and with the difference from the sound, the speed of the object can be calculated.
The first possibility is a quite good app which can be used on the smartphone:
www.rcspeedo.info/
It is worth the few bucks, you have nothing to do then turn it on and make a straight fly over it.
RC Speedo is automated very much, so unfortunately, there are a few models which can't be meassured with the app because they are too silent etc...
if that happens, you should get an audio recorder for your smartphone (I'm using tape-a-talk) and use that with a very high sample method and a not comprimated format. 44100kHz and wav. seems to be very good.
Now you are going to record the whole flight including the low straight passes and when you are at home, you are going to cut out only the passes. With that data, you are going to use wave-os-scope and then you know the achived speeds.
Seems easy, or? Once you know how fast your model is really flying, it will be easier for you to estimate the speed of other models and you will have a smile in your face when you hear some estimates from other guys.