**JayBird
Design Page**

Page
3

>>>>
Go to Page [1] [2]
[3] [4]
[Glossary] <<<<

Click
Here to download the Volo View Viewer. This viewer is essential
for you to be able to view the drawings of the JayBird airplane.

**I****MPORTANT:**
All drawings, photos, and information presented on these web
pages are the sole property of Jay S. McMullan and are ©
2002. Nothing on these pages can be copied or reproduced in
any way without the written, expressed permission of Jay S.
McMullan and/or his assigness.

**Calculating
Tail Volume Coefficients**

Now
that the fuselage and wing have been "sized" for my
airplane, I have to calculate the **volume
coefficients** for the horizontal and vertical tails.
I have sketched them out as seen on the previous page but now
I have to make sure they are the correct size. As with all the
steps in designing an airplane, there are trade-offs. I can
make the tail surfaces very large and they will give great stability
and control but on the down-side, the control surfaces may be
very hard to move and the airplane may be very hard to control
directionally in a strong crosswind. Using the software that
comes with "Preliminary Design - Modern Aircraft Design
for the Non-Engineer" I will plug in the required data
and the size of the tail surfaces I have drawn. The software
will then give me the **volume
coefficients** of the tail surfaces as they are drawn.
You will find out that airplane design is 90% math. This software
greatly simplifies all of the calculations that would have to
be done over and over. If I had not already drawn the tail surfaces,
the software will tell me what the coefficients should be and
I could draw the tail surfaces accordingly. So the software
gives me two ways to go about determining the calculations we
need. "Preliminary Design" gives more instruction
and examples on how to go about determining the **volume
coefficients**.

Using
the software with "Preliminary Design - Modern Aircraft
Design for the Non-Engineer" makes calculating volume coefficients
easy.

The
software asks for a "**moment
arm**". This measurement is taken from the **center
of gravity** of the aircraft to the center of the
tail surfaces. The **moment
arm** is a distance from a point at which the airplane
rolls or pitches. In our case it is 156 inches. An airplane
with a longer **moment
arm** will require smaller tail surfaces and an airplane
with a shorter **moment
arm** will require larger tail surfaces. A longer
**moment arm**
will also provide more longitudinal and directional stability
in the aircraft. The reason this works is that a longer **
moment arm**,
when moved, gives more torque at the point of rotation. Mechanics
understand this principle. If they have a bolt that is too tight
for them to remove with a normal wrench, they can add a piece
of pipe, sometimes called a cheater, to the end of the wrench
and it makes it easier for them to remove the bolt. By using
the same amount of force that would not remove the bolt with
the wrench, the added piece of pipe allows the bolt to be easily
removed.

The
horizontal tail **volume
coefficients** for light airplanes ranges between
0.3 and 0.7. The drawings I have made have a horizontal tail
**volume coefficient**
of 0.55. The vertical tail **volume
coefficients** on most light airplanes range from
0.02 and 0.05. Mine comes in at 0.04. The horizontal and vertical
tail ** volume
coefficients** both fit suitably well without having
to make any changes. If they did not, I would need to scale
the drawings of the tail surfaces up or down, depending on the
size I am looking for.

The tail
moment arm determines the size of the airplane's tail surfaces.
Directional and longitudinal stablility are affected by the
length of the moment arm also.

Next,
we will check the aspect ratio of our horizontal and vertail
tails and then we will size the **rudder**
and **elevator**.
As in the main wing of an airplane, a higher aspect ratio is
desired for the tail surfaces of the airplane. The **horizontal
tail**, as drawn, has an area of 19.7 square feet
and a span of 9' 7 1/6'. Using our formula, I get an aspect
ratio of 4.66. The **vertical tail's** aspect ratio
is 2.17.

[Previous
Page] [Next
Page]