lml@lancaironline.net Mailing List Archive

<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> Eric has incorrectly presented several basic concepts of Newtonian physics and algebra. These concepts are not difficult to understand but seem to be the source of endless confusion. As people who enjoy hurtling through space with kinetic and potential energies orders of magnitude larger than what is required to terminate our lives, we owe it to ourselves to understand the concepts of mass, force, gravity, work, and power. If you think I incorrectly state the laws of nature then I would invite you to dust off that High School Physics text and see for yourself. This is not a physics forum and I do not wish it to turn into one but these concepts are useful in aviation when discussing the dynamics of flight. If you are already versed in these matters (npi) or prefer bliss then please skip to the bottom. Eric writes: << V at or near zero invalidates the basic thrust equations of T = np * 550 * HP / V and... np = ( T * V ) / (HP * 550) because we would have to divide or multiply by zero. Gee, that means that np would be either infinite or zero. >>> Gee... NO, that means that as your velocity drops to zero np also drops to zero. So, at zero velocity you are expending all your energy and going nowhere. Your propulsive system has zero efficiency. I have no idea how you conclude that np becomes infinite as V reaches zero. <<In your "aircraft tied to a tree" scenario, useful work IS being done: A great deal of air is being accelerated from zero to some velocity.>> Work is being done but, in reference to the plane, it is not useful as neither the kinetic or potential energy of the plane is changing. If your objective is to push air around and ultimately produce heat then the propeller is 100% efficient at that task. <<Would you also say that a hovering aircraft is doing no "useful" work?>> Yes that is exactly right, no work is being done in relation to the aircraft or the ground. The fact that the aircraft's kinetic or potential energy is not changing proves this. All the work is going into thin air. <<Gravity is an acceleration, not a static force>> WRONG! Gravity, or gravitation is the intrinsic attraction between two lumps of matter. The acceleration due to the gravity present at the earth's surface varies slightly from place to place and varies with the mass of the attracted object. To simplify life the constant "g" was introduced that represents a fixed acceleration due to gravity of 32.174 ft/sec^2. <<When we are in level flight, we are continually accelerating "up" at 1 G.>> << To maintain 1 G level flight, we are accelerating the air flowing over the aircraft DOWN at some combination of mass and velocity capable of supporting 1 G flight of our current aircraft weight.>> You can't have it both ways. Correctly stated, in level flight you are accelerating mass (air) down to create a force that opposes the force of gravity. <<That's 32 fps/sec; NOT 32 fps(Change your equation to represent lb.f of force instead of lb. of weight and this might be clearer>> I don't see how, as a pound IS a force and weight IS force. The mass of an object is sometimes referred to as Lbm or "pounds mass" but the correct term for mass when using feet , pounds and seconds is Slugs. Important basic concepts in physics: Mass is an intrinsic property of matter. Force is what you have to apply to get matter to change its velocity. Gravity is the attraction two lumps of mass have for each other. Gravity is proportional to mass. We do not know what causes gravity or how it is transmitted. The effect of gravity is a force that draws the two bodies together. The force is proportional to the mass of the two bodies and the square of the distance between them. This force can result in an acceleration if it is not opposed by an opposite and equal force. Since the earth is a lot bigger then most of the stuff on it and we tend to stay near the surface we consider the terrestrial acceleration of gravity to be constant at 32.174 ft/sec^2 or 9.80665 M/sec^2. We refer to this constant with the term "g". A Pound is the force one Slug of mass exerts on the surface of the earth. AKA weight. Work is the application of force for a given displacement (Foot Pounds). If you are not moving, you are not doing work. Power is the application of work for a given period of time (foot pounds/second). And now for the mind bender: There is no such thing as centrifugal force. When a body revolves about an axis a force is needed to constantly deviate it towards the axis. This force is called centripetal force. The equal and opposite resistance offered by the body is called centrifugal force but it isn't a force at all. It is simply a manifestation of the body "wanting" to remain on a fixed tangential trajectory. What we call centrifugal force, the force pushing you down into your seat in a coordinated turn, is in fact the airplane pushing you up (centripetal force), changing your trajectory, accelerating you in a new direction. <<There are MANY dimensionless coefficients that typically come out to some fraction of 1; do we represent all of them as percentages???>> No, but they all CAN be. I am 99 & 44/100 % sure of that. Having books on the shelves is a good idea. A better idea is to read them but the best idea is to understand them and be able to correctly apply the knowledge they contain. Regards Brent Regan LML website: http://members.olsusa.com/mkaye/maillist.html LML Builders' Bookstore: http://www.buildersbooks.com/lancair Please remember that purchases from the Builders' Bookstore assist with the management of the LML. Please send your photos and drawings to marvkaye@olsusa.com.