A few days ago, I watched an episode of “Cops” that was shot in Nome, Alaska. At the end, the state Troopers were in the mountains recovering a woman that had died a few years prior. They had gone up in helicopters. Here’s my question.
Given that the air is much lighter at high altitudes, does that affect propeller flight? (choppers/planes) I know it could fly, but is there a loss of power/speed/etc? Is there a noticable difference in the physics at high altitudes?
Air is not lighter it is less dense at high altitudes. That affects the effectiveness of the rotors (there is less air to work on), which reduces carrying capacity and speed. Indeed, helicopters have a ceiling, as do any aircraft.
Altitude, temperature and even humidity affect the performance of aircraft propulsion systems. I am no longer a pilot, but I can give some background information.
As you noted, air is thinner at higher altitude. That provides less air for an airplane’s propeller to push against. It also provides less air for a helicopter’s rotor to push downward against.
The atmosphere at 18,000 feet is only half as dense as the atmosphere at sea level. Only a few helicopters can fly that high, but many nicer airplanes can fly that high.
To make matters worse, thinner air also provides less oxygen to the engine, so there is less power available, which worsens performance. A turbocharger on a piston engine can rectify all or part of that problem, up to a certain altitude where the atmosphere is so thin that even a turbo can’t find enough air to compress. As a VERY rough estimate, that usually happens at about 20,000 feet.
Cold air is denser than warm air, so that’s a plus. It isn’t a huge difference, but important. Each model of aircraft comes with a “performance envelope” diagram. It is a graph with a polygon area that shows limits on flight for combinations of altitude and temperature. High and hot is a serious limit. Flying out of Las Vegas, Nevada, in a small airplane when the air temperature is over 100 degrees is suspect, especially if a person wants to continue climbing westward to get over the Smith Range.
Humidity doesn’t have much effect on performance, but there is an effect. Humid air is less dense, so it provides slightly less lift than dry air. However, I have not seen a performance envelope graph that takes humidity into account. There may be some that do, but I haven’t seen one.
Finally, with helicopters there is the worry of “settling under power.” When hovering close to the ground, the air pushed down by the main rotor strikes the ground, flows radially outward, then can rise again, get sucked into the void above the rotors, then get forced down through the rotor again to complete the circle. The helicopter is then trying to hover in a column of air that is moving faster and faster downward. The helicopter settles into the ground. I believe that was the explanation for the drop of the Blackhawk helicopter that crashed last Winter on Mount Hood, Oregon, when trying to rescue two or three groups of climbers who had fallen.
To avoid or reduce settling under power, helicopter pilots maintain some forward motion instead of staying in one place when “hovering” near the edge of the performance envelope graph. The helicopter is therefore moving out of the column of descending air.
SteveAtlanta, that’s about the best answer I could have hoped for. Thanks.
A couple corrections to that answer:
First, air temperature does have a significant effect. The difference in density altitude between cold and warm days (think of that as “apparrent altitude” that correlates to the performance specs), can easily be several thousand feet.
The above explanation of settling with power is incorrect. In ground effect, the airflow pattern is indeed changed, but the cushion of air improves the situation. Blade tip vortices are reduced in size, which gives the rotor disc an improved efficiency. A vortex ring state (the actual technical term) happens when you satisfy three criteria: power applied, lack of translational lift (i.e. in a hover), and, depending on the type of helicopter, about a 300-800 foot per minute descent rate. If any of those conditions are not met, you don’t have a vortex ring state.
The most common means of entering a vortex ring state is by attempting to hover at too high an altitude. There is not enough lift provided by the rotor to remain in level flight, and the helicopter begins to sink and can become uncontrollable.
Duffer, to answer the question a little more, a rescue helicopter might be able to fly as high as 18,000 feet, but it would have to be in forward flight (taking advantage of translational lift). That same helicopter might be unable to hover higher than 8,000 feet. So a high mountain resue can become very difficult, not being able to hover. Unless there is a convenient long, flat, smooth surface right on the mountain, the helicopter wouldn’t have any safe way to land or take off at that altitude.
Actually, the main problem is with “retreating blade stall”. As noted, the density of the atmosphere is less at altitude. An airfoil needs a certain amount of air flowing over it to generate enough lift to maintain a given airspeed. An airplane can fly faster (given certain limitations) to maintain the needed airflow. A helicopter has a problem that airplanes don’t: One of their “wings” is going backwards.
Imagine a helicopter in a hover. Let’s say the rotor tips are turning at 300 knots. The airspeed is the same on both the advancing and retreating blades. Now let’s take the helicopter to 100 knots forward speed. The advancing blade has an airspeed of 400 knots, and the retreating blade has an airspeed of 200 knots. Obviously, the blades will generate different amounts of lift at a given pitch. What keeps the helicopter from rolling into the retreating blade? The rotor hub has hinges. The feathering hinge decreases the pitch onthe advancing blade, and increases the pitch on the retreating blade. Blades also “flap”. Two-blade systems (“semi-rigid rotor systems”) have a “teeter hinge” that allow the advancing and retreating blades to flap as a unit. Multi-blade systems (“fully articulated”) have flapping hinges, or they are “rigid systems” that use the flexibility of the blades to deal with flapping. This “flapping” allows the rotor blades to reach equilibrium. And then there are the “lead-lag” hinges. Due to the coriollus effect, these flapping blades will speed up or slow down. On a semi-rigid system (as on a Robinson R-22) or a Bell JetRanger) there is no need for lead-lag hinges because the blades are in line with each other. On multi-bladed systems the individual blades must be able to move forward and backward to compensate for the coriollus effect.
