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#1
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Equal Power (not!)
To All:
When two identical engines are turning at the same rpm it does NOT mean they are producing the same amount of power. It is the LOAD on the engine that determines how much power is being produced at a particular rpm, not the rpm itself. For an automobile engine converted for flight the handiest 'load-meter' is a simple vacuum gauge attached to the intake manifold. Converted car engines, from the Volkswagen to the Reo (yes, 'Reo' with an R) do not have a set of factory specs telling you how well they perform as an AIRCRAFT powerplant. Fortunately, an engine can't lie. Attach a few instruments, determine your local atmosphere and let her rip -- the engine will tell you how much power it can produce when fitted with a given prop. ------------------------------------- In the same vein, be damn careful about DEFINING the prop. Most home-builders have the idea that when it comes to props, diameter and pitch is all you need to know. Alas, with only those two factors you can miss by a mile. In addition to its diameter and the geometrical pitch of its blades, in order to define the LOAD a particular prop will impose you need to know the weight of the prop, the total area of the blades, and how that area is distributed relative to the prop's diameter. Lemme give you an example: You can take a "54x42" prop that needs a 140cid engine to spin it 3200 rpm and shave that puppy down to a tooth-pick that will turn 3200rpm on the nose of a stock (96cid) VW engine. But it won't fly the plane, a fact you'll discover the instant you're out of ground effect. ----------------------------------------------------------------------------- It's really all about thrust, as in the Production Of. But folks tend to hear what they WANT to hear, whereas the engine, the prop and the airframe can only respond to the reality of physics, mechanics and aerodynamics. Vast Mystery, of course. Or at least, half-vast. -R.S.Hoover |
#2
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Equal Power (not!)
To All:
When two identical engines are turning at the same rpm it does NOT mean they are producing the same amount of power. It is the LOAD on the engine that determines how much power is being produced at a particular rpm, not the rpm itself. For an automobile engine converted for flight the handiest 'load-meter' is a simple vacuum gauge attached to the intake manifold. Converted car engines, from the Volkswagen to the Reo (yes, 'Reo' with an R) do not have a set of factory specs telling you how well they perform as an AIRCRAFT powerplant. Fortunately, an engine can't lie. Attach a few instruments, determine your local atmosphere and let her rip -- the engine will tell you how much power it can produce when fitted with a given prop. ------------------------------------- In the same vein, be damn careful about DEFINING the prop. Most home-builders have the idea that when it comes to props, diameter and pitch is all you need to know. Alas, with only those two factors you can miss by a mile. In addition to its diameter and the geometrical pitch of its blades, in order to define the LOAD a particular prop will impose you need to know the weight of the prop, the total area of the blades, and how that area is distributed relative to the prop's diameter. Lemme give you an example: You can take a "54x42" prop that needs a 140cid engine to spin it 3200 rpm and shave that puppy down to a tooth-pick that will turn 3200rpm on the nose of a stock (96cid) VW engine. But it won't fly the plane, a fact you'll discover the instant you're out of ground effect. -------------------------------------------------------------------------- --- It's really all about thrust, as in the Production Of. But folks tend to hear what they WANT to hear, whereas the engine, the prop and the airframe can only respond to the reality of physics, mechanics and aerodynamics. Vast Mystery, of course. Or at least, half-vast. -R.S.Hoover I am not exactly how to define "weight of the prop", but the rest need to be said and repeated. Thanks for posting. Peter |
#3
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Equal Power (not!)
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