Hi Roddy, you are very good at explaining stuff. great that you go through all the steps when you reduce the equations. In a variable pitch propeller case, you have about a 0.6 static efficiency for a well-designed propeller system, raising to about 0.85 at forward flight. I think the example propeller you used was fixed pitch, then everything is dependent on where you put the propeller pitch, optimize for static thrust and low speed or higher speed.
What is seriously missing from this propeller thrust equation is area in the airfoil equation where L = (1/2) * ρ * V² * C_L * A, since thrust is a function of, if not purely proportional to, cord length of a propeller. I've 3d printed several props varying only cord length, and find that the thrust appears proportional to cord length for multiples of 2, 3, 4 and 5 of the cord length. Similarly since area in the lift equation includes multiple wings, I've also 3d printed props using blade counts of 2, 5, 7, 11, 13, 17 ... and up to 37 on larger props to find that thrust also increases proportional to blade count as long as there isn't a significant overlap of blades inducing channel turbulence between the blades as predicted by the Re. Smaller 200mm props overlap early, 300mm props after 19, and 450mm props after 37, since the majority of the lift is in the outer perimeter of the prop due to V². When thrust is held constant on the test stand, RPM drops as blade area increases, and with that vortex generation is significantly reduced. This should improve efficiency for props with a longer cord length and a large number of blades. Lowering RPM does increase required torque for the same thrust and power. This does significantly change design points for the motors, both ICE and Electric, to maintain optimal efficiency. And obviously changes the Re and Mtip points in the Ct function. Is this why the turbofan compressors all have nearly 100% of the intake area covered by a blade? John Bass Sr Engineer, semi-retired Blue Sky Aviation ATV's
Hello I should calculate the theoretical thrust of a quad copter for my graduation paper, but I just cant get the same results for the example calculations. Ive set the area S = (0.127m)^2 * pi , which yields a disc area of approx. 0.0507 m^2. (The propeller used in the video has a diameter of 10 inches => radius 5 inches = 12.7 cm = 0.127m) I assumed the air density to be standard ρ = 1.225 kg/m^3 For the power I used the value from the table (P=U*I). If plugged into the equation it yields: T = (2*1.225 kg/m^3 * 0.0507m^2 * P^2)^1/3 -> it works for the first value, but not for the following ones. Thanks
How do i figure out the Velocity of fluid by just knowing the RPM? I know that the thrust is the mass flow rate * the velocity of the fluid I know air density is 1.225kg/m3 I don't even know if this is the right question to ask, my brain is spaghetti right now trying to figure this out all day long. The more i try to understand the more confused i get.
You just have to make some assumptions. In this video I assumed the propeller was static, e.g. fitted in an engine test cell, or on an aircraft on the ground with the brakes on. With that assumption, the velocity of air sufficiently far in front of the propeller is zero. So now you want to work out what velocity is induced by the propeller from the RPM. The power provided by the engine will be the engine torque times the RPM. If you have the RPM and torque you can then conclude that power in from the engine equates to the power given to the air, or at least a large percentage of it. If you don't know the Torque, then you can look up the moment of Inertia for the propeller + engine combination. The engine power will then relate to the product of half the moment of inertia and the angular velocity squared.
If I wanted to optimize a propeller for static thrust, like the fan in a wind tunnel for example, would I then use the V0 value to calculate the resultant relative wind for setting the AOA of my blade elements?
So if I design a propeller 4 - 5m in diameter (for a human powered airplane) with 300W power input for a 45km/h flight and I want to test it, I should expect only 30% efficiency? I am just wondering, how to test the propeller then and what numbers to expect. What do you think?
45km/h is approximately 12m/s. So, you might expect to get a day when the wind blows at 12m/s. Put the propeller into the wind, apply your 300W of power and measure the thrust. That thrust x 12m/s equates to the power out. Divide this by 300W and you'll have your efficiency.
@@AG-kh5ct No, but Eqn 14-64 takes the previous equation and multiplies it by 0.85(1-As/Ad) where As is the area of the spinner and Ad is the area of the disc. The 0.85 is an efficiency factor I assume.
