How Gear Combinations Affect the Revolutions of the Rotor Blade

Researched by Tanner C.





The purpose of this experiment was to find out how changing the size and combination of gears increases or decreases the number of revolutions of a helicopter rotor blade.

I became interested in this idea because my dad is a helicopter pilot.

The information gained from this experiment will help people better understand how gears work in our everyday life.

For my hypothesis I predict the large drive gear will turn the rotor blade more revolutions.

I based my hypothesis on the type of gears that are used in the engine and transmission of the helicopter that my father flies, a Bell Jet Ranger 206 BIII.

In order to accomplish this experiment I used a flat wood platform. Gear support bases attached to the platform with screws. I then used three different size gears (small, medium, and large) in combinations of three. (See data table)

The constants in this study were:
The size of the rotor blade
The size of gears
How many revolutions the crank handle was turned of the drive gear
 The angel of the rotor blade

The manipulated variable was:
 The different combinations of gears.

The responding variable was:
 The revolutions of the rotor blade

To measure the responding variable I counted the number of revolutions the rotor blade gear turned when I turned the drive gear five (5) turns.

6 Big gears* 16 teeth 11cm diameter
6 Medium gears* 12 teeth 8.5cm diameter
6 Small gears* 8 teeth 6cm diameter
3 Yellow handles
6 Big platforms 11.5cm wide
6 Medium platforms 9cm wide
6 Small platforms 6.5cm wide
1 Piece of wood 85cmx29.5cmx2cm
14 Screws #6 1.9 cm long 
2 Wood dowels 23cm
1 Wood dowel 25.5cm
3 Wood stir sticks 23cmx2.5cm
1 All purpose Goop glue
3 Nail brads 2.5cm
* Gears are made by Learning Resources.

1. First, I picked the three sizes of gears that are shown in the data table.
2. I placed the gears on their bases in three gear combinations.
3. I attached the crank handles to the drive gears.
4. Next, I turned the drive gear around five times slowly.
5. As I turned the drive gear I counted how many times the rotor blade gear went around.(see Appendix to make rotor blade)
6. I recorded that information in the data table in the space for that gear combination.
7. I kept repeating this procedure until all the combinations were completed.

The original purpose of this experiment was to see which combination of gears would turn the rotor blade gear around the most in five (5) turns of the drive gear. Each gear group had a drive gear (with handle) and a rotor blade gear.

The results of the experiment were that the large to small gear combination made the rotor blade gear turn the fastest (10 revolutions). The medium to small gear combination came in second with the rotor blade gear turning 7.5 revolutions. The large to medium gear combination came in third with the rotor blade gear turning 6.75 revolutions. The slowest gear combination was the small to large gear combination with the rotor blade gear turning only 2.5 revolutions.

See the data table and graph.

Small Small 5
Small Medium 3.3
Small Large 2.5
Medium Small 7.5
Medium Medium 5
Medium Large 3.75
Large Small 10
Large Medium 6.75
Large Large 5

My original hypothesis was to determine which combination of gears would make the rotor blade go slower or faster.

I based my hypothesis on information about the type of gears used in the helicopter that my dad flies. The helicopter’s transmission and engine use many different types of gears to help deliver power to the rotor blades.

What I have learned from this experiment is if a person wants to increase the speed or revolutions of a gear or blade he/she would use a larger drive gear with a smaller gear. If a person wants to decrease the speed, he/she would just reverse the gear combinations.

The results indicate that my hypothesis was correct.
With the results of this experiment, I wonder if the same thing could be done using worm gears and bevel gears.

If I were to conduct this project again, I would use steel gears instead of plastic gears because the gears would run smoother. I would also use a motor to run a drive gear instead of a hand crank.


Research Report

Gears are used in every day machines. Gears turn on shafts that mesh with other gears that turn other drive shafts. Gears have cogs, which are teeth. Some cogs are slanted or straight. There are four types of gears. They are spur gears, rack and pinion gears, beveled gears, and worm gears. Gears mesh together to rotate wheels, shafts, blades, and other things. Gears are used to increase or decrease the speed of drive shafts, which are measured in revolutions per minute or Rpm’s. When a gear is turned one direction the other gear that it is in contact with will rotate in the opposite direction.

Spur Gears

Spur gears are a common type of gear. Spur gears are a loud gear because there is more surface area touching when they mesh. Spur gears are used with sun gears and planet type gears and work like a clock. When the sun gear turns, the smaller planetary gears rotate around the sun gear. Helicopter transmissions use this type of gear combination as well as other types. The bigger the drive gear to the smaller rotor gear the faster the rotor blade will turn. (Please see diagrams in appendix.)
 Beveled Gears

Beveled gears are used for changing the direction of shafts that drive the wheels of cars. In helicopters the engine drive shaft enters the main rotor blade transmission horizontally then the transmission changes the rotor blade shaft 90° vertically. Beveled gears are slanted so they can mesh with other gears when a change in direction is needed. This type of gear changes the direction by 90°. 

Rack and Pinion Gears

Rack and Pinion gears are used on cars and other equipped with steering systems. The rack has cogs or teeth on a long rod or bar. The rack moves back and forth when the pinion gear rotates right or left in a stationary position. For example, a steering wheel in a car is attached to the pinion gear and the rack is attached to the wheels to allow the car to turn right or left.

Worm Gears

Worm Gears are like a screw.  The main shaft is a screw, which turns a spur gear. This type is used on guitars to adjust the string tension. The worm gear repeats its job over and over.
 Cranking Action

Cranking action is a force that transfers power from one shaft to another shaft. This action is completed by the use of gears, which can change the direction or the speed of a shaft.

Revolutions Per Minute

Revolutions per minute (rpm) stands for the number of times an object such as a wheel turns in a minute. It is usually used to measure how much power or how fast something is turning. It is used to measure the speed of a shaft, which is turned by the gears attached to it.


There are many types of gears. There are cogs or teeth in every type of gear. All gears do something to help mankind. Gears are used in almost anything mechanical. With out the use of gears our every day lives would not be as easy or simple.


Marshall, Brain, Plant Engineering, HOW GEARS WORK Oct. 29 2002 <>

Glover, David, MAKE IT WORK MACHINES, THE HANDS ON APPROACH TO SCIENCE New York Thomson Learning pages 18 & 19

Lafferty, Peter, FORCE & MOTION DK 1992 pages 20 & 21

Macaulay, Dave, THE NEW WAY THINGS WORK Houghton Mifflin Company Boston 1998 page 37

Clarke, Mark, Personal Interview, December 1, 2002



My interview with Mark Clarke

David Mark Clarke is 50 years old. He flies for a fruit growing company in Yakima. He first started flying 20 years ago in 1983 when he took his first helicopter lessons. From 1983 to 1987 he work towards his helicopter ratings. He went to flight school to get his private license, commercial license, flight instructor’s certificate, and instrument rating. For every hour of flight instruction a person needs 2 to 3 hours of ground instruction. He worked around helicopters fighting forest fires in the middle 1970s. From 1979 to 1981 he worked around helicopters that helped build chair lifts at a ski resort in Colorado. He likes flying because it is challenging, it’s something different to do as a job, and enjoys the independent feeling. He has flown these types of aircraft: Bell JetRanger 206B/L, Robinson R-22, Boeing Vertol BV 107 a tandem rotor helicopter, and Astar 350, Hiller 12E, and Cessna airplanes 152 and 172 series. He has over 4500 hours flying aircraft.

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