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Thread: A question about airplanes and helicopters

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    A question about airplanes and helicopters

    This is something I've been wondering about: why airliners fly higher than helicopters. I know that one reason is that for airliners, the fuel benefits of flying higher are very good, and also they fly above weather and other traffic. Helicopters are typically not designed for high-altitude flight because their benefit over airplanes is that they are good for short-distance flights with little landing space, and are not designed to be flown at high altitudes. Also, I think that the efficiency falls more than it does on airplanes, where the limitation is the availability of oxygen for engines.

    What I also wondered is whether safety is a consideration. Since an airplane needs a flat area to land, and has a relatively high glide-ratio, say 20:1, flying high gives you much more choice in an emergency. Whereas a helicopter only has a glide ratio of say 3:1 and can land in a relatively small flat area, so it's not as much of a benefit to be high when the engine fails. Is this also a consideration?
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    The "wing" area of helicopters is quite small, and limited in airspeed to how fast the rotor can turn.
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

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    The helicopter blade has velocity range from low at the hub to high at the tip.this compromises the design relative to fixed wing lift. So the wing area is smaller and the lift non linear. Then the pitch of each blade must be variable to give control in forward flight, changing as the blade rotates into and out of the wind. This further compromises design because stall must still be avoided. So there is a lower ceiling and it is for zero forward speed, since at max height any blade adjustment must reduce total lift. Conversely a fixed wing must fly faster to get lift at high altitude.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
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    I guess I should have been clearer with the question: In addition to other factors, is the safety issue (the fact that an airplane can glide significantly whereas a helicopter cannot) a reason for choosing higher altitudes for airplane flights?
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    A reason why jet planes fly high is that they face different speed constraints on takeoff and in cruise.
    The takeoff speed of planes is limited by length of available runways. Whereas cruise speed is limited by the sound barrier.
    A plane flying low is going to be inefficient.

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    Quote Originally Posted by Jens View Post
    I guess I should have been clearer with the question: In addition to other factors, is the safety issue (the fact that an airplane can glide significantly whereas a helicopter cannot) a reason for choosing higher altitudes for airplane flights?
    No the thinner air means fixed wing can go faster to maintain lift but the helicopter has both a tip speed limit and angle of attack variation so its effective limit is when not moving sideways (excluding any fixed wings it might also have) Sonic speeds further complicate the helicopter and the blades are well below sonic, that's another limitation. They can fly high enough to operate so the design compromise is to get fast forward speed with the pitch of the blades changing every half rev. to deal with the changing air flow. A low speed helicopter could be designed to get higher but it's not much use.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    No the thinner air means fixed wing can go faster to maintain lift but the helicopter has both a tip speed limit and angle of attack variation so its effective limit is when not moving sideways (excluding any fixed wings it might also have)
    I’m sorry but there seems to be a disconnect.i don’t understand how your answer relates to my question...


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    As above, so below

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    Quote Originally Posted by Jens View Post
    I guess I should have been clearer with the question: In addition to other factors, is the safety issue (the fact that an airplane can glide significantly whereas a helicopter cannot) a reason for choosing higher altitudes for airplane flights?
    Helicopters can autorotate. The higher you are, the more choice you have in safe landing spots. Too low, and you're likely to crash before you can adjust.

    A plane can fly faster to get enough lift from thinner air. A helicopter can only spin its rotor faster, and there are mechanical limits to its ability to do so. Also, a jet plane's engines have the air being rammed into their intakes, compensating somewhat for the thinner air.

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    Quote Originally Posted by Jens View Post
    I guess I should have been clearer with the question: In addition to other factors, is the safety issue (the fact that an airplane can glide significantly whereas a helicopter cannot) a reason for choosing higher altitudes for airplane flights?
    No, it's not a defining factor. Airliners are designed for efficient mass transit whereas helicopters are designed for utility in vertical take off and landing. The different design goals result in the altitude capabilities and limitations, as described by others above.
    ETA. Note there is a difference between airplane and airliner. Airplanes can of course fly quite low too. For example, a Cessna is not capable of high altitude flight for various technical reasons.
    Last edited by headrush; 2019-Apr-11 at 04:55 AM.

