## Science Projects with Toy Parachutes

By Dr. Jean Potvin

## Parks College Parachute Research Group

There are many interesting scientific experiments that can be done to study the physics of parachutes. These experiments involve dropping test parachutes of different designs in order to see their effects on the payload's descent speed. Such design modifications include:

• changing the payload weight
• lengthening or shortening the length of the suspension lines
• changing the number of suspension lines
• increasing or decreasing the radius of the parachute
• cutting holes and/or slits in the parachute fabric

#### Some theory:

By virtue of being blunt and large, parachutes generate a lot of air resistance or "drag". When attached to a freefalling payload, the large drag generated by a parachute helps in reducing the payload's fall speed by acting against the payload's weight. In general, parachute drag will increase with the radius of the chute and thus reduce the payload's speed. Also, drag will be reduced when holes or slits are cut in the canopy fabric, i.e., the more holes, the faster the descent. Finally, adding suspension lines will reduce the effective parachute surface area; in contrast, lengthening them will increase it somewhat. In terms of fall speeds, this means that adding suspension lines will increase the descent rate while lengthening them will decrease descent rate.

The descent speed of a payload attached to a parachute is calculated by a well-known formula discussed in another page on this web site, Calculating the Descent of a Round Parachute.

#### How to make small parachutes for science projects:

Toy parachutes can be purchased or made by using simple materials such as strings, plastic from garbage bags or nylon fabric from old (discarded!) tents. These pictures show some of the designs we have tested, a parachute without holes and a parachute with holes.

#### Other sources of small parachutes:

Hobby stores that sell model rockets usually sell small rocket recovery parachutes, made from both plastic for small rockets, and from fabric for the larger rockets. A parachute supply company called Para-Gear also sells a small parachute. Item: A3333, Name:Parachute Toy. Note: Before simply purchasing a parachute, make sure your research or science project allows you to do so! An important part of your project might be to make your own, which is easy, fun, and inexpensive anyway.

#### How to measure descent rates:

A typical experiment would consist in dropping different parachute designs and measure the descent speed of each design. The easiest way to measure descent speed is by suspending under the payload a long weighted line called a "plumb line", and by measuring the time between the landing of the plumb line weight and the landing of the payload. The length of the plumb line should be measured before or after the drops. The descent speed is simply given by dividing the plumb line length be the landing time interval of the payload and plumb line weight. An important note: the weight at the end of the plumb line should be small relative to that of the payload so to not change the desired descent speed of a given design. The weight is used to keep the plumb line straight during the descent.

#### Some final remarks:

* In general, chutes that do not have holes and slits cut in the fabric tend to oscillate wildly during their descent. This is due to the fact that the excess air caught by the descending canopy spills to one side when there is too much of it in the canopy. This sideways spill in turns pushes the canopy in the opposite direction to one side of the payload, thus generating the oscillation when the spill is repeated. One way to reduce the oscillations is to cut holes and slits that let some trapped air through. There are all sorts of hole and slit patterns that can be cut as shown in an article by Dr. C. W. Peterson that was published in Physics Today, August 1993 (the magazine Physics Today can be found at university and college libraries).

* Tests chutes may be covering some horizontal distance while descending. This may be due to the presence of winds, and/or to the parachute being unevenly trimmed. Uneven trim arises whenever the suspension lines are not of the same length or when the holes/slits are not of the same size or located symmetrically with respect to the parachute's center.

In the event that the chutes go forward, a proper measurement of the descent rate should involve the extra measurement of the horizontal distance covered. This is easily done by measuring the horizontal distance between the drop point and the landing point of the payload.

Then the descent speed is given by:

```
travel distance
v =  ------------------------
landing time interval

where

(travel distance)^2  = (horizontal distance)^2  +  (plumb line length)^2
```