Why do helicopters use a stabilizer rotor while airplanes with propellers don't?

[TShock]

why do helicopters need a stabilizer rotor but airplanes that use propellers don't?

 

[russ_watters]

Airplanes have wings and always have forward speed.

 

[ManDay]

The more interesting question you could have asked is why a helicoper does whereas a gyrocopter doesn't. Or: Why do helicopters need to spent energy on hoovering, although dE = F * ds. Why can we not invent something, which doesn't waste 100% of its power for a process which actually requires none, yet?

Therefore:

http://www.yugatech.com/blog/wp-content/uploads/2007/02/hot-air-balloon05.jpg [/URL] > http://k.b5z.net/i/u/6049565/i/Helicopter_Pictures_012.jpg

 

[rcgldr]

why do helicopters need a stabilizer rotor but airplanes that use propellers don't?

Depends on the airplane. In a P57, if a pilot made the mistake of going full throttle on takeoff, it could lead to a crash; takeoff speeds are slow and the ailerons need a lot of throw to counter the torque from the prop, and just after the plane leaves the ground, the landing gear no longer supply a counter torque force, so suddenly it's up to the airlerons. Plus a lot of aileron throw can create adverse yaw, so then rudder inputs are required to compensate for that.

For most lower powered single engine aircraft, the landing gear provides enough counter torque when first moving, and at take off speed, there's enough natural resistance in the wings to resist the prop torque, and it takes little if any aileron input to compensate.

For a helicopter, the only component that resists the rotor torque is the tail rotor. Since the tail rotor creates a side force as well as a countering torque, a true hover requires a slight lean of the main rotor. This is more evident with radio control helicopters, as there is a quick correction just after leaving the ground to stop the helicopter from drifting sideways. While in flight, if forward speed is fast enough, then weather vane effects help counter the main rotor torque.

 

[russ_watters]

Why can we not invent something, which doesn't waste 100% of its power for a process which actually requires none, yet?

That was invented long before the helicopter: it's called a balloon.

 

[ManDay]

That was invented long before the helicopter: it's called a balloon.

Oh seriously? You didn't accidently happen to click those two links I gave below my post, did you?

Still, a balloon spends energy and owing to a lack of isolation it will keep spending energy altough it will not ascent...

 

[Danger]

Only a single-rotor chopper needs the tail. A tandem twin such as a Chinook, or one with two counter-rotating blade sets on the same axis, doesn't have the torque problem.

 

[ivanetski]

A good website which explains the bashttp://www.helicopterpage.com/" t.

 

[rcgldr]

One thing not covered is how gyroscopic effects mean that the cyclic controls are set to be 90 degrees "earlier" than the desired pitch or roll input.

http://helicopterflight.net/gyroscopic%20P.htm

update - might as well include one of my favorite radio control aerobatic helicopter videos. The action starts 25 seconds into the video:

http://jeffareid.net/rc/rcheli.wmv

 

[pchalla90]

jeff:
helicopters work because they push air downwards fast enough to propel the helicopter upward.

in your video, however, when the helicopter is upside down, air is being pushed upwards, so it should be falling faster than an object solely under the influence of gravity, yet it remains airborne.

can you explain that to me?

thanks.

 

[pchalla90]

wait... can the pilot change the direction the blades are spinning? that would explain everything, except the few moments when it should plummet before flipping over and then being caught...

 

[rcgldr]

Helicopters work because they push air downwards fast enough to propel the helicopter upward.

Radio control helicopters can also push air upwards. In aerobatic mode the blades mostly rotate at constant speed, and the overall pitch (collective) can be changed from + 10 to +13 degrees to - 10 to -13 degrees depending on the power of the radio control helicopter. The high thrust to weight ratios (around 8 to 1 or so) allows these models to do impressive stunts. The model has separate throttle and pitch control, but these are combined into a single programmable control on the transmitter, to maintain near constant rpm of the main rotor in aerobatic mode. In addition, the tail rotor control is also mixed into prevent yaw reaction to pitch command inputs from the transmitter, but the tail rotor also has it's own control to allow the pilot to control the yaw axis.

Radio helicopters have 3 main rotor modes: normal - rotor speed is variable, range is -1 pitch idle, then throttle is increased greatly at 0 pitch to allow safe take off, and then throttle / pitch mixed to prevent loss of speed in rotor; idle-up - aerobatic mode. Rotor is set to run a high speed at all pitch inputs, range is max negative pitch to max positive pitch with 0 pitch in middle; throttle hold - throttle is reduced to idle, simulating a loss of power. The tail rotor has a gyro to stabilize the yaw axis and has 2 basic modes - normal - yaw axis is stabilized but not held, sideways flight will result in weather vane response to yaw the heli so it faces direction of flight; heading hold mode - yaw axis is held at a fixed heading or fixed rate of yaw, despite any side forces from flight, used for smoother aerobatic manuevers. Generally heading hold and idle up mode are selected by the same switch.

As mentioned, an electronic gyro is used to stabilize the yaw axis. The cyclic axis, pitch and roll, are stabilized via a fly bar which looks similar to a small rotor perpendicular to the main rotor. There are weights attached to the end of the fly bar which responde to cyclic (roll or pitch) changes, countering these changes with opposing inputs directly to the cyclic. The size and mass of the weights on the fly bar determine the amount of cyclic stability. Some older real helicopters used flybars as well.

The main rotors on radio control helis rotate clockwise as viewed from above rather than counter clocwise (as with real American type helicopters, according to the link posted ealier).

Classis real helicopters also have throttle and collective controls, and these are independently controlled by the pilot (left stick moves up and down for pitch, and twist grip on left stick is throttle) but the pitch can't be set negative at all except on a few models, and it's a very small amount only used for auto-rotate (loss of power) recovery. Classic helicopter required the pilot to balance everything, throttle, collective, tail rotor (yaw). Most modern helicopter use computerized assists, stabilizing cyclic (roll/pithc) and yaw axis, to relieve the burden on the pilot (last thing you want in a rescue helicopter is a pilot using most of his concentration trying to keep the helicopter stable). Real helicopters don't have the thrust to weight ratio of the models, maybe 2 to 1 is the uppper limit.

Can the pilot change the direction the blades are spinning?

No, too much momentum, instead the pitch angle is changed as described above.

Link with picture and description of swash plate (combined cyclic and collective control)

http://en.wikipedia.org/wiki/Swashplate_(helicopter)

 

[rcgldr]

Took a while, but I finally found a link with a description and a picutre of a fly bar:

http://www.rc-airplane-world.com/flybar-balance-and-paddle-pitch.html

In one of the forums I post at, someone posted a picture of a real but older small helicopter that also used a flybar, but I can't find it now.

update - found it, from a racing game forum (started off with assists used in games to "assists" used with models and real life.)

Picture of a Hiller H-23, an early type of helicopter: http://tri.army.mil/LC/CS/csa/h23d001.jpg

Although the "paddles" on the end flybars appear to be wings, they aren't aerodynamic (too close to center of rotation to have much aerodynamic effect) it's their momentum that is important. A change in roll or pitch causes the weight to pivot on the flybar axis, which in turn is connected to the cyclic in order to provide negative feedback to dampen (stabilize) a helicopter along it's roll and pitch axis (cyclic dampening / stabilizing). The weights at the end are called paddles, but again, note they don't behave like wings, just weights that pivot on an axis.