help on prop size....

[eekin]

hi..

currently my motor is recommended for 11 prop size.. range frm 10x4.7sf on 3s pushing 1250g thrust to 10x6 on 3s pushing 1140g thrust.

its 1180kv max watt is 312w.


my qn is, why is it wen its on sf prop 10x4.7, it produces more thrust than wen its on a higher pitch prop like 10x6?? im quite confused, isnt it wen we up the pitch, it gives more thrust??


and im planning to put a 3 bladed prop, but i dunno wat size shud i go for tat is suitable for the motor.. shud i down the size and pitch too??

relli hope someone can help.. thx..

[joe yap]

No, when you increase the pitch, you increase the prop velocity, or dynamic thrust. You are only measuring static thrust on ground.

Furthermore, you are just increasing the pitch alone without reducing the diameter, thus increasing the prop load.

If you really want to compare fairly, you should do so by keeping the prop load constant as much as possible. Prop diameter, pitch and blade design will affect the loadings.

Practically, if you want to change the prop diameter or pitch without drasticaly changing the loads, we compensate by keeping the summation of both constant.

For example, for a 10 x 5 prop,

Add 10 to 5 = 15

If you want to increase the pitch to 6",
Take 15 - 6 = 9
So, use a 9" diameter prop with 6" pitch.

If you want to increase the prop diameter to 11",
Take 15 -11 = 4
Thus, use a 11 x 4 prop.

If you want to increase the number of prop blades,
reduce either the diameter or pitch by the number of blades you add.

To use a 3 bladed prop, using the previous example,
go for either 9x5 or 10x4.

[eekin]

k... thx alot joe... but lets say a 2628 motor tat spins a 9x5 prop at 1200kv produces thrust of 800g. then a bigger motor like 2830 also spinning 9x5 at 1200kv. does it also produce same thrust of 800g?

[joe yap]

The 2830 motor should be able to produce more thrust and spins the 9x5 prop faster than the 2628.

Although both motors are rated at 1200kv, you'll only get the calculated rpm (kv x voltage) without any load. The moment you put a prop on it, the actual rpm will drop. And of course the larger motor will have get higher rpm with the same prop, compared to the smaller one.

[eekin]

ok.. now i understand better.. thx joe. aniwae.. im trying to mount this new motor. not the stick mount.. first time using x mount. if nitro.. so easy.. haha.

aniway.. the plane that i have, already had a piece of wood abt 1.5cm epoxied onto the foam of the front of the plane. my plane is foam btw. its not balsa and nt ply.. thk its basswood. so how do i mount this xmount? i mean, wats the orientation of the mount wen we look directly form the front? is it "x" or "+"? and do i need to angle the mount? i heard about motors needing to have a thrust angle. can u enlighten me abt this part? and wats the rational of it?? some ppl say nitro need angle, electric dun..

[joe yap]

The cross radial mount should be orientated in 'X' configuration. I am not sure what 1.5cm basswood are you talking about. Is it a 1.5cmx 1.5cm stick protruding in front of the firewall? If so, we call it a beam mount, and it is completely useless if you are going to use you usual cross radial mount for outrunner motors. If that's the case, remove it completely and glue a piece of 1.5mm plywood in front of the foam plane's firewall. Make a hole in the middle if necessary to accomodate the outrunner motor shaft if it protrudes behind. Mount the motor directly onto the plywood using 4 wood screws.

If the motor is too far back with respect to the engine cowling, you can stack up some hardwood like baswood in between the motor mount and firewall.

Who says that engine planes needs side and downthrust and electric planes don't need? If you see him again, maybe you'll like to ask him the reason behind such theory.

Actually, as long as you have rotating propellor, you'll need downthrust and sidethrust in most cases, except for some cases, irregardless of source of energy. The spinning prop, always impart gyroscopic effect that tends to turn your plane towards the left on its own. The faster the prop spins, the greater the effect and a little right thrust will cancel or reeuce this effect.

Most planes tends to pitch its nose up with the application of power, especially for high wing planes, which the thrust line is way below the wings that creates a rather big moment. Downthrust on the motor will again, cancel or reduce the effect.

For the cross radial mount, you can adjust the side and down thrust simply by adding washers in between the mount and the firewall.

[eekin]

The cross radial mount should be orientated in 'X' configuration. I am not sure what 1.5cm basswood are you talking about. Is it a 1.5cmx 1.5cm stick protruding in front of the firewall? If so, we call it a beam mount, and it is completely useless if you are going to use you usual cross radial mount for outrunner motors. If that's the case, remove it completely and glue a piece of 1.5mm plywood in front of the foam plane's firewall. Make a hole in the middle if necessary to accomodate the outrunner motor shaft if it protrudes behind. Mount the motor directly onto the plywood using 4 wood screws.

If the motor is too far back with respect to the engine cowling, you can stack up some hardwood like baswood in between the motor mount and firewall.

Who says that engine planes needs side and downthrust and electric planes don't need? If you see him again, maybe you'll like to ask him the reason behind such theory.

Actually, as long as you have rotating propellor, you'll need downthrust and sidethrust in most cases, except for some cases, irregardless of source of energy. The spinning prop, always impart gyroscopic effect that tends to turn your plane towards the left on its own. The faster the prop spins, the greater the effect and a little right thrust will cancel or reeuce this effect.

Most planes tends to pitch its nose up with the application of power, especially for high wing planes, which the thrust line is way below the wings that creates a rather big moment. Downthrust on the motor will again, cancel or reduce the effect.

For the cross radial mount, you can adjust the side and down thrust simply by adding washers in between the mount and the firewall.

the piece of wood is not a stick mount bro.. its a 5x5cm hardwood. thickness 1.5cm. epoxied to the front of the plane. and motor needed to be pushed forward to fit the cowling.. but i didnt use stacking method. intead i bought 2 x mounts and 4x35mm extension rods. like the one in the pic.. my configuration is the first on the left..

so i guess the way to mount it is using self tapping screws straight to the firewall?? coz i do not have access to the other side of the firewall. so cant use bolt and nut. im planning to use self tapping screws and reinforce with some eopxy. it would be somewhat permanent tho.. any suggestions?

and how do i knw if my plane needs downthrust or right thrust or both?? i knw one way is to fly it and observe.. but if i take out the mount again, then the hole in the firewall will get loose and wun be secured like before. and how much thrust should i adjust to?? eg?? how many washers etc..

if i need downthrust, i put washer on the top part of the mount. if right thrust, out on the left. correct? and when u say a plane will naturally turn to the left, u mean banking to the left or yawing to the left??

[joe yap]

Your method should work. Just use wood screw or machine screw long enough to hold the mounting system.

You are right about the shimming for side/down thrust. But check with the original stock motor mounting system. The fuselage itself may already have some form of side and down thrust incorporated which your eyeball can't tell. Stay with the original thrust angle first and fly before changing. You can sometime tell when you install the engine cowling.

Gyroscopic effect should tend to yaw your plane to the left, but dihedral effect can induce a yaw induced bank. Anyway, this effect may not be significant in most flight phases until you try to pull some precision aerobatics. Not much of a concern for most sport planes, but 2 degrees right thrust is pretty common in most designs.