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drone with variable pitch propeller

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marcofa

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I am designing a variable step propeller system for a small drone of 2.5m wingspan and motor 50cc with 8000 rpm.
as a propeller three blades found in the modeling field from 26" step 10" that weigh each 64 grams.
because I am not an aircraft designer, in the calculation of the resistance of the carrier hub I have come across a calculation problem that leads me to unexpected results and to intuition abnorms.
due to the centrifugal force/centripeta the single shovel rotating to 8000 rpm will be prompted to traction in its attack to the hub, and in the case of failure of the bond it would leave for the tangent doing damage.
and here comes my doubt in the calculation of the force applied to the shovel.
shovel weight 64 grams with baricentro to 160 mm from the axis
rotation 8000 rpm
I get a force of about 7000n per shovel, that is, the hub will be urged by three forces at 120° each of 7000n.
Is it a correct calculation or I've lost a factor of 10 on the street? I think for a weight of 64 grams a result of 7000n is abnormal.
if someone let me out of the swamp will have my eternal gratitude
Mar
 
Well also 8000 rpm are not few; however the newtons come 7,100 to me too. consider that for cylindrical symmetry the transverse loads on the shaft are zero: the vertical component is 7000 + 2*(-7000 cos(60°)) and the horizontal component -7000 sen(60°) + 7000 sen(60°).
 
If you can sacrifice a propeller just try to apply 7000n to the shovel, if it breaks then the calculation is definitely wrong.

p.s. in addition to the traction you also need to dimensional for the torsion of the base of the shovel due to both the propulsive thrust and the resistance to the air due to the rotation.
 
then it is just as I feared, so the hub support that holds the propeller from going into orbit must bear three forces at 120° of 7000 and broken n.
at this point it is necessary to calculate this value by integrating dm into dr on the whole shovel (I do not have the constructive design of the shovel)
since using the physical center is worsening.
I just have to slice a shovel and for every slice find mass and radius after which to make the sum of the forces. A piece of work! !
Is there a better procedure system?
Mar
 
a fem analysis, but what? I only have the physical shovel in my hand (carbon) and I don't know how the internal structure is.
I could try to make a fem at a photo....ha has.
I just need to slice! ! !
 
marcofa said:
I am designing a variable step propeller system for a small drone of 2.5m wingspan and motor 50cc with 8000 rpm.
(cut)
if someone let me out of the swamp will have my eternal gratitude
Click to expand...
the change of the step you need to be able to do continuously or you need more simply a step not variable but adjustable to the ground and fixed before takeoff?
I ask you because it seems to me that in the first case perhaps it will cost more the hub and the system of regulation of the step and control of the motor turns (if the propeller you want also to constant turns) than the entire drone. I add that, unless you have a sophisticated control system like that of military drones (well bigger and from absurd costs) with a radio control from aeromodels also risks to put in stall the blades for too much incidence. In short, control incidence of the blades, motor turns all in function of the conditions of flight (velocity of the aircraft, incidence etc.) from the ground by means of a radio remote control seems to me confused Do you have experience flying on real planes with variable step props and manual control of motor turns?

if instead you are aware of a regulation of the step from the ground, step that then remains fixed but that you can vary from time to time according to the scheduled flight conditions, then it seems to me to remember that they are on the market hubs and complete propellers tripala or quadripala beautiful that made, without being so much to bang to invent the wheel.

ah, if what you call drone is not a common aircraft but is a real drone, which must fly in inhabited areas to make video shoots or photos then I think getting certification to make it fly in those contexts will be more complicated than building the drone itself. a aircraft of 2.5 meters wingspan can make a massacre.

Here, for today I have finished caxxare doubts, but I hope you are still useful:
 
so much to add figurines to figures, why use the center of gravity should be different from making an integral along the shovel?
what about density distribution?

ah, to my partial justification the error on the calculation of the centrifugal force was given by the commas in excel, which is damned!
 
at lightning.
the reason for which integration even if rudimentary is necessary in the fact that the force of each section goes with the square of the speed, then 10 gr to 100 mm will exercise a force enormously less than 10 gr places to 400 mm from the axis.
Try to count in two cases and you're gonna be scared.

a marcof, almost my eponymous since I am marcofa, I confess that it would be a drone on the generas: a giant model (2.5 m) equipped with micro camera, to see you from above while pilots.
for the helical step is precisely in flight controlled by a very precise proportional remote control that allows to adjust the step in the cabrates and the beaten ones, in the phases of takeoff and landing and also allows the brake (with reverse of the blades) in the phase of final rolling; and if not enough it allows to put the helix in the flag without having to minimize the engine.
second step will be to get the automatic step change to the change of engine regime, but for this you will see.
Mar
 
