Tench745
Master member
You will need to spin props small enough for the motor to handle. As an example, let's assume the motor can usually handle an 8x6 prop. You wouldn't be able to run two 8x6 props because now the load on the motor would be doubled (plus a little extra for friction and losses in the gear or belt drive).
agupt108
Member
I see, but would is still be feasable? Also, then, what props to use? I am using a power pack c-motor (rated for 10x4.5 prop); so maybe 6 x 3 or by 4 props now?
Tench745
Master member
Lemme do some math... I have this equation for figuring out comparable prop sizes.
Load= (D^3)*P*sqrt(N-1)
D= Diameter
P= Pitch
N= # of blades
a 10x4.5 2-blade prop has a load of 1000*4.7*1= 4700
Presumably you need a prop that has a load of half that; 2350.
Let's assume you want the same speed, so your pitch will still need to be around 4.7
That gives us 2350= (D^3)*4.7*1
Deriving that out we see that the D= the cube-root of (2350/4.7) Which is 7.937 inches.
Since they don't make props in that diameter and the 10x4.7 was probably near the upper limits for that motor we'll play it safe and look for something smaller either a 7" or a 7.5" prop.
I found a 7.5x5 prop from HQ prop. https://www.hqprop.com/hqprop-75x5-light-grey-2cw2ccw-poly-carbonate-p0337.html
Plugging that into our equation it has a load of 2109 which is pretty close to what we were looking for.
Load= (D^3)*P*sqrt(N-1)
D= Diameter
P= Pitch
N= # of blades
a 10x4.5 2-blade prop has a load of 1000*4.7*1= 4700
Presumably you need a prop that has a load of half that; 2350.
Let's assume you want the same speed, so your pitch will still need to be around 4.7
That gives us 2350= (D^3)*4.7*1
Deriving that out we see that the D= the cube-root of (2350/4.7) Which is 7.937 inches.
Since they don't make props in that diameter and the 10x4.7 was probably near the upper limits for that motor we'll play it safe and look for something smaller either a 7" or a 7.5" prop.
I found a 7.5x5 prop from HQ prop. https://www.hqprop.com/hqprop-75x5-light-grey-2cw2ccw-poly-carbonate-p0337.html
Plugging that into our equation it has a load of 2109 which is pretty close to what we were looking for.
agupt108
Member
Wow! Thank you very much! I would think a drive belt would have less friction right? I will try to make some designs tommorow for such a belt.
Tench745
Master member
Belts tend to be quieter than gears. How much friction they have depends on belt tension, pulley size, and a number of other things. Doesn't hurt to try it, and I look forward to seeing what you come up with!
agupt108
Member
Also for a plane like the ft sea duck, would scaling it down to 75% work for the props? Since:
7.5/10 = 0.75
So since the prop is 75 % of the size of the props on the sea duck, will reducing the size of the plane by that much work?
agupt108
Member
Ok thank you!
Tench745
Master member
Short answer: maybe.
As far as size scaling, yes 75% would be right.
When you scale a design up or down you need to remember that area and volume don't scale linearly.
Area scales by the square (length x width) and volume scales by the cube. (length x width x height).
If your wingspan is 75% of the original, the wing area will be .75² or approximately 56% of the original wing area. (And volume would be 42%) The electronics don't necessarily get that much lighter for a plane only 75% of the original, so you still have quite a bit of weight to carry around on considerably less wing.
Also worth nothing that the folds and tabs on the plans won't be the right width anymore because they've been scaled down, but the foam thickness hasn't, so you need to adjust for that while building.
If the plane has enough power or is build light enough, or the original wing loading was light enough it might work fine. Other times the result flies like a dog; it all depends.
Many twin-engine planes have props that spin the same direction, but they have more torque-roll at low airspeeds. Counter-rotating props eliminate this tendency and are preferred for this reason. Either way will probably work for a proof of concept plane but spinning the same way probably will be lighter and less complex. It comes down to what you're comfortable with and what you want to achieve.
quorneng
Master member
agupt108
You can indeed use two props of a size that just one would normally be suitable for the motor by using a gear or belt reduction. This is indeed why indirect drives are used in planes and cars.
