Speed of Delta Dart Indoors
An interesting activity might be to calculate the average speed of your Delta Dart as it flies in a circular path indoors. Outdoors calculations would be difficult with any breeze at all. Knowing the speed or "velocity" of your model is needed for other calculations such as lift or kinetic energy. I have supplied calculations for the same procedure using a two-cell electric plane that would most likely fly slower than the Delta Dart / AMA Cub.
The formula for velocity is very simple, just think about how we express the speed of an automobile; example: the car was traveling 55 miles per hour. Breaking this down we have the distance,55 miles and the time, one hour. From this we can conclude that:
Velocity = Distance / Time.
No matter how well we have our Delta Dart trimmed, it will Not fly for one hour indoors. Do you know how we can figure velocity for a shorter time period? Yes, we measure the distance it flies during part of a flight, and the corresponding time it takes to fly that distance. This will give velocity in feet per second, for miles per hour; further conversions are needed, which I will explain later.
Measuring the Distance of One Circle of Flight Path
The distance around a circle is known as the "circumference".
Using simple geometry and measuring across the circle of the flight path (the diameter), we can calculate the circumference. Measure the diameter by marking the floor under the flight path and using a tape measure. It might be easier if you have a helper stand on the opposite side of the circle and watch the plane passing directly overhead to determine the edge of the circle, mark the spot, masking tape works well. You mark your side as well, then measure across the circle, I used a tape measure.
Marking Diameter of Circle Formula for Circumference of Circle
Computing Circumference of the Circle, and What is PI?
The wavy symbol with two vertical lines is the symbol for "PI". PI is the ratio of circumference to diameter which is a constant that is close to 3.14. PI is the same no matter how large the diameter of the circle.
Diameter of Example Flight Circle: 28 feet
Circumference:
87.92 feet = 28 feet x 3.14
(Distance)
Timing the Flight
The flight can be timed by using a stopwatch and picking a point on the circle directly in front of you. When the plane passes by the point, click the stopwatch on, and just as it passes the same spot again, click the stopwatch off.
Time for Example Circle: 13.6 seconds
Using the Velocity Formula
We now have the two components needed for the velocity formula; distance and time.
Speed in Feet per Second: 6.46 feet per second = 87.92 feet / 13.6 seconds
To put our calculations into Miles Per Hour, additional steps are needed to convert the units of feet to miles and seconds to one hour.
Speed Per Hour
Converting seconds to one hour is easy if we know two things; how many seconds in one minute and minutes in one hour.
Using those facts we multiply 60 by 60.
Change seconds to an hour - 60 x 60 = 3600 seconds in one hour
60 seconds = 1 minute
60 minutes = 1 hour
Next we multiply the feet per second by 3600 to arrive at feet per hour.
Example Speed in Feet per Hour
Feet per hour: 23,256 = 6.46 feet x 3600 seconds
Change to miles
There is yet another step to get to the units we are looking for, converting feet to miles. We must know the number of feet in one mile, 5280 feet.
Dividing feet per hour by the number of feet in one mile should give us the answer we are looking for.
Example Speed in Miles per Hour
Miles Per Hour: 4.4 miles per hour = 23,256 feet / 5280 feet
Is the Result Reasonable?
When doing calculations it is a good idea to check for major errors by asking yourself if the answer is reasonable. If we know that the average human walks at around 3 mph, we can compare the speed of our plane to walking. Does our plane appear to fly at a speed slightly faster than a person walking? If the answer is yes, than our calculations were most likely done correctly.
Can you think of where inaccuracies might be?