Understanding RC Helicopter Lift

by John Salt - Updated March 2025

How is RC helicopter lift generated and controlled?

Great question! Without lift after all, an RC helicopter or any aircraft for that matter won’t fly.

For anything to fly – the amount of lift produced has to be greater than the force of gravity pulling it back to the ground.

"Why Won't My RC Helicopter Lift Off / Take Off?"

This is one of the most common questions I get asked; there are two primary reasons:

• Not enough rotor speed (relative wind speed).
• Not enough rotor blade pitch angle (angle of attack).

With those two points in mind, let's first look at how lift is produced by all wings and rotor blades. This should then help you understand and figure out why your heli is not overcoming the pull of gravity.

The Airfoil

The oversimplified airfoil diagram is only telling part of the story - a small part.

Most things that fly, from birds to supersonic aircraft and yes helicopters too, rely on airfoils (also called aerofoils) to create lift.

How an airfoil (the shape of all wings, or rotor blades if looked at edge on) generates lift is much more complex than the oversimplified low pressure above the wing theory most of us were taught in school; even private pilot ground school for that matter.

To fly an RC helicopter, plane, glider or quad-copter, you certainly don't have to fully understand all the physics going on here. Not only that, covering all the fluid dynamics going on with airfoils right down to the quantum level of molecule interactions between the airfoil and air is well beyond my pay-grade. I will however show a great instructional video shortly that takes a deep dive into all that if you want a college level understanding. 

I'll just cover the simple to grasp concepts that are important in relation to our understanding of RC helicopter lift control. Hint - generalizations galore! 

Here we go... 

The Two Main Theory's of Lift

2. "Bernoulli's" Theory of Lift (the minor factor)

Bernoulli's theory as it applies to fluid dynamics, states faster moving air/liquid creates lower pressure due to the conservation of energy. Nothing wrong with that and it's proven. 

The misconception however is that Bernoulli's principle alone is responsible for airfoil lift, when in fact it's only one of 3 contributors. 

The second misconception has to do with the air molecules being forced to speed up as they travel over the longer top distance of the airfoil, to meet up at the same time with the ones below. This is known as "equal transit-time" and is a fallacy.   

Airfoils do not produce equal transit-time flow.

The air flowing over the top of an airfoil is definitely moving faster than below.

Much faster than the equal transit-time hypothesis would suggest as shown in this animation based off actual wind tunnel tests with pulsed smoke.

Regardless of what causes the air to move faster over the top of the airfoil, and how low pressure above the wing is formed, low pressure above the wing has been determined to be the smallest component of total lift generated by a rotor or wing.

Newton Wins!

The majority of lift is generated from the redirected airflow; both from the air under the wing being deflected downward, along with the air above the wing also being diverted downward behind the wing (laminar flow exit angle). For both to occur, the airfoil requires a positive angle of attack.  

Don't believe this?

Well, if the majority of lift was in fact created by a low pressure area above the wing or rotor, there really should not be much, if any air movement below it - right?

Displaced cloud from wing induced redirected airflow.

If you've ever hovered your heli over tall grass, or watched a helicopter hovering over open water, snow, or a dusty surface, we know this is far from reality. 

There is a massive volume of air being pumped downward by the rotor system.

Plane wings are also pumping massive amounts of air downward as this very cool image illustrates.

Low pressure above the wing creating all of the lift would also not explain ground effect. 

A flat slab of balsa still produces lift thanks to Newton's laws.

Bernoulli's theory literally falls out of the sky when trying to explain how a wing can produce lift when inverted. 

It also doesn't address how a totally flat wing on a toy rubber band powered plane that many of us had as kids is able to fly.

Many simple flat wing RC airplanes also exist that RC modelers are building these days such as the "May Bee"; inspired of course by the ever popular Gee Bee. 

Flat wing RC Airplanes such as this May Bee are popular due to their simplicity and ease of building.

Another example of this that many of us have "first hand" experience with is when you hold your hand out the window of a fast moving vehicle with a slight upward angle. There is a surprising amount of force pushing your hand upward.

The downside to this simple flat / slab wing and relying only on the redirection or air underneath is flat wings are not efficient. 

This is due to large increase in drag that occurs over the top leading edge or the wing where the air becomes detached & turbulent as the angle of attack increases. An airfoil shape on the other hand is very good at keeping the air flow attached to the top surface and creates laminar flow (parallel/organized flow) due to something called the Coanda effect.   

To sum up, your RC rotor blades create lift due to 3 different forces all reliant on relative wind speed & angle of attack.