marcofa said:
a marcof, almost my eponymous since I am marcofa, I confess that it would be a drone on the generas: a giant model (2.5 m) equipped with micro camera, to see you from above while pilots.
Click to expand...
ok, a 2.5 m wing opening aircraft with a video camera mounted on it.
for the helical step is precisely in flight controlled by a very precise proportional remote control that allows to adjust the step in the cabrates and the beaten ones, in the phases of takeoff and landing and also allows the brake (with reverse of the blades) in the phase of final rolling; and if not enough it allows to put the helix in the flag without having to minimize the engine.
second step will be to get the automatic step change to the change of engine regime, but for this you will see.
Mar
Click to expand...
I give you my best wishes, but I have no idea what casino you are going to stick to, both for the design of the mechanics that controls the change of the step and for the electronic control (Let's make sure if you want to make it at constant turns) and finally for the piloting. the step changes you refer to should be done manually, and an account is to be in the cockpit and have both the tools under the eye and the physical sensations of the flight, and an account is to have in hand a radio remote control.
constructive tolerances must then be extremely reduced if you do not want the first brusca virata to be removed all the abaradam (you take into account the gyroscopic effect, because it is only straight flights like a fuse and the rate of beaking and landing of the aeromodels are very high). an account is to dimensional a hub that must hold a fixed shovel, another speech is to realize a mechanics sufficiently precise, robust and reliable to hold the same shovel that rotates to vary the step. My memories of variable-step propellers (tourist or acrobatic planes) disassembled for review are those of extremely complex and high precision objects.
I allow myself to add that in an aircraft the variable step creates, as mentioned above, more complications than actual advantages. You don't have to optimize engine consumption and propeller performance in anticipation of making 800 km of cruise. there are aeromodels that have such a power weight ratio so they are in hovering like helicopters and helix montage from 50 euros, then see you
If you do it by delight you have all my approval and you can invest a fortune too:smile:
 
marcofa said:
at lightning.
the reason for which integration even if rudimentary is necessary in the fact that the force of each section goes with the square of the speed, then 10 gr to 100 mm will exercise a force enormously less than 10 gr places to 400 mm from the axis.
Try to count in two cases and you're gonna be scared.
Click to expand...
but the total force that "sees" the hub on the attack of a single shovel should not be equivalent to that produced by the mass of the shovel placed in its center?
 
no because the shovel is variable, there is more weight near the axis and little weight away.
different for the helicopter shovel that is practically to constant section, then to account with the center is not very wrong
Mar
 
marcof said:
but the total force that "sees" the hub on the attack of a single shovel should not be equivalent to that produced by the mass of the shovel placed in its center?
Click to expand...
We imagine three balls from 50 gr each at distances 50, 100, 150 mm from the center, and then one from 150 gr to 100 mm from the center, all rotating to 100 rpm.

100 rpm = 100 x 6.28 rd / 60 sec = 10.5 rad/sec

first case (50+50+50)

= 0.05 kg * 10.5^2 * 0.05 mt = 0.27 n
= 0.05 kg * 10.5^2 * 0.1 mt = 0.55 n
= 0.05 kg * 10.5^2 * 0.15 mt = 0.83 n
fct = fc1 + fc2 + fc3 = 1.64 n

second case (150 gr to 100 mm)
fc = m * omega^2 * r = 0.15 kg * 10.5^2 * 0.1 mt = 1.65 n

actually seems to be the same thing.
 
Of course it's the same in case you considered it.
You did a constant section propeller.
Now try to make this account:
three balls for a total weight of 150 gr, one of 20 gr one of 50 gr and one of 80 gr at distance of 50, 100, 150 mm from the center.
first calculation:
20 gr to 50 mm, 50 gr to 100 mm and 80 gr to 150 mm
according to calculation:
20 gr to 150 mm, 50 gr to 100 mm and 80 gr to 50 mm
you will see that the results will no longer be equal, nor at least equal to a sphere of 150 gr to 100 mm from the axis.
Mar
 
marcofa said:
Of course it's the same in case you considered it.
You did a constant section propeller.
Now try to make this account:
three balls for a total weight of 150 gr, one of 20 gr one of 50 gr and one of 80 gr at distance of 50, 100, 150 mm from the center.
first calculation:
20 gr to 50 mm, 50 gr to 100 mm and 80 gr to 150 mm
according to calculation:
20 gr to 150 mm, 50 gr to 100 mm and 80 gr to 50 mm
you will see that the results will no longer be equal, nor at least equal to a sphere of 150 gr to 100 mm from the axis.
Mar
Click to expand...
It is obvious that in both cases the center of gravity moves, which in neither case is half the length, which would instead be the case of a constant section propeller.

So you in the first post when you gave the value of the radius to 160 mm meant half the maximum radius, not the position of the centerpiece. If you'd told us right away, we'd save a little effort... .
 
marcofa said:
Of course it's the same in case you considered it.
You did a constant section propeller.
Now try to make this account:
three balls for a total weight of 150 gr, one of 20 gr one of 50 gr and one of 80 gr at distance of 50, 100, 150 mm from the center.
first calculation:
20 gr to 50 mm, 50 gr to 100 mm and 80 gr to 150 mm
according to calculation:
20 gr to 150 mm, 50 gr to 100 mm and 80 gr to 50 mm
you will see that the results will no longer be equal, nor at least equal to a sphere of 150 gr to 100 mm from the axis.
Mar
Click to expand...
eh thanks to the piffer, so you moved the center of gravity doing the counts with two completely different mass propellers equal! is like throwing the hammer once taking it from the handle and the time after from the iron bowl and complaining that the results in terms of distance and physical effort are different:rolleyes:
 

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