Things you always wanted to know the answer to [Vol. 3]
Discussion
AstonZagato said:
On the fuel consumption theme, does a helicopter use more fuel traveling forward in level flight than when in the hover?
I assume that forward travel has drag and that the body provides negligible aerodynamic lift so I assume that the hover is more fuel efficient (in terms of gallons per hour rather than miles per gallon) as the lift requirement would be the same. However I couldn't work out if the forward motion somehow made the blades more efficient.
Don't the blades provide lift when moving forwards too? As in autogyros that have non-powered blades. I always thought hovering was the least efficient way of staying in the air.I assume that forward travel has drag and that the body provides negligible aerodynamic lift so I assume that the hover is more fuel efficient (in terms of gallons per hour rather than miles per gallon) as the lift requirement would be the same. However I couldn't work out if the forward motion somehow made the blades more efficient.
Shaolin said:
AstonZagato said:
On the fuel consumption theme, does a helicopter use more fuel traveling forward in level flight than when in the hover?
I assume that forward travel has drag and that the body provides negligible aerodynamic lift so I assume that the hover is more fuel efficient (in terms of gallons per hour rather than miles per gallon) as the lift requirement would be the same. However I couldn't work out if the forward motion somehow made the blades more efficient.
Don't the blades provide lift when moving forwards too? As in autogyros that have non-powered blades. I always thought hovering was the least efficient way of staying in the air.I assume that forward travel has drag and that the body provides negligible aerodynamic lift so I assume that the hover is more fuel efficient (in terms of gallons per hour rather than miles per gallon) as the lift requirement would be the same. However I couldn't work out if the forward motion somehow made the blades more efficient.
Bit of googling reveals:
"The efficiency of the hovering rotor system is improved with each knot of incoming wind gained by horizontal movement or surface wind. As the incoming wind enters the rotor system, turbulence and vortexes are left behind and the flow of air becomes more horizontal. All of these changes improve the efficiency of the rotor system and improve aircraft performance."
http://www.dynamicflight.com/aerodynamics/translat...
Can't find anything that gives relative figures, but there are suggestions that hovering uses twice as much power to stay in the air as forwards flight.
"The efficiency of the hovering rotor system is improved with each knot of incoming wind gained by horizontal movement or surface wind. As the incoming wind enters the rotor system, turbulence and vortexes are left behind and the flow of air becomes more horizontal. All of these changes improve the efficiency of the rotor system and improve aircraft performance."
http://www.dynamicflight.com/aerodynamics/translat...
Can't find anything that gives relative figures, but there are suggestions that hovering uses twice as much power to stay in the air as forwards flight.
Dr Jekyll said:
I don't see how an engine can be more efficient at 3000 rpm in one gear than another.
You really think your mpg would be the same at 3,000 rpm in 1st as it would be in 6th??The whole point is that while some elements working against the speed of the car will rise with the square of that speed, there are so many more issues at play.
The drag on the car from air resistance my be specifically minimised at 60mph for example so could be much less than 4x the drag at 30mph owing to efficiencies from spoilers or turbulent-air flow etc...
Lift might also significantly lower rolling resistance.
Timmy40 said:
I always thought manufacturers recommended shifting to a higher gear as soon as possible in order to reduce fuel consumption, implying 4th gear 3k rpm will result in lower fuel consumption.
No, higher gears result in lower fuel consumption for the same vehicle speed, the question was if both had an engine speed of 3000rpm, a car in a higher gear will have a higher vehicle speed for the same engine speed.walm said:
By that logic, the most efficient way to drive a car would be to drive as slowly as possible (in this case 1st at 3,000rpm) and that isn't true.
You want lower rpm and higher gearing typically.
Therefore for a given rpm you want the highest gear.
I can't help but wonder at your logic (not a question).
He said at the same engine speed.You want lower rpm and higher gearing typically.
Therefore for a given rpm you want the highest gear.
I can't help but wonder at your logic (not a question).
To cover a given distance using the least fuel possible, the slowest vehicle speed that allows stable combustion would be about right, depending really on the car, a high resistance (aero and rolling) car with a low variation in bsfc, you'd be better off at a low engine speed and hence low vehicle speed, for a low resistance car you might be better off just running at peak bsfc.
thismonkeyhere said:
Yes, but it will also finish the mile sooner, so only if 'Fuel increase due to more torque' > 'Fuel saving due reduced run time' will the car in 4th use more for the mile.
I'm assuming you're trolling.scarble said:
walm said:
By that logic, the most efficient way to drive a car would be to drive as slowly as possible (in this case 1st at 3,000rpm) and that isn't true.
You want lower rpm and higher gearing typically.
Therefore for a given rpm you want the highest gear.
I can't help but wonder at your logic (not a question).
He said at the same engine speed.You want lower rpm and higher gearing typically.
Therefore for a given rpm you want the highest gear.
I can't help but wonder at your logic (not a question).
But no.
Because then you will be travelling faster and so there will be greater resistive forces acting on the vehicle, requiring more power to hold the mandated engine speed.
It actually would be more efficient to drive in 1st.
It would be very slow and boring and an utterly absurd situation, but at the same engine speed it would burn less fuel.
Because then you will be travelling faster and so there will be greater resistive forces acting on the vehicle, requiring more power to hold the mandated engine speed.
It actually would be more efficient to drive in 1st.
It would be very slow and boring and an utterly absurd situation, but at the same engine speed it would burn less fuel.
scarble said:
But no.
Because then you will be travelling faster and so there will be greater resistive forces acting on the vehicle, requiring more power to hold the mandated engine speed.
It actually would be more efficient to drive in 1st.
It would be very slow and boring and an utterly absurd situation, but at the same engine speed it would burn less fuel.
Just nonsense. Aerodynamic drag is negligible below about 40mph. Because then you will be travelling faster and so there will be greater resistive forces acting on the vehicle, requiring more power to hold the mandated engine speed.
It actually would be more efficient to drive in 1st.
It would be very slow and boring and an utterly absurd situation, but at the same engine speed it would burn less fuel.
Driving in first means a huge number of ignition cycles/distance covered.
walm said:
Dr Jekyll said:
I don't see how an engine can be more efficient at 3000 rpm in one gear than another.
You really think your mpg would be the same at 3,000 rpm in 1st as it would be in 6th??The whole point is that while some elements working against the speed of the car will rise with the square of that speed, there are so many more issues at play.
The drag on the car from air resistance my be specifically minimised at 60mph for example so could be much less than 4x the drag at 30mph owing to efficiencies from spoilers or turbulent-air flow etc...
Lift might also significantly lower rolling resistance.
The only way you could use less fuel at higher speed is if the engine is more efficient. It is conceivable that an engine could be so much more efficient at the higher rpm as produce more MPG at a higher speed [b]in the same gear[b]. Though this would only occur at very low revs. But if the higher road speed is achieved in a higher gear such that the rpm is exactly the same, the engine will be equally efficient therefore must burn more fuel to produce the extra power.
As for lower drag at 60mph than 30mpg, WTF?
SpeckledJim said:
singlecoil said:
SpeckledJim said:
Just nonsense. Aerodynamic drag is negligible below about 40mph.
Yes, just ask any cyclist 
singlecoil said:
SpeckledJim said:
singlecoil said:
SpeckledJim said:
Just nonsense. Aerodynamic drag is negligible below about 40mph.
Yes, just ask any cyclist If the conversation was about cycling, where the power and speeds involved are much lower, then what was negligible for a car is no longer negligible.
Where is the squabble?
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