1. Air diversion downward under the wing (strongest).
2. Downward laminar flow exit angle behind the wing (medium).
3. Lower air pressure above the wing (weakest).

Here's one of the best instructional video courses I've come across that explains wing and rotor lift fundamentals right down to molecular interactions. 

Before totally dismissing Bernoulli's theory of lift, here's an interesting question I often get asked that Bernoulli can help explain. 

"John, whenever I'm flying my micro RC helicopter indoors and I get close to a wall or window, the helicopter is often pulled into them. In fact, it's sometimes impossible to overcome this strange wall or window attraction even with full cyclic. Is this normal or is there something wrong with my helicopter?"

Yes it's very normal, and has nothing to do with your heli; it's due to Bernoulli's principle. To better understand this, let's try a simple experiment to demonstrate Bernoulli's principle. 

Take a normal 8.5”x11” piece of paper and hold it between both hands length wise at the leading edge so the back edge of the paper flops down. 

Now blow down and across the top of the paper, at about a 45 degree angle. What happens?

The back edge of the paper lifts up, even though you are blowing it downwards. That's Bernoulli's principle in action; the fast moving air over the top of the sheet of paper is creating a lower pressure area above than underneath. 

Ever wonder why a shower curtain gets sucked inwards when you turn the shower on? The flow of water from the shower head is also creating faster air movement down the side of the curtain creating a lower pressure area inside the shower than on the outside. 

This is why when your micro heli is near a wall, the fast moving downwash of air flow from the rotor creates a lower pressure area up against the wall compared to the other side of the helicopter. This pressure differential then pushes your little heli into the wall or window. It has nothing to do with your heli or piloting skills :-)

Our Two RC Helicopter Lift Control Variables

We Need The Speed!

Newton's second law we just learned about shows us how the velocity of this air flow / relative wind over the wing or rotor is proportional to the downward air flow pushing the wing upward.

Thus, the faster the air flows the more lift. Again, this is a very basic explanation; there are other forces and factors involved such as drag, turbulence, flow separation, air temperature, density & viscosity, along with the shape of the airfoil that limit the speed of air over a wing and ultimately limit the amount of lift produced.

We Need The Pitch!

The other component of lift is angle of attack or in helicopter control terminology “BLADE PITCH”.

This is where Newton's 3rd law comes into play. As you angle the rotor blade or wing up (leading edge higher than the trailing edge) you start generating diverted downward airflow; both from under the wing deflection as well as trailing edge laminar flow diversion, which both push the rotor blade/wing upward. 

This is called increasing the angle of attack of the wing. What do you think we call it in the heli world as it applies to rotor blades? Increasing the collective pitch angle. 

This picture shows a typical rotor blade edge on with a special RC helicopter tool called a pitch gauge measuring the angle of attack or pitch of the rotor blade. As you can see, this blade is showing a positive pitch angle of about +13 degrees.

Negative Pitch

So now let’s take the airfoil and pitch idea one step further.

If you look at the shape of that rotor blade airfoil in the above photo with the pitch gauge, it's identical on both sides. Thus the name symmetrical wing or rotor blade, same on the top and bottom.

Having a symmetrical airfoil shape on a wing or rotor blade now means airplanes and helicopters can efficiently fly upside down. By changing the angle of attack or pitch to negative when upside down, the rotor now produces lift in the opposite direction – pretty neat.

Here's our RC helicopter pitch gauge now showing a negative pitch angle of about -9 degrees. Because this is a symmetrical rotor blade, if the helicopter was upside down, the rotor would actually be creating a good amount of lift. 

I demo how this "negative" pitch change occurs in the video below.

Airfoil / Rotor Blade Shapes

We now know the flat slab wing (like on our little balsa rubber powered plane or our home built May Bee), while producing lift, is also very inefficient because the air flow detaches behind the leading edge and creates all sorts of chaotic turbulence, vortices and drag.

On the other hand, that nice rounded flowing shape of an airfoil is used to keep the air flow attached / bound (boundary layer) to the wing or rotor behind the leading edge thanks to the Coanda effect as the angle of attack is increased - up to a point.

If the angle of attack is increased too much or the air speed is reduced too much, the boundary layer becomes detached causing a large increase in drag. This is called a "stall" where lift is greatly reduced or lost completely.

It's also what causes that scary "fluffling" sound you get from your RC heli rotor when over pitching it during hard aggressive flying. 

There are also different airfoil shapes to improve performance and lift efficiency for specific types of flying & applications. 

