why cant we just add more blades to propellers and rotors to increase thrust?
In: Physics
Well presumably it would add load to the motor and interfere with the airflow of the other blades and thus diminish returns. There’s a happy medium to be found and engineers are good at determining that and what we see is generally optimized.
There are more ways of increasing thrust of a propeller. You can change the angle of the propeller blades to make them steeper, you can widen the blades, or increase the speed. Exactly what to do depends on more complex aerodynamic properties. Usually propeller tips are causing most of the issues with sound, turbulence and lost efficiency so you want as few propeller tips as possible. But this subject is way too complex for this subreddit.
The reason propellers do not have more thrust is that they need to match the motor that powers them. If you increase the thrust of a propeller then you need more power to maintain its speed. If your motor is not able to provide this power the propeller will slow down decreasing its thrust and the efficiency of both the propeller and the motor.
Thrust is limited by the power of the engine. If you add more blads and keep the same engine it will rotate slower and the thrust is not increased.
The advantages of more blades are the same diameter propeller can produce more thrust or a smaller diameter can produce equal trust.
The drawback is the thrust when the airplane is stationary depends a lot on the diameter so if you want an airplane that quickly accelerates and takes off larger diameter and fewer blades are more efficient.
More blades alos result in a higher cost.
Adding more blades to an airplane model is common when engine power is alos increased to be able to have the same diameter
So it is a compromise between multiple factors.
For most applications, the thing to optimize or maximize isn’t thrust. In almost any commercial application, one of the biggest factors is fuel efficiency. To be even more specific it is fuel efficiency at cruising speed.
No one wants a jet turbine to produce 1million pounds of maximum thrust but burns ten thousand gallons per minute at cruising speed (when max thrust isn’t needed). Nor would they pay 50million dollars per engine.
There are other factors – maintenance, noise, reliability, size, weight…
Engineers have to find a useful compromise.
Each additional blade lowers the efficiency of the rest of the blades because they travel a smaller distance to the air disturbed by the blade in front of it.
You get diminishing returns in efficiency as you add rotor blades. To retain efficiency you would ideally rather just increase diameter for the same number of blades if more lift is desired. But you do see a fair amount of “just one more blade, bro” in derivative models because it’s cheaper to redesign a hub to add one more of the same blade than it is to put in an all new rotor system of a larger diameter and increase the length of the tail boom. Although with an extra blade you’ve shifted all of the resonant frequencies and may need to stiffen up areas to avoid N/rev vibration. Plus with an extra blade you need more power/torque so you usually do a gearbox and engine upgrade as well. Also in a lot of cases, increasing solidity gets you the same results. Like if you’ve got 4 blades, making each blade 25% wider (chord) does essentially the same thing as adding a 5th blade without changing the structural dynamics as much.
CH-53 and MD-500 are examples of adding an additional blade to gain more lift.
Propellers use blades that have an airfoil shape, just like aircraft wings. If you put them too close together the airflow from one interrupts the other, so they need to be spread out. In practice that means planes usually have 2 or 3 blades but some have as many a 6.
But let’s imagine that we did add as many propellers as we can fit on a rotor. The airfoil shape doesn’t work as well so we can simplify the blades to be thin angled plates. This creates a bunch of channels that the air can be forced through. But those channels are open on the outside so air is spilling out, let’s put the rotor and blades inside a tube the same diameter so the air is blocked from spilling out the ends. This casing also forces the air to all flow backwards instead of out to the side. But it’s also twisting because of the rotor so let’s put more blades that don’t move inside the casing that will straighten out the flow so all the air moves straight back. We’ve just reinvented the ducted fan.
You can add more blades to increase thrust, but it’s usually more efficient to keep fewer blades an just make the blades longer, at least until the blade tips start getting close to the speed of sound.
You can and do. Look at the fans on the GE90, which is the most powerful commercially available engine.
There are a lot of blades. So many blades close together.
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