Vertical takeoff aircraft require a far more powerful engine than a helicopter to lift the aircraft at a given vertical speed because they are shooting high velocity jets of air downwards. An engine “sufficiently powerful” for a helicopter would not be sufficient for VTOL.
To counteract gravity, for each kg of helicopter you need to impart 9.8 units of momentum per second to the surrounding air. You can either do that by moving a lot of air very slow, or a little air very fast. Because energy goes like velocity squared, it’s way more efficient to push down a lot of air slowly.
I know helicopters and VTOL exist. I had previously assumed that they were less efficient than planes (requiring more powerful engines and/or more fuel per unit mass maintained aloft per minute) and that that’s why they weren’t nearly as common as planes. But I had noticed my confusion about that before.
Now this article is claiming that there shouldn’t be any power (or, I think, fuel efficiency?) difference at least in theory. ”...it is also capable of lifting the same weight straight up...”
I tentatively agree that in theory there should be no difference in fuel efficiency at the task of remaining in the air, i.e., providing lift.
The reason the US military is switching from helicopters to VTOLs for transporting soldiers is that VTOLs are more fuel-efficient at making trips of more than 100 miles or so. Of course, the way they do that is by covering ground faster than a helicopter.
tl;dr: For a hovering aircraft, upward thrust equals weight, but this isn’t what determines engine power.
I’m no expert, but the important distinction is between power and force (thrust). Power is work done (energy transferred) per unit time, and if you were just gliding slowly in a large and light unpowered glider at a fixed altitude (pretending negligible drag), or to be actually realistic, hovering in a blimp, with lift equalling weight, you’re doing no work! (And neither is gravity.) On the other hand when a helicopter hovers at a fixed altitude it’s doing a great deal of work accelerating a volume of air downwards. (See also Gravity loss for a rocket.)
Now the interesting part: although for a hovering airplane, blimp or helicopter the upward force produced is equal to the weight, the power needed is different because the formulas for thrust and power aren’t directly linked. Thrust: F=ddtmv=ma. To compute work done on the air, consider the kinetic energy imparted on the air pushed down in one second. Power: P=ddt12mv2. Let’s say your helicopter is 1000kg, and simplify the gravitational constant as g=10ms−2, so your weight is 1000g=10000N. To create an equal upward thrust you could push 200kg of air per second downwards at 50ms−1… or 400kg of air at 25ms−1. But the former requires a power of P=12200⋅502=250kW=335hp while the latter is only P=12400⋅252=125kW=168hp! (This is a lower bound on, and directly proportional to, the energy in the fuel the engine must burn.)
So, to be fuel efficient a helicopter would have to have long blades that turn slowly, moving a large volume of air down slowly. But they don’t, apparently it’s not feasible. I imagine lighter helicopters can be more efficient though? And I’m not going to do any calculations for fixed wing aircraft. IANAAE.
This is also why turboprob and turbofan engines are more efficient than plain turbojet engines: they can produce the same thrust while expelling air at a lower velocity, hence with less work done, by using the jet engine to drive a propeller or fan.
Isn’t that the principle behind vertical takeoff aircraft? Or do you mean something else?
Vertical takeoff aircraft require a far more powerful engine than a helicopter to lift the aircraft at a given vertical speed because they are shooting high velocity jets of air downwards. An engine “sufficiently powerful” for a helicopter would not be sufficient for VTOL.
It’s one of them energy vs momentum things.
To counteract gravity, for each kg of helicopter you need to impart 9.8 units of momentum per second to the surrounding air. You can either do that by moving a lot of air very slow, or a little air very fast. Because energy goes like velocity squared, it’s way more efficient to push down a lot of air slowly.
I know helicopters and VTOL exist. I had previously assumed that they were less efficient than planes (requiring more powerful engines and/or more fuel per unit mass maintained aloft per minute) and that that’s why they weren’t nearly as common as planes. But I had noticed my confusion about that before.
Now this article is claiming that there shouldn’t be any power (or, I think, fuel efficiency?) difference at least in theory. ”...it is also capable of lifting the same weight straight up...”
I tentatively agree that in theory there should be no difference in fuel efficiency at the task of remaining in the air, i.e., providing lift.
The reason the US military is switching from helicopters to VTOLs for transporting soldiers is that VTOLs are more fuel-efficient at making trips of more than 100 miles or so. Of course, the way they do that is by covering ground faster than a helicopter.
tl;dr: For a hovering aircraft, upward thrust equals weight, but this isn’t what determines engine power.
I’m no expert, but the important distinction is between power and force (thrust). Power is work done (energy transferred) per unit time, and if you were just gliding slowly in a large and light unpowered glider at a fixed altitude (pretending negligible drag), or to be actually realistic, hovering in a blimp, with lift equalling weight, you’re doing no work! (And neither is gravity.) On the other hand when a helicopter hovers at a fixed altitude it’s doing a great deal of work accelerating a volume of air downwards. (See also Gravity loss for a rocket.)
Now the interesting part: although for a hovering airplane, blimp or helicopter the upward force produced is equal to the weight, the power needed is different because the formulas for thrust and power aren’t directly linked. Thrust: F=ddtmv=ma. To compute work done on the air, consider the kinetic energy imparted on the air pushed down in one second. Power: P=ddt12mv2. Let’s say your helicopter is 1000kg, and simplify the gravitational constant as g=10ms−2, so your weight is 1000g=10000N. To create an equal upward thrust you could push 200kg of air per second downwards at 50ms−1… or 400kg of air at 25ms−1. But the former requires a power of P=12200⋅502=250kW=335hp while the latter is only P=12400⋅252=125kW=168hp! (This is a lower bound on, and directly proportional to, the energy in the fuel the engine must burn.)
So, to be fuel efficient a helicopter would have to have long blades that turn slowly, moving a large volume of air down slowly. But they don’t, apparently it’s not feasible. I imagine lighter helicopters can be more efficient though? And I’m not going to do any calculations for fixed wing aircraft. IANAAE.
This is also why turboprob and turbofan engines are more efficient than plain turbojet engines: they can produce the same thrust while expelling air at a lower velocity, hence with less work done, by using the jet engine to drive a propeller or fan.