In the Example from RUclips, the efficiency is 0.3 or 30% while you explained efficiency of a propeller is around 85% which is, 85% of the engine power is delivered to the aircraft movement. My question, what is possible the reason the efficiency in that example is so low compared to your many calculation?
The reason is because the propellers are acting at different angles of attack. When the propeller is static, the angle of attack is basically the blade angle (if we ignore inflow factors). At cruise, the angle of attack is dependent upon the RPM and the aircraft velocity. The blade angle is chosen so that the angle of attack at cruise is at the optimum position, i.e. where the propeller airfoil has the max lift to drag ratio.
can I ask a question ? is there any material can not be repaired in aircraft?? I mean the materials contain wood,steel,aluminum and composite. thx a lot ur video +after watching ur video about al-prop repair , I think the point of prop repair that is 'Prop repair' is limited in Minor repair. So I ask that ... can the al-prop be an answer on my question?
@@RoddyMcNamee as I get it, the crack or dent of al-prop can not be allowed. if so- that case is not minor repair, right? then - is there any minor repair of al-prop? thanks for ur reply - always
@@paiisebutton6322 you are correct. A minor repair on an al prop would be something like a dent from a small stone or some light corrosion. However, as a rule of thumb no repairs are allowed near the shank of the propeller.
Hi Roddy, you are very good at explaining stuff. great that you go through all the steps when you reduce the equations. In a variable pitch propeller case, you have about a 0.6 static efficiency for a well-designed propeller system, raising to about 0.85 at forward flight. I think the example propeller you used was fixed pitch, then everything is dependent on where you put the propeller pitch, optimize for static thrust and low speed or higher speed.
Thanks Bjorn. I was using data from a fixed pitch RC Aircraft propeller.
Keep making these videos. They are great!]
What is seriously missing from this propeller thrust equation is area in the airfoil equation where L = (1/2) * ρ * V² * C_L * A, since thrust is a function of, if not purely proportional to, cord length of a propeller. I've 3d printed several props varying only cord length, and find that the thrust appears proportional to cord length for multiples of 2, 3, 4 and 5 of the cord length.
Similarly since area in the lift equation includes multiple wings, I've also 3d printed props using blade counts of 2, 5, 7, 11, 13, 17 ... and up to 37 on larger props to find that thrust also increases proportional to blade count as long as there isn't a significant overlap of blades inducing channel turbulence between the blades as predicted by the Re. Smaller 200mm props overlap early, 300mm props after 19, and 450mm props after 37, since the majority of the lift is in the outer perimeter of the prop due to V².
When thrust is held constant on the test stand, RPM drops as blade area increases, and with that vortex generation is significantly reduced. This should improve efficiency for props with a longer cord length and a large number of blades.
Lowering RPM does increase required torque for the same thrust and power. This does significantly change design points for the motors, both ICE and Electric, to maintain optimal efficiency. And obviously changes the Re and Mtip points in the Ct function.
Is this why the turbofan compressors all have nearly 100% of the intake area covered by a blade?
John Bass
Sr Engineer, semi-retired
Blue Sky Aviation ATV's
Great video! As a rookie I understood everything explained, thank you!
Really interesting! Could you make a video about hovercrafts ? I really want to know how it works
Hello
I should calculate the theoretical thrust of a quad copter for my graduation paper, but I just cant get the same results for the example calculations.
Ive set the area S = (0.127m)^2 * pi , which yields a disc area of approx. 0.0507 m^2. (The propeller used in the video has a diameter of 10 inches => radius 5 inches = 12.7 cm = 0.127m)
I assumed the air density to be standard ρ = 1.225 kg/m^3
For the power I used the value from the table (P=U*I).
If plugged into the equation it yields: T = (2*1.225 kg/m^3 * 0.0507m^2 * P^2)^1/3 -> it works for the first value, but not for the following ones.
Thanks
How do i figure out the Velocity of fluid by just knowing the RPM?