There are advantages in doing this as two bigger but slower revving props with a bit more pitch are more efficient and can largely offset the losses from using gears or belts in the first place.
The difficult question is by how much should you reduce the prop speed so two together absorb what the motor can handle. The power absorbed by a prop is roughly proportional to the square of the rpm. Halve the revs it only absorbs 1/4 the power so off the top of my head this suggests you should reduce both prop rpm by 0.7:1 to absorb the same power as a single prop would running at motor speed.
You may have to set up the drive system in a "test rig" to actually find out what works best.
Good luck!
Historical note.
The Wright brothers knew all this well. The big twin props on the 1903 Wright Flyer were driven by a single engine via a reducing chain drive with a ratio of 3:1 so the props rotated at 330 rpm. The engine rotated at 1000 rpm. By crossing over the chain drive on one side the props also rotated in opposite directions.
It only took the brothers a year or so of experimentation to get it all to work effectively!
You can indeed use two props of a size that just one would normally be suitable for the motor by using a gear or belt reduction. This is indeed why indirect drives are used in planes and cars.
There are advantages in doing this as two bigger but slower revving props with a bit more pitch are more efficient and can largely offset the losses from using gears or belts in the first place.
The difficult question is by how much should you reduce the prop speed so two together absorb what the motor can handle. The power absorbed by a prop is roughly proportional to the square of the rpm. Halve the revs it only absorbs 1/4 the power so off the top of my head this suggests you should reduce both prop rpm by 0.7:1 to absorb the same power as a single prop would running at motor speed.
You may have to set up the drive system in a "test rig" to actually find out what works best.
Good luck!
Historical note.
The Wright brothers knew all this well. The big twin props on the 1903 Wright Flyer were driven by a single engine via a reducing chain drive with a ratio of 3:1 so the props rotated at 330 rpm. The engine rotated at 1000 rpm. By crossing over the chain drive on one side the props also rotated in opposite directions.
It only took the brothers a year or so of experimentation to get it all to work effectively!
agupt108
Member
Ok thank you! Trying to make design now.
quorneng
Master member
agupt108
V belts can transmit a lot of torque for their size but are less efficient than a flat belt. The problem with a flat belt is their limited torque capability for their size and weight. Crossing a flat belt also introduces losses as the belt is twisted through180 degrees twice for each revolution. It all takes energy to do.
It would interesting to know the details of the motor you intend to use and the size of the props. You may find any belt of a reasonable size will be unable to transmit the torque.
Remember also belt tension adds a load to the bearings which could be significant unless low friction ball bearings are used.
Just some observations.
V belts can transmit a lot of torque for their size but are less efficient than a flat belt. The problem with a flat belt is their limited torque capability for their size and weight. Crossing a flat belt also introduces losses as the belt is twisted through180 degrees twice for each revolution. It all takes energy to do.
It would interesting to know the details of the motor you intend to use and the size of the props. You may find any belt of a reasonable size will be unable to transmit the torque.
Remember also belt tension adds a load to the bearings which could be significant unless low friction ball bearings are used.
Just some observations.
agupt108
Member
I am using a C-pack radial from FT, so a 2218 1180 kv motor. It's rated to use a 10x 4.5 motor, and Tench745 (thank you!) calculated to get the same load I should use around a 7.5 x 5 prop on each pulley end. Also what are bearings? Thank you!
Also, I found this:
This type of belt drive [cross belt drive] employs a belt when two parallel shafts have to rotate in the opposite direction. At the junction where the belt crosses, it rubs against itself and wears off. To avoid excessive wear, the shafts should be kept at maximum distance from each other and operated at very low speeds.
"operated at very low speeds" is a bit concerning...
I found this too, not sure though:
Synchronous drive belts and pulleys use teeth to prevent slippage and unwanted speed variations. AutomationDirect SureMotion 5M and 8M (5-mm and 8-mm pitch) high torque synchronous drive/timing belts have a curvilinear HTD (High Torque Drive) tooth profile which allows the belts to have a much larger pulley contact area for improved performance.
Thank you!
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agupt108
Member
Yes! I modeled it from that only. I am thinking wooden pulleys and some good quality v-belts. However, any ideas as to how to make the wooden pulleys without a lathe? I have a drill maybe something using that?
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