Phase 2: Airframe Design
A Note to the Media
If you represent a broadcaster, news service or publisher
who is interested in obtaining a scoop on this project,
its construction, testing/deployment and the frightening implications
it represents then please contact me to discuss
obtaining exclusivity.
I am presently keeping a video diary of the project's progress
and this can be supplied in mini-DV or MPEG2 format on disk
or tape.
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Designing a suitable airframe for the LCCM project has necessitated a sea of
compromises.
Unlike an expensive Raytheon Tomahawk, the DIY LCCM won't have a solid-rocket
booster to launch it into the air and get it rapidly up to the optimum flying
speed of several hundred miles per hour. Instead, it will be catapulted from
the back of a speeding pickup truck so will have to start flying at a mere
70mph or so.
What's worse, at launch, the craft will be carrying a full load of fuel,
adding significantly to the amount of lift needed to keep it in the air.
Of course it would be nice if we could just use our own solid-rocket booster
but that would significantly complicate the whole project. The design of
reliable, effective solid rocket boosters is not a trivial affair and often
requires the use of materials that aren't readily available "off the shelf".
So, we're stuck with a relatively low-speed launch mechanism and this makes
the task of designing a suitable airframe a whole lot more difficult. That's
because, due to the laws of aerodynamics, there are some significant
differences in design between a craft designed to fly efficiently at 70mph
and one designed to do so at 400mph.
Aircraft intended for high-speed flight are designed to create the absolute
minimum disturbance (drag) as they slice through the air. This design
objective generally manifests itself as a long slender body and relatively
small broad yet thin tapered wings that have just enough area to provide the
lift needed to keep the craft airborne at the optimum flying speed.
By comparison, an aircraft designed to fly most efficiently at low speeds
tends to have larger, thicker and more cambered wings for any given weight in
order to generate plenty of lift at those lower speeds.
Just compare a high-speed fighter plane such as the rather extreme F-104A
Starfighter to something like a modern microlight designed for much slower
flight. Note the remarkable similarities between the F-104A and the world's
first cruise missile, the V-1 Flying Bomb.
The problem with big, thick, cambered wings is that although they work well
at low speeds, they generate so much drag and excess lift as velocity
increases, that they make it very difficult to obtain the high speeds we're
seeking with our LCCM.
As you can see - this isn't going to be simple and no matter how much thought
is applied, our LCCM is going to be a set of aerodynamic compromises.
If you're unfamiliar with the basics of aerodynamics then this may be a bit
of a learning exercise.
You'll see why when we analyze the goals:
- a very low drag to enable maximum flight speed and range
- a comparatively low stall-speed to allow the use of a simple launcher
- a good level of inherent stability across the entire flight envelope
- a form that lends itself to simple construction techniques
- a form that allows the use of a pulsejet engine which radiates significant heat
Let's take an overview-type look at the factors that will affect our design
choices. Note that in order to make this material more understandable by
those without much prior knowledge, I've (over)simplified things a little,
used analogies, and avoided the use of complex math wherever possible.
1. Drag
Drag is one of the four key forces that act on any aircraft in flight
(the others are gravity, lift and thrust).
Read more....
2. Low Stall Speed
Since the DIY LCCM won't have a rocket-booster or be dropped from an airplane
already moving at several hundred miles per hour, it will have to tollerate
being launched at less than one quarter it's designed flying speed.
More to be uploaded shortly
3. Inherent Stability
Without some degree of inherent stability, our LCCM would be solely reliant
on the guidance system to correct every little change in attitude or direction
that might be caused by turbulence or other factors. To simplify the
guidance system, the craft must be designed so that the aerodynamic layout
provides a basic level of inbuilt stability.
More to be uploaded shortly
4. Simplicity Of Construction
In accordance with the projects "DIY" title, the construction techniques used
for the airframe must be relatively simple and not reliant on complex or
expensive tools and materials. This goal is met by using modern composite
materials based on foam and fiberglass.
More to be uploaded shortly
4. Form Considerations
Bearing in mind all the other design factors previously discussed, the
airframe must be designed to take into account several other factors --
not the least of which is the very high operating temperature of a
pulsejet engine.
More to be uploaded shortly
Final Decisions
Accepting that the LCCM's aerodynamic design will be a mix of compromises,
the following choices were made which, I believe, are the most sensible
balance of factors:
- While attention will be paid to the requirements of low-speed, low-drag
flight, more emphasis will be given to the dictates of the high-speed
cruise-mode. As you will see when the issue of the engine design is covered,
a simple but effective method of temporarily doubling the engine's thrust at
low speeds will be used to help overcome the high drag intrinsic to the
slow-speed flight of such a craft.
- Obvious enhancements that represent very little compromise (such as
wingtip fins) will be used to reduce drag.
- Wing-loading will be kept as low as possible, contingent on maintaining
structural integrity and minimal form-drag.
- A low-drag laminar flow airfoil section will be used.
CAD drawings of the resulting airframe design will be posted in the next few
days, along with the calculations used to determine key aspects of the
design.