I know that the thrust is the mass flow rate * the velocity of the fluid
I know air density is 1.225kg/m3
I don't even know if this is the right question to ask, my brain is spaghetti right now trying to figure this out all day long. The more i try to understand the more confused i get.
You just have to make some assumptions. In this video I assumed the propeller was static, e.g. fitted in an engine test cell, or on an aircraft on the ground with the brakes on.
With that assumption, the velocity of air sufficiently far in front of the propeller is zero.
So now you want to work out what velocity is induced by the propeller from the RPM. The power provided by the engine will be the engine torque times the RPM. If you have the RPM and torque you can then conclude that power in from the engine equates to the power given to the air, or at least a large percentage of it.
If you don't know the Torque, then you can look up the moment of Inertia for the propeller + engine combination. The engine power will then relate to the product of half the moment of inertia and the angular velocity squared.
@@RoddyMcNamee Thank you for quick reply. I subscribed and thanks for this video really helped out. I should be able to work this out now.
Nice bro 👏
It was a detailed derivation
can you help me the way calculate area of disc
What does S in the equation stands for?
Propeller disc area.
If I wanted to optimize a propeller for static thrust, like the fan in a wind tunnel for example, would I then use the V0 value to calculate the resultant relative wind for setting the AOA of my blade elements?
Yes, that is what I would do.
So if I design a propeller 4 - 5m in diameter (for a human powered airplane) with 300W power input for a 45km/h flight and I want to test it, I should expect only 30% efficiency? I am just wondering, how to test the propeller then and what numbers to expect. What do you think?
45km/h is approximately 12m/s. So, you might expect to get a day when the wind blows at 12m/s. Put the propeller into the wind, apply your 300W of power and measure the thrust. That thrust x 12m/s equates to the power out. Divide this by 300W and you'll have your efficiency.
Good idea, thanks.
How many blades is this for?
The prop was treated as a single disc, so it doesn't consider the number of blades.
@@RoddyMcNamee Is there a reference to this? Perhaps a book that has this final equation?
@@AG-kh5ct Eqn 14-63 in General Aviation Aircraft Design : Applied Methods and Procedures by Snorri Gudmundsson
@@RoddyMcNamee thank you. Does it show the equation multiplied by the propeller efficiency?
@@AG-kh5ct No, but Eqn 14-64 takes the previous equation and multiplies it by 0.85(1-As/Ad) where As is the area of the spinner and Ad is the area of the disc. The 0.85 is an efficiency factor I assume.
cheers this was really helpful
In the Example from RUclips, the efficiency is 0.3 or 30% while you explained efficiency of a propeller is around 85% which is, 85% of the engine power is delivered to the aircraft movement. My question, what is possible the reason the efficiency in that example is so low compared to your many calculation?
The reason is because the propellers are acting at different angles of attack. When the propeller is static, the angle of attack is basically the blade angle (if we ignore inflow factors). At cruise, the angle of attack is dependent upon the RPM and the aircraft velocity. The blade angle is chosen so that the angle of attack at cruise is at the optimum position, i.e. where the propeller airfoil has the max lift to drag ratio.
Great video!
Anybody watched young sheldon and watched this?
can I ask a question ?
is there any material can not be repaired in aircraft??
I mean the materials contain wood,steel,aluminum and composite.
thx a lot ur video
+after watching ur video about al-prop repair , I think the point of prop repair that is 'Prop repair' is limited in Minor repair.
So I ask that ... can the al-prop be an answer on my question?
Yes, generally only minor repairs can be performed in the field, and with composite propeller blades, very little can be done in the field.
@@RoddyMcNamee as I get it, the crack or dent of al-prop can not be allowed. if so- that case is not minor repair, right?
then - is there any minor repair of al-prop?
thanks for ur reply - always
@@RoddyMcNamee I want to know the reason why al-prop can not be repaired in field.
I mean - about minor repair
@@paiisebutton6322 you are correct. A minor repair on an al prop would be something like a dent from a small stone or some light corrosion. However, as a rule of thumb no repairs are allowed near the shank of the propeller.
@@RoddyMcNamee and the reason is that can be affected in airworthiness?
muah