Airfoil shapes dictate RC helicopter lift efficiency

Symmetrical airfoils like most of us are very familiar with on our RC helicopters, while being much more efficient than a flat slab, are not as efficient at producing lift as a semi or flat bottomed airfoil. They are however capable of faster head and flight speeds, along with the ability to fly inverted. Basically being the best compromise for all areas of RC helicopter lift and control, both upright and inverted.

The "under-cambered" airfoil takes lift efficiency to the extreme. This shape is ideal for light weight micro fixed pitch applications and produces lots of lift - they are sometimes called high lift rotors, but these are absolutely no good for inverted flight.

Many toy & micro and coaxial RC helicopters use this type of airfoil rotor design. The obvious reason is because it produces the most amount of lift with the least amount of power (high lift to power & weight ratio).

Fixed Pitch RC Helicopter Lift Control

Under-Cambered Rotor Blades Used On This Micro Coaxial RC Helicopter

In our helicopter hobby, most fixed pitched helicopters (like our little micro coaxial above) will use under-cambered airfoils to create the most amount of lift for the low amount of power available. 

The speed of these fixed pitched blades is altered by you the pilot, to adjust how much lift is produced. Faster head speed - more lift. Slower head speed - less lift.

Lift control by way of rotor speed adjustment is somewhat vague and imprecise because motors and rotors can't speed up and slow down instantaneously.

So... If you have a fixed pitch RC helicopter that won't lift off, the problem is not enough rotor speed. Some common things to check:

Collective Pitch RC Helicopter Lift Control

With collective pitch, we change the angle of attack of the rotor blades while the speed of the rotor blade remains fairly constant to adjust how much lift is produced.

Less angle of attack = less lift. Greater angle of attack = more lift.

This method of controlling lift is both very precise and more or less instantaneous. 

Speed of the rotor on collective pitch RC helicopters is also adjusted, but more as a way to control power output for a specific type of collective pitch flying. The majority of lift control on all collective pitch helicopters comes from adjusting the blade's angle of attack, not from the rotor speed (also referred to as head speed or rotor RPM).

Normal or scale flying for example we will usually be running lower rotor RPMs, because efficiency, docile flight characteristics, and longer flight times are preferred. Opposed to maximum head speeds used for fast & aggressive aerobatic flying (3D), when maximum power is needed over efficiency & longer flights. 

Scale RC Helicopter Flat Bottom Rotor Blades

There are RC helicopters with collective pitch that use flat bottomed or semi-symmetrical rotor blades to benefit specific areas of flight, such as auto rotations, or specific types of flying such as scale where inverted flight is not performed.

It's all a compromise of efficiency vs. performance!

So... If you have a collective pitch RC helicopter that won't lift off, the problem could be not having enough blade pitch, not enough rotor speed, or a combination of both! Common things to check:

Summing Up RC Helicopter Lift

• Newton explains airfoil lift much better than Bernoulli.
• Positive angle of attack is required to produce lift.
• Airfoil shapes help keep air attached to the wing/rotor reducing drag.
• By increasing the speed of the rotor, we can increase lift (up to a point).
• By increasing the angle of attack or positive pitch of the rotor, we also increase lift (up to a point).
• Lift is controlled exclusively by rotor speed on fixed pitch RC helicopters.
• Lift is controlled (mainly) by angle of attack on collective pitch RC helicopters. 
• Simi-symmetrical, flat bottomed, and under-cambered airfoils are more efficient at creating lift than symmetrical airfoils.
• To fly upside down, we use symmetrical rotor blades and reverse the angle of attack (meaning it's positive while inverted).
• Most micro fixed pitch RC helicopters use under-cambered rotor blades, don’t require as much power to fly, can't fly that quickly & can't fly inverted.
• Most collective pitch RC helicopters use symmetrical rotor blades, need more power to fly, can fly fast & can fly inverted.
• To lift off the ground or gain altitude, the helicopter must produce more lift force than the force of gravity pulling it down.
• To sustain a hover, the lift force must equal the force of gravity. The force of lift and the force of gravity are balanced. In this case the helicopter remains at a fixed altitude.
• To descend, the lift force has to be less than the force of gravity so gravity can pull the heli down.
  

As I cover more areas of heli flight theory in the Direction And Cyclic Control section, you'll see there are other forces acting upon the helicopter that causes the amount of lift being produced by the rotors  to constantly be changing. This is why you have to keep adjusting the amount of lift produced while flying.  

Collective Pitch Lift Control Makes Precise Hovering Of This Beastie Possible.

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