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    Quote Originally Posted by headrush View Post
    No, it's not a defining factor. Airliners are designed for efficient mass transit whereas helicopters are designed for utility in vertical take off and landing. The different design goals result in the altitude capabilities and limitations, as described by others above.
    ETA. Note there is a difference between airplane and airliner. Airplanes can of course fly quite low too. For example, a Cessna is not capable of high altitude flight for various technical reasons.
    Yes. And DC-3 is an airliner and, like Cessna, not capable of high altitude flight.
    The thing is, DC-8 unlike DC-3 is capable of high altitude flight, and faster than DC-3. And about as efficient. Whereas helicopter maximum speed is limited precisely by the rotor geometry and sound speed to a much smaller fraction of sound speed than in case of a jet plane.

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    Quote Originally Posted by Jens View Post
    I’m sorry but there seems to be a disconnect.i don’t understand how your answer relates to my question...


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    sorry if I did not explain well but others have reinforced the answer. I could put it another way. In designing a helicopter you can choose whether to go for height or speed and forward speed is usually what you want. Going forward the blades have to adjust their pitch and this is a mechanical limit. A fixed wing plane gets lower drag at height and can fly faster to get lift so long distance planes fly high. There are planes where large propellers are rotated to give vertical take off like a helicopter and then rotate to point forward like a prop plane. This design does not require the prop blades to adjust every revolution so avoids the helicopter problem while adding other complexities.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    sorry if I did not explain well but others have reinforced the answer. I could put it another way. In designing a helicopter you can choose whether to go for height or speed and forward speed is usually what you want. Going forward the blades have to adjust their pitch and this is a mechanical limit. A fixed wing plane gets lower drag at height and can fly faster to get lift so long distance planes fly high. There are planes where large propellers are rotated to give vertical take off like a helicopter and then rotate to point forward like a prop plane. This design does not require the prop blades to adjust every revolution so avoids the helicopter problem while adding other complexities.
    No problem at all. Actually, what I meant to ask is, starting with the awareness that:

    (1) There are technical differences between airplanes helicopters that make high-altitude flight more attractive for airplanes.

    and

    (2) Because of the landing capabilities of helicopters, it is more efficient to use them for short routes and therefore they are not designed for high flight.

    Is there an additional factor, i.e.:

    (3) For helicopters higher altitude is not safer.

    And I wanted to ask about the third point. And cjameshuff answered my question with:

    Helicopters can autorotate. The higher you are, the more choice you have in safe landing spots. Too low, and you're likely to crash before you can adjust.
    But in any case, I'm grateful for your answer because you provided better detail on point (1).
    As above, so below

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    Quote Originally Posted by Jens View Post
    No problem at all. Actually, what I meant to ask is, starting with the awareness that:

    (1) There are technical differences between airplanes helicopters that make high-altitude flight more attractive for airplanes.

    and

    (2) Because of the landing capabilities of helicopters, it is more efficient to use them for short routes and therefore they are not designed for high flight.

    Is there an additional factor, i.e.:

    (3) For helicopters higher altitude is not safer.

    And I wanted to ask about the third point. And cjameshuff answered my question with:



    But in any case, I'm grateful for your answer because you provided better detail on point (1).
    For (1): In an airplane, higher speed gets you more lift. Higher altitude gets you less drag. Go high, go fast.
    For a helicopter, higher speed just makes the problems associated with the advancing and retreating blades more pronounced. Airplanes can approach Mach .9 without major problems. But helicopters exceed that on the advancing blade even at low speed. High forward speed isn't helpful, except in certain hybrid cases. So they can't gain lift by going fast high up, and the thin air reduces lift.

