× Welcome to the CPL Aerodynamics question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

## Maximum still air range for turbocharged engine

• scanwinder
• Topic Author

### scanwinder created the topic: Maximum still air range for turbocharged engine

Hi,

Just going through the aerodynamics syllabus and saw the following point:

6.7.2 List/identify aerodynamic and engine considerations which are required to achieve maximum still air range and endurance when operating aeroplane with a:
(a) normally aspirated engine
(b) turbocharged/supercharged engine

For the normally aspirated engine I know the maximum still air range should be when flying at the minimum drag speed and endurance is the speed for minimum power. Is this any different in a turbocharged/supercharged engine? I'm assuming it isn't but, the way the syllabus point is written, it sounds like it is, so it would be good to get clarification if anyone knows for sure.

Would the effect that turbocharging/supercharging has on engine efficiency change these speeds?

• Posts: 2311

### bobtait replied the topic: Maximum still air range for turbocharged engine

There are two things to consider for achieving the theoretical maximum still air range.

The airframe: as far as the airframe is concerned, maximum range will be achieved if the aircraft is flown at the IAS for minimum drag [best lift/drag ratio]. That means it is also flying at the best lift/drag ratio angle of attack [about 4° for conventional wings]. That remains true no matter what the height is. An aircraft flying at the best lift/drag ratio is enjoying it's maximum efficiency i.e. it is passing through the air with the least effort. It should be remembered that IAS and angle of attack are the only variables that the pilot has control over in level flight. Since, at any moment in level flight lift must equal weight, it follows that any particular angle of attack, say 4°, there will always be one particular IAS no matter what the height may be.

The engine: considering only the engine, even if it wasn't in an aeroplane, the maximum efficiency is achieved when the throttle is wide open - maximum volumetric efficiency. However the actual power produced at full throttle and therefore the resulting IAS, will vary with height.

When we put the airframe and the engine together, theory tells us that we should fly at the height where full throttle [now the engine is happy] produces the IAS for best lift/drag ratio [now the airframe is happy]. In practice, of course, that would be quite ridiculous. Imagine flying a Cessna 172 at a height where full throttle resulted in an IAS of 65 knots! That would probably be near 20,000 feet. What about the fuel used to get to that height and the possibility of very strong winds at that height.

You will find that Kermode's 'Mechanics of Flight' Page 175 supports this view. As far as turbo/supercharged engines are concerned, as far as I know, the same argument would apply and the height required would be much greater. I would be interested in hearing from anybody who has further information on turbo/supercharged engine requirements in flying for range.