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 Gs increase. See ENCLOSURE І: TECHNICAL INFORMATION later in this document.

Perhaps these individuals dont 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 mile radius (1320 foot) turn requires a hefty 4.56g pull at 300 mph under standard, sea level conditions while its 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.

Contact Us
All graphics, photos, and text Copyright 2011 BVM, Inc.
Use of graphics or photos without written permission from BVM is strictly prohibited.