Understanding RC Helicopter Lift
by John Salt - Updated February 2020
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?"
This is one of the most common questions I get asked by people who contact me for help. There are two primary reasons:
- Not enough rotor speed (head 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 rotors. This should then help you understand and figure out why your heli is not overcoming the pull of gravity.
The oversimplified airfoil diagram is only telling part of the story - a small part.
All 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 this lift is much more complex than the oversimplified low pressure above the wing theory most of us were taught in primary school, or even 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 (as I said, lift it's complicated stuff). Not only that, covering all the fluid dynamics going on with airfoils is well beyond my ability and the scope of this article. There are plenty of other good "lift articles" out there already - here's one.
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
1. "Newton's" Theory of Lift (the main factor)
Newton's second and third laws are arguably the primary contributors that explain airfoil lift. They are also the two that we have direct control over while we fly our RC helicopters.
Newton's second law is commonly represented by the formula F=MA (force equals mass times acceleration). Applied to an airfoil, this can be interpreted as the lift of a wing being proportional to the amount of air diverted down times the vertical velocity of that air.
Newton's third law states that for every action, there is an equal and opposite reaction. The action of the wing on the air is to force the air downwards while the reaction is the wing being pushed upwards. In short, an airfoil is a very effective air direction diverter or air pump.
To simplify all that - if we want more RC helicopter lift, our rotor blades can either increase their velocity, divert more air (mass), or a combination of both.
We will look at how this is achieved while flying our little egg-beaters shortly. I do however want to cover the second theory of lift. Partly because it's what many of us were incorrectly taught; and secondly because its the underlying principle responsible for one thing you have likely experienced while flying micro RC helicopters indoors.
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 lift, when in fact it's a small contributor.
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 again, 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.
Regardless of what causes the air to move faster over the top of the airfoil, and how low pressure above the wing is ultimately formed (both still not fully understood); low pressure above the wing has been determined to be a rather small component of total lift generated by a rotor or wing. For the average RC aviator, the argument is therefore moot.
The majority of lift is generated from the redirected downwash airflow.
Displaced cloud from wing induced downwash.
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.
If you've ever hovered your heli over your head, 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, or even something as simple as a paper airplane can generate lift.
So why am I still talking about Bernoulli's theory, and what does it have to do with your little micro RC heli?
First, let's try a simple experiment right now 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.
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. The result? The higher pressure on the outside pushes the shower curtain towards the shower flow.
Which finally brings us to your micro RC helicopter.
Have you noticed the walls or windows in your house almost seem to be magnetic when your micro heli is getting fairly close to them? No matter what you do, it's very hard or perhaps even impossible to overcome this strange magnetic wall or window attraction.
It's a very real force the micro heli is experiencing, but it has nothing to do with magnetism - it's all due to Bernoulli's principle.
When a micro heli (or any size helicopter for that matter) is near a wall, the fast moving downwash 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 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 is proportional to the downward air flow pushing the wing upward.
Thus, the faster the air flows over the wing the more lift. This is a
very basic explanation; there are other forces and factors involved such as drag, turbulence, flow separation, 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, which pushes the rotor blade/wing upward. The more you angle the rotor blade, the more air you divert downward and more lift is created.
We call this positive or high angle of attack. What do you think
we call it in the heli world? “Positive Pitch”.
This picture shows a typical rotor blade edge on with a
special RC helicopter tool
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.
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.
a symmetrical airfoil shape on a wing or rotor blade now means
airplanes and helicopters can 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
The rounded shape of the airfoil is responsible for keeping the air flow "bound" to the wing or rotor (boundary layer) as the angle of attack is increased - up to a point. If just a flat slab was used instead (like on our little balsa rubber powered plane), the airflow would easily separate above the wing, and create all sorts of chaotic turbulence, vortices and drag.
That's exactly what happens when a wing stalls. It's also what causes that scary "fluffling" sound you get from your RC heli rotor when over pitching it during hard aggressive flying.
Airfoil shapes dictate RC helicopter lift efficiency
Symmetrical airfoils like most of us are very familiar with on our RC helicopters are not as efficient at producing lift as a semi or flat bottomed airfoil. They are however capable of faster head and flight speed.
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.
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, and has the least amount of weight (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 amount of power available. As a result, smaller
motors/batteries can be used for longer flight times and
lighter helicopters. This is a good compromise seeing that fixed
pitched helis can’t sustain inverted flight anyways.
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 can be 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:
- Bad/weak battery (usually the most common culprit).
- Faulty motor (worn out, bearings shot, magnets shot, etc).
- Worn main shaft bearings.
- Something dragging in the drive train, or overly tight/binding gears.
- Throttle channel output from radio too low (check your channel travel limits and throttle curve/s).
- Bad connectors.
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 very fast.
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).
Tame normal 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 usually performed.
Once again, efficiency over performance!
However, most out of the box collective pitch, single rotor helicopter
kits will come with symmetrical rotor blades because they are the best
compromise for all areas of RC helicopter control.
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:
your collective pitch range with a pitch gauge! This is by far the most
common reason why collective pitch RC helicopters are not producing
enough lift - not enough blade pitch. Your manual should give you
recommend collective pitch angles (set with your pitch curves), but most
CP helis should be producing enough lift to hover at around +5 degrees
of collective pitch. If none of that makes sense - my setup & tips eBook will explain it all.
- Not enough rotor speed. Same items to check as with fixed pitch.
Summing Up RC Helicopter Lift
- Newton's laws explain airfoil lift much better than Bernoulli's.
- Airfoils & positive angle of attack are required to produce lift.
- By increasing the speed of the wing or rotor we can increase lift (up to a point).
- By increasing the angle of attack or positive pitch of a wing/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 have to use symmetrical rotor blades and reverse the angle of attack (meaning it's positive while inverted).
- Most fixed pitch RC helicopters use under-cambered rotor blades, don’t require as much power to fly, and can't fly that quickly.
- Most collective pitch RC helicopters use symmetrical rotor blades, need more power to fly, and can fly fast.
- To lift off the ground or gain altitude, the helicopter must produce more lift force than the force of gravity pulling it down.
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.
are very basic examples of lift theory and how it applies to RC helicopter lift. As we examine more areas of flight theory in the
Direction And Cyclic Control
section, you will see there are many other forces acting upon the
helicopter that cause the amount of lift being produced by the rotors to
be changing constantly.
This is why there is no such thing as a
simple “up-hover-down” RC helicopter lift button on your radio or in a
real helicopter – the dynamics are always changing and you have to keep
adjusting the amount of lift produced to stay in control.
Collective Pitch Lift Control Makes Precise Hovering Of This Beastie Possible.