Anyway. As the helicopter gains more forward speed, there comes a point where the retreating blade can no longer compensate for it. It’s just like a stall on an airplane wing; the flow of air over the airfoil is not enough to sustain lift. Since air density is lower at altitude, a helicopter must slow down in order to prevent the retreating blade from reaching the critial airspeed. The higher a helicopter flies, the slower it must go.
Not quite.
When hovering glose tothe ground, you’re “hovering in ground effect”. The downward rush of air hits the ground and provides a “cushion” that allows the helicopter to fly more effeciently. This is why helicopters can hover at higher altitudes “in ground effect” than they can “our of ground effect”.
“Settling with power” (AKA “vortex ring state”) can occur at any altitude, but I’ve never personally heard of hit happening while in ground effect. Settling with power happens at low airspeeds and a descent rate of around 300 feet per minute. At low airspeeds (less than about 15 knots) you are descending into your own rotor wash. This isn’t a problem until you start descending more than about 300 fpm. Less than 300 fpm, your downwash is still going away from the helicopter. Greater than 300 fpm, the helicopter is descending into the column. Adding power to try to arrest the descent only aggrevates the situation. To get out of it, the pilot must get out of the sinking column of air. He can do this by lowering the collective and getting out of the bottom of it, but typically helicopters are flying low anyway. The better option is to move in any direction, usually forward, to get out of it.
I believe that the highest successful mountain rescue landing by a helicopter was in 1992 when the Pakistanis snatched Stephen Venables off Panch Chuli V at 19,000 feet. I think it only got one skid down…
Regret I cannot find a cit that it is the highest but (of course) he wrote a book about it:
http://www.scotlandonline.com/outdoors/essential_books_3.cfm?book_id=240&book_cat_id=4
Rotor, johnny, slloooww doowwnn…I’ll have to reread those a few times. Keep in mind I don’t have a pilot’s license, but these posts all seem to have the answer I was looking for.
Again, thanks for the effort everyone. Dons the reading glasses
Good god, I’ve just re-read my post. I really should get a nice dose of caffeine into my system before I start typing!
In a nutshell:
[ul][/]Helicopters must fly more slowly at higher altitudes because of retreating blade stall. Airplanes must fly faster to maintain a given altitude due to the lower density at high altitudes.
[/]Helicopters can hover at higher altitudes while in ground effect than they can while out of ground effect.
[/]Sloped surfaces rob ground effect, so a helicopter at high altitude that tries to land on a slope may not be able to hover.
[/]At altitudes that are greater than the helicopter’s ability to hover in ground effect (which is affected by actual altitude, temperature, humidity, aircraft weight, etc.) a helicopter will have to perform a “run-on landing” where forward speed is carried through touchdown.
[/*]“Settling with power” occurs at a hover or very low airspeed when out of ground effect, and when the helicopter is descending through its own rotor wash (approximately 300 fpm).[/ul]
Let me try that again…
In a nutshell:
[ul][li]Helicopters must fly more slowly at higher altitudes because of retreating blade stall. Airplanes must fly faster to maintain a given altitude due to the lower density at high altitudes.[/li][li]Helicopters can hover at higher altitudes while in ground effect than they can while out of ground effect.[/li][li]Sloped surfaces rob ground effect, so a helicopter at high altitude that tries to land on a slope may not be able to hover.[/li][li]At altitudes that are greater than the helicopter’s ability to hover in ground effect (which is affected by actual altitude, temperature, humidity, aircraft weight, etc.) a helicopter will have to perform a “run-on landing” where forward speed is carried through touchdown.[/li]“Settling with power” occurs at a hover or very low airspeed when out of ground effect, and when the helicopter is descending through its own rotor wash (approximately 300 fpm).[/ul]
Sorta true. As density decreases, a given indicated airspeed equates to a higher true airspeed. (The indication is via a needle on the airspeed instrument. It’s affected by air molecules ramming into a forward-facing tube, much as a wing is: when there are fewer molecules per liter of air, it takes a higher speed to give the same indication.)
The power needed to maintain that indicated airspeed changes very little with altitude. So, at least up to an altitude where the engine can produce power with reasonable efficiency, it’s more efficient to fly higher (other things, such as winds, being equal).
But for any air-breathing engine, there will come an altitude at which the maximum power available is equal to the minimum power necessary to maintain altitude (about the same power needed at sea level). The plane has reached its absolute ceiling and (unless it finds rising air) can’t climb higher.
I have.
Humidity DOES have a noticable effect on performance if you bother to look for it. I notice in part because I have flown aircraft considered “underpowered” by many, and because I actually do bother to calculate take-off and and landing distances on occassion.
In addition to humidity increasing the density altitude of an air mass, there is also the matter of carbuerator ice, which will definitely impact aircraft performance… but that’s getting a little off topic.
Right. I meant to put “indicated airspeed”. I was trying to simplify because I have a tendency to become pedantic, and I left out too much.
Broomstick: This reminds me of that little girl who crashed a few years ago. High airport elevation, overgross aircraft, high humidity, and she was in a C-177; which was considered to be underpowered. (Jeez, the only thing her instructor missed was high temperature to complete the mix! Oh, and he could maybe have added a downwind takeoff. :rolleyes: )
Actually, the instructor did better than high temp - he flew an overloaded 177 into a thunderstorm:
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001208X05676&key=1
[Nitpick]
“lighter” equates to “less dense” . Always. Trust me.
[/Nitpick]