To gain a basic understanding of the principles of propeller aerodynamics, and how they are used to create thrust for unmanned aircraft.
At the end of this briefing you will be able to:
- Describe the naming convention for propellers
- Discuss the meaning of the pitch of a propeller
- Discuss propeller pitch and performance
Propellers in Simple Terms
The aircraft propeller consists of two or more blades and a central hub to which the blades are attached.
Each blade of an aircraft propeller is essentially a rotating wing.
As a result of their construction, the propeller blades are like aerofoils and produce forces that create the thrust to pull, or push, the aircraft through the air.
The engine furnishes the power needed to rotate the propeller blades through the air at high speeds, and the propeller transforms the rotary power of the engine into forward thrust.
Propellers for Unmanned Aircraft
Pitch is the displacement a propeller makes in a complete spin of 360° degrees.
This means that if we have a propeller of 40” pitch it will advance 40 inches for every complete spin as long as this is made in a solid surface; in a liquid environment, the propeller will obviously slide with less displacement.
The pitch concept is not exclusive to propellers, other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn of the screwdriver.
Propeller Blade Angle
Fixed-pitch and ground-adjustable propellers are designed for best efficiency at one rotation and forward speed.
They are designed for a given aircraft and engine combination. Since the efficiency of any machine is the ratio of the useful power output to the actual power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. Propeller efficiency varies from 50 to 87 percent, depending on how much the propeller “slips.”
Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch.
Geometric pitch is the theoretical distance a propeller should advance in one revolution; effective pitch is the distance it actually advances.
The Twist on a Propeller
The reason a propeller is “twisted” is that the outer parts of the propeller blades, like all things that turn about a central point, travel faster than the portions near the hub.
If the blades had the same geometric pitch throughout their lengths, portions near the hub could have negative AOAs while the propeller tips would be stalled at cruise speed.
Propeller Pitch and Efficiency
The pitch of the propeller is generally chosen to provide the speed characteristic of the aircraft for the purpose required.
Increasing the blade pitch increases the blade drag, and decreasing the blade pitch decreases the blade drag.
A larger (coarser) blade angle, for a given RPM, will adsorb more power and require more torque to turn it at the requested RPM.
Usually 1° to 4° provides the most efficient lift/drag ratio, but in flight the propeller AOA of a fixed-pitch propeller varies — normally from 0° to 15°. This variation is caused by changes in the relative airstream, which in turn results from changes in aircraft speed. Thus, propeller AOA is the product of two motions: propeller rotation about its axis and its forward motion.
Propellers are designated by two numbers:
A propeller designated as a 12-6 propeller is therefore:
- 12″ in diameter
- 6″ of pitch.
…where pitch is the distance a propeller will move forward in one revolution in a perfect fluid (which air is not).
Theoretically a 6″ pitch will move forward 6″ with each complete (360°) revolution of the propeller.
How Pitch Affects Propulsion
The properties of a propeller with high pitch:
- High speed flight
- Poor Acceleration
- Poor Climb
- Can be difficult to slow down for landing
The properties of a propeller with low pitch:
- Low speed flight
- Good Acceleration
- Good Climb
- Finer speed control throughout throttle range – particularly at low throttle settings
Pitch in Simple Terms
An easy way grasp the concept of propeller pitch is to draw a parallel to the gearing in a car.
Low pitch propellers = low gear in your car.
It will get you up hills well but will not take you any where fast.
High pitch propellers = Beginning your drive in fifth gear.
It will take forever to accelerate to speed but the plane is cruising when it gets there.
An out of balance propeller can be the cause of a lot of problems. Some of these problems manifest as:
- Prevents the engine from developing full power.
- Causes excessive vibration through the airframe.
- Causes excessive vibration through on-board electronics, leading to premature failure.
- Causes fuel foaming which can cause the engine to run ‘lean’.
- The result is the engine loosing performance and power, to stall, or just not run smoothly.
This is all amplified in a smaller aircraft.
Before you attempt to balance a propeller, be sure to clean it.
Most propellers are close to being in balance when purchased, so they should only need a small amount of work to bring them into perfect balance.
If the propeller is severely out of balance – return it because too much material would have to be removed which would significantly change the shape of the blade.
If one blade is heavier than the other, then the usual method to bring the propeller into balance is to remove material from the heavy blade using sandpaper.
Trimming Heavy Propellers
Do not trim the tip of the heavy blade!
Although the blade may balance statically, it will not be balanced when it starts to spin, because of unequal mass distribution. Material is generally removed from the face (front) of the propeller or from the back of the propeller.
Generally all that is required is to sand the face a little.
Occasionally you may encounter a propeller that does not track properly. Either the mounting hole is off-centre or the hub is not square to the plane of rotation.
In either case, if the propeller is noticeably out of track you should not use it.
It is easy to see if the propeller is tracking correctly. Stand back for safety and look at the propeller from the side and from the rear.
From the side, both tips should be clearly visible in the same line. If you see two lines, then the hub is not square to the plane of rotation.
The recommended procedure is to return the propeller – it is defective, and may require too much modification to ‘repair’.
Selecting Motors & Propellers
Beware the “hobby mentality“
The propeller should be chosen to match the aircraft — not the engine. An appropriate engine should then be chosen!
Consider this. Mounting a racing propeller to a scale WWI aircraft will severely limit the model as an early warbird has so much airframe drag that the propeller will never be able to deliver it’s full potential, with the result that the aircraft will be a sluggish flyer at best.
Additionally, using too ‘slow’ a propeller – one with low pitch – on a model intended to go fast may prevent the aircraft from gaining enough speed to fly at all. A basic mistake is finding a propeller that works great – with a certain engine in a certain aircraft – and then imposing that propeller on that engine regardless of the aircraft!
Number of Blades
With ‘model’ sized aircraft the most efficient propellers are two bladed. Because the diameter of our propellers tend to be small, multiple blade propellers disturb the air that the trailing blade is entering tending to make them less efficient.
Generally, with smaller aircraft, for best overall performance, it is recommended to utilise 2-blade propellers.
Matching Propeller and Motor
The Reality of Motor and Propeller Selection
Test & Verification Procedure (Actual procedure – V-TOL Aerospace):
- The propeller was fitted to the motor, which was mounted on the scales to provide down thrust, pushing against the scales.
- The current meter was setup in line with the battery and Electronic Speed Controller to measure current drawn by the motor.
- The servo checker was setup to control the motor through the ESC.
- A camera was mounted such that it could see the current meter and the display on the scales.
- For safety the video display and servo checker were operated from behind a safety barrier.
Given the test data, and noting that cruise speed for the condor aircraft is attained at 8A/100W with a 9 x 4.5 propeller, it can be seen that the most efficient propeller to produce the required amount of thrust is the 9 x 4.5 propeller.
Hazards of Propellers
Safety must always be first and foremost in your thoughts when handling unmanned aircraft, or components from unmanned aircraft.
- Propellers spin at relatively high speed.
- Propellers are made from hard material.
This is why should we pay special attention to the propeller when completing an inspection of an aircraft!
What happens when things go wrong?
All that stopped the blade segment from passing all the way through the partition was the fabric on the other side!
You should now be able to:
- Describe the naming convention for propellers.
- Discuss the meaning of the pitch of a propeller.
- Discuss propeller pitch and performance.