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IN DEFENSE OF OUR SPORT
Question:
Why do we have a 200 mph
limit on turbine powered model airplanes?
Answer:
Because it is recognized by
knowledgeable and experienced people in the A.M.A. and JPO (Jet Pilots
Organization) that the introduction of gas turbine engines into model
aircraft facilitated speeds previously unobtainable.
The exhaust gas velocity of
some model jet engines can exceed 1000 mph and the thrust to weight ratios
can easily exceed 1:1.
NOTE: Other
model airplane propulsion systems do not have the potential for excessive
speed and therefore are not speed restricted.
For sure, some model jet airframes can exceed 200 mph and
survive, and a specifically designed model aircraft could yield speeds in
the 300 to 400 mph range, but common sense and experience tells us to
operate within our A.M.A. Turbine Waiver limits.
The Jet Pilots Organization, along with the A.M.A. safety
committee established the 200 mph limit for turbine model operation in 2000
for all of the reasons stated within this informational document. The
physics and facts have not changed since then.
Recently, internet posts and jet event reports have surfaced
confirming that a few individuals do not respect the A.M.A. regulations or
are ignorant as to why they were adopted and in force today.
Additionally, these individuals do not possess the engineering
background to fully comprehend the forces that the model is subjected to as
the velocity and “G’s” increase. See ENCLOSURE І: TECHNICAL INFORMATION
later in this document.
Perhaps these individuals don’t realize that we live in a post
9/11 era and that our sporting activity is under scrutiny by agencies of
local, state, and federal governments. The turbine powered model airplane is
of particular interest to the entities.
HOW TO DEFEND OUR SPORT
A model airplane is
defined by the A.M.A. as a line of site, non-autonomous vehicle. In the case
of a gas turbine powered model, its maximum velocity is restricted to 200
mph. We can best defend our sport by self policing and abiding by this
definition and other restrictions defined in the A.M.A. Turbine Waiver
document (see below). We must not give any indication or examples to
agencies of higher authority that the model airplane crosses these lines.
Unfortunately, some
individuals have already violated the definition of the model jet and have
bragged about it on the internet. Some damage has been done because actions
do have consequences. Is it fair that our sport should be put in jeopardy
for the purpose of self promotion of a few individuals?
The model industry does
not have, nor could we afford, a certifying bureaucracy such as the F.A.A.
Should we attract further attention of this or similar governmental
agencies, we are finished as a sporting entity.
If you enjoy this sport
as much as I have since its inception, you might consider being more
proactive in protecting it. This means, operating within our defined limits,
encouraging your fellow jet pilots to do so as well, and avoid
irresponsible, flaunting, internet posts.
I am sure there will be
some critics of this plea to defend our sport, but I hope they will give
sufficient thought before they inflict more damage via the internet. Please
do not let opinions out speak the facts.
We have been a bit lucky
with our good safety record and freedom of operation. Let us not rely on
luck in the future.
Thank you for your
consideration,
Bob Violett
POST SCRIPT:
After showing this
document to a few other accomplished jet modelers, the question arose: Do
these individuals still have their A.M.A. Turbine Waivers?
Reference:
Please read:
A.M.A.
Turbine Waiver: www.modelaircraft.org/files/510-a.pdf
Technical Information
Enclosure І
Speed Control Devices
ENCLOSURE І
TECHNICAL INFORMATION
FORCES ON AN AIRFRAME
The dynamic pressure on an airframe is defined as the maximum
pressure force available in a flow field relative to the true airspeed.
These forces are defined by the equation:
q = 1/2
σ V ^2 with "σ"
defined as air density on a vehicle and “V” as the velocity in ft/sec.
An example of force (pressure) acting on the forward facing
surfaces of a typical model jet such as a BVM Ultra Bandit is:
25.2 lb/sq ft at 100 mph
56.7 lb/sq ft at 150 mph
100.8 lb/sq ft at 200 mph
157.5 lb/sq ft at 250 mph
226.8 lb/sq ft at 300 mph
308.8 lb/sq ft at 350 mph
The effect of speed and radius of the turn on “G” forces is:
F = (mv^2)/r where
“F” is force, “r” is radius, “m” is mass and “v” is velocity
As the true airspeed of an aircraft doubles from 150 mph to
300 mph, it experiences four times the “G-loading” when performing a simple
level turn of a similar radius. This means that even a large ¼ mile radius
(1320 foot) turn requires a hefty 4.56g “pull” at 300 mph under standard,
sea level conditions while it’s only 1.14g at 150 mph. Additionally, when
this turn radius is halved to 1/8 mile, the load factor doubles to 9.12g!
Thus it is easy to envision how more aggressive maneuvering required to keep
a 300 mph aircraft in comfortable visual range or to set up for speed runs
can easily impact huge aerodynamic loads on a structure.
Every control surface has a critical flutter speed dependant
on its area, weight, hinge moment, and electro/mechanical control system.
Every flying surface has a deflection resistance to a control
input that is velocity and “G” dependant.
Full scale aircraft are properly funded to afford professional
engineering and wind tunnel testing. This data and flight testing establish
the VNE (Velocity to Never Exceed) and maximum positive and negative “G”
limits. These vehicles are then safe to operate as published.
Model jets are sporting vehicles that do not have these
advantages nor are the assemblers, pilots, and maintenance crews similarly
trained.
For a model company to be able to honestly publish a V.N.E. of
300+ mph, the product would not be affordable as a sporting device. Model
airplane developers rely primarily on comparative and intuitive engineering;
therefore, the end product is just a model airplane with corresponding
performance limitations.
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