    The highest altitude I've ever heard of for a helicopter is 23,000 feet for a rescue on Mt. Everest. That's really pushing it. You've probably flown higher on an airliner without even thinking about it.
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

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    Quote Originally Posted by profloater View Post
    sorry if I did not explain well but others have reinforced the answer. I could put it another way. In designing a helicopter you can choose whether to go for height or speed and forward speed is usually what you want.
    You have the same choice in case of planes, but high speed at low altitude is little use, because it requires long runways.
    Quote Originally Posted by profloater View Post
    Going forward the blades have to adjust their pitch and this is a mechanical limit. A fixed wing plane gets lower drag at height
    Actually not true.
    Quote Originally Posted by profloater View Post
    and can fly faster to get lift so long distance planes fly high.
    Thatīs the major reason.

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    Quote Originally Posted by chornedsnorkack View Post
    You have the same choice in case of planes, but high speed at low altitude is little use, because it requires long runways.

    Actually not true.

    Thatīs the major reason.
    I am not sure if you mean by "not true" that drag is less at height, ie less dense air, or that helicopters also have less drag at height? Or that the blades adjust pitch? So I am confused by your objection.
    A helicopter flying forwards has one blade at high air speed, going forwards, while its opposite one has much less airspeed because it is going backwards and therefore the pitch must change mechanically.
    The angle of the blade also changes with radius and these combine to limit design possibilities. So it is in the nature of helicopters to fly lower than what is optimum for fixed wing planes. The comparison should be with prop aircraft because it is the design of the prop which dictates that height difference.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    I am not sure if you mean by "not true" that drag is less at height, ie less dense air, or that helicopters also have less drag at height? Or that the blades adjust pitch? So I am confused by your objection.
    That drag is less at height.
    It is true that for the same speed, drag would be smaller in less dense air.
    But that is applicable to trains, whose weight is supported on wheels.
    Planes must needs rest their weight on lift. And at the same speed, lift also is smaller in less dense air.
    A plane can compensate for air density by flying faster in less dense air. In first approximation, increasing speed in less dense air such that the product of air density and square of speed stays constant, and lift stays equal to weight, leaves drag constant - does NOT decrease it. In second approximation, with Mach and Reynolds effects, drag increases slightly.

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    Quote Originally Posted by chornedsnorkack View Post
    That drag is less at height.
    It is true that for the same speed, drag would be smaller in less dense air.
    But that is applicable to trains, whose weight is supported on wheels.
    Planes must needs rest their weight on lift. And at the same speed, lift also is smaller in less dense air.
    A plane can compensate for air density by flying faster in less dense air. In first approximation, increasing speed in less dense air such that the product of air density and square of speed stays constant, and lift stays equal to weight, leaves drag constant - does NOT decrease it. In second approximation, with Mach and Reynolds effects, drag increases slightly.
    indeed and this explains why planes choose to fly high where it is very difficult to design a helicopter to do that, which is why they fly lower. It is not a safety consideration as such but a consequence of how helicopters work.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    indeed and this explains why planes choose to fly high where it is very difficult to design a helicopter to do that, which is why they fly lower. It is not a safety consideration as such but a consequence of how helicopters work.
    There are basically two ways to get a plane, or helicopter, to fly higher into thinner air:
    Make it lighter, or
    make it faster.
    There is space to make planes faster in cruise, because the near-sonic cruise speed is uselessly fast for takeoff.
    There is not space to make helicopters faster, because the advancing blade already is near-sonic.
    The only way to design a helicopter to fly higher is make it lighter. But unlike making a plane faster, it does not have other advantages.

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    Quote Originally Posted by chornedsnorkack View Post
    There are basically two ways to get a plane, or helicopter, to fly higher into thinner air:
    Make it lighter, or
    make it faster.
    There is space to make planes faster in cruise, because the near-sonic cruise speed is uselessly fast for takeoff.
    There is not space to make helicopters faster, because the advancing blade already is near-sonic.
    The only way to design a helicopter to fly higher is make it lighter. But unlike making a plane faster, it does not have other advantages.
    I think we agree then. the answer to the OP is in the way these two types fly.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  20. #20
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    The best of both worlds?
    http://www.eaglespeak.us/2005/02/ret...gyroplane.html
    https://sites.google.com/site/stingr...arpa-gyroplane

    I would think a heliplane might be one of the safer vehicles to ride in.

    The Plimp
    https://www.livescience.com/64186-pl...-aircraft.html

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