Improving Performance
The
Wright Brothers built and tested several gliders before attempting powered
flight.
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Improving Performance
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December 17, 1903 first successful powered flight for the Wright brothers. Covered only 120 feet and lasted 12 seconds. This flight was the result of years of experiments, calculations, and improvements. |
Introduction See Video Clip of my Wright Flyer Electric RC Model Flying - 4.3 mb mpeg
The most important steps you can take in improving the performance of a plane such as the Delta Dart, revolve around reducing the weight of the plane. Performance of the model is dependent on the wing loading, which can be calculated mathematically. Before the Wright Brothers made their first powered flight they have done endless calculations and experiments to improve the efficiency of the lift of the wings of their aircraft. Like your rubber powered plane, their airplane had little power to spare. On larger aircraft, the airfoil of the wing can make a big difference, so the Wright's spent considerable effort testing different airfoils. On a small plane such as the Delta Dart, a flat wing is fairly efficient, more important is the wing loading.
Material presented here will demonstrate how to calculate wing loading, how to reduce the weight of your plane, and how to record important data. Also presented, is a slightly modified Delta Dart of much lighter construction.
Figuring Wing Area - First step to calculating Wing Loading
Comparison of Standard Dart With Light Weight Dart
What is so Bad About a Heavy Airplane
Working With and Handling Lightweight Structures
Figuring the wing area is the first step in computing the wing loading. To compute the wing area of the Delta Dart, each wing half is a triangle. Therefore the following formula can be used:
Area of Triangle = 1/2 Base x Height
Area of Triangle = .5 x 6.25" x 6.25" Area of Triangle = 19.53 square inches
Total Wing Area = 2 x 19.53 Total Wing Area = 39.06 square inches
One of the most important factors in the performance of a rubber powered airplane is the wing loading.
Wing Loading = Weight / Wing Area
Lew Gitlow in his book, "Indoor Flying Models" stated "the Power required for a model's flight is equal to its weight times its sinking speed." This means a lighter plane will require less power to keep it in flight. With smaller power requirements, smaller rubber can be used, which means more turns can be put in the rubber.
You might of figured out that increasing the wing area would also reduce the wing loading. Increasing wing area will also increase drag, which might not be desirable.
Calculating Wing Loading
For small rubber models it is most common to work in metric unit, grams
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28.35 grams in one ounce |
Don Ross in his book "Rubber Powered Model Airplanes", uses .33 grams per square inch as a good wing loading for a small model.
A typical Delta Dart might weigh 10 grams with rubber:
10 grams / 39 square inches = .256 grams per square inch
Your 10 gram Delta Dart should fly well at this wing loading, and it does!
Designing a Lightweight Delta Dart
To test the theory that reducing the wing loading, will increase flight times and reduce power requirements, I built a lighter Delta Dart. Almost the entire structure was built of 1/16" square balsa. Covering was condenser paper. As the tail structure was much lighter, the tail moment was lengthened 1/2".
Comparison of Standard Dart With Light Weight Dart
10 grams / 39 square inches = .256 grams per square inch Flight time 35 seconds
7 grams / 39 square inches = .180 grams per square inch Flight time 75 seconds
10 - 7 = 3 3 / 10 = 30% reduction in weight
75 - 35 = 40 40 / 35 = 114% increase in flight time
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| Dart surfaces made from 1/16" balsa - truss structure adds strength | Light weight Dart weighs 6 grams without rubber. |
Light weight Delta Dart in
flight.
Flight Log of First Session for the Lightweight Dart
The 1/16" square structure proved rather fragile. On the first flight the plane went to the right instead of left as I had expected, hitting a wall. Result was a break in three places on the right wing panel.
Even with a longer tail moment, the plane proved to be slightly nose heavy. The plane would climb but was racing around much too fast. Cut off 1/4" from nose and added small amount of clay to tail.
Started with 3/32"rubber, which is .093". This proved to be more power than needed and the plane was quickly bumping the ceiling and using up turns too fast. Changed to .080" rubber which proved to be closer to ideal.
What is so Bad About a Heavy Airplane
Tips for Reducing Weight (Of Your Model)
Lighter covering material - covering can be a substantial portion of the total weight. Refer to Building Tips section for discussion on Japanese tissue Japanese tissue would probably be the lightest covering material you would want to use on the Delta Dart, especially if it is to be flown outside.
Scraping material from prop - material can be removed from the prop by carefully scraping the prop with a razor blade. This technique is used extensively with Science Olympiad models. Be sure to check the balance of the prop, both blades must weigh the same.
Selecting lighter wood, balsa wood is grouped by the weight of a cubic foot of balsa. Lightweight balsa would weigh in the range of 4 to 6 pounds per cubic foot. Lighter weight balsa would be more expensive because there is a limited amount of it.
Rounding
the edges of balsa and cutting material from motor stick – see Building Tips
Build model so no extra weight is needed for balance. If you can build your plane so that little extra weight is needed to get the CG in the proper place, that is less total weight. This is can be more difficult than it would seem as material weight varies and two identical planes can balance differently. Center of Gravity would also vary for flying indoors or outdoors.
Do not use anymore glue than necessary. It is important to use adequate glue on all joints, glue dripped outside of the joints, only adds weight.
Note: with lighter plane, less rubber is needed, this means less total weight as rubber adds to significantly to total weight
Working With and Handling Lightweight Structures
As
you build lighter planes, these planes will be increasingly fragile and
breaking the plane while handling it is a very real possibility. It takes
practice handling lightweight planes, and the experience of breaking a few
planes. With practice you will learn where to grab the plane, and how much force
you can apply without breaking the plane.
Common sense, should tell you to pick up your plane where the wood is
the heaviest. It is easy to forget and pick up the plane quickly by a wing tip,
crunching the wood.
Building
your plane, it becomes increasingly easy to break the delicate balsa wood as
the wood size decreases. Extreme care
should be used while sanding or cutting.
Storage and Transportation of Your Model
Many
model airplanes have been broken while either storing the model or in the process
of getting it to the flying site. Find a cardboard box and label what is inside
the box, otherwise the box can be thrown away, thought to be empty. Family pets have destroyed many models, as
have people sitting on planes that were left on chairs.
To get an accurate indicator on the flight performance of your plane, flights must be timed and the information recorded. Data that you might record could be total flight time, rubber length and width, and number of rubber turns. Additionally, trim adjustments such as CG changes, trim tabs, etc. should be recorded also. A small notebook and pencil should always be in your field box.
Stopwatches - the easiest and most accurate method for timing your flights would be by using a stopwatch. With a stopwatch it is easy to start from zero at anytime.
Desirable
features; large display, large buttons, and buttons that start and stop
reliably every time.