Helicopter Main Rotor Blade in slow-mo

mikewint · Feb 21, 2020

DerAdlerIstGelandet said:
Its ground speed, not airspeed.

It might be easier to think in terms of a motorboat in a river. The motor has the ability to move the boat through the water at a given speed, say 10mph or kph. That is fixed and never changes. The boat does not know if it is a river going upstream or down or in a still unmoving lake, it continues to provide a steady 10mph/kph through the water no matter what the water happens to be doing. In other words the boat's through the water speed is independent of what the water is doing.
If the boat is in a river and heads downstream, the water all by itself is moving the boat downstream at, let's say 5mph/kph. The boat's motor moves it through the water at 10mph/kph while the water itself is moving at 5mph/kph in the same direction. Look over at the river's bank and you will be moving along the bank at 15mph/kph in the downstream direction. Turn the boat around 180 degrees and head upstream. The boat continues to move through the water at 10mph/kph. This time however the river's motion is opposite the boat's motion so looking over at the river's bank it will pass by at just 5mph/kph in the upstream direction.
You can easily demonstrate this with a toy motorized car and a rug. Moving the rug moves the car even with the motor off. Turn the car on and point it towards one end of the rug. Pull the rug at just the right speed opposite the car's motion and the car will remain in the same spot as the rug moves out from under it. The car's motion and the rug's motion are independent of each other

DerAdlerIstGelandet · Feb 21, 2020

mikewint said:
It might be easier to think in terms of a motorboat in a river. The motor has the ability to move the boat through the water at a given speed, say 10mph or kph. That is fixed and never changes. The boat does not know if it is a river going upstream or down or in a still unmoving lake, it continues to provide a steady 10mph/kph through the water no matter what the water happens to be doing. In other words the boat's through the water speed is independent of what the water is doing.
If the boat is in a river and heads downstream, the water all by itself is moving the boat downstream at, let's say 5mph/kph. The boat's motor moves it through the water at 10mph/kph while the water itself is moving at 5mph/kph in the same direction. Look over at the river's bank and you will be moving along the bank at 15mph/kph in the downstream direction. Turn the boat around 180 degrees and head upstream. The boat continues to move through the water at 10mph/kph. This time however the river's motion is opposite the boat's motion so looking over at the river's bank it will pass by at just 5mph/kph in the upstream direction.
You can easily demonstrate this with a toy motorized car and a rug. Moving the rug moves the car even with the motor off. Turn the car on and point it towards one end of the rug. Pull the rug at just the right speed opposite the car's motion and the car will remain in the same spot as the rug moves out from under it. The car's motion and the rug's motion are independent of each other

Mike, I don't need airspeed and ground speed explained to me. I'm a pilot...

gumbyk · Feb 21, 2020

tyrodtom said:
When you go from a headwind to a tailwind your airspeed doesn't change, just your ground speed.

After a time it does, but your groundspeed doesn't instantly change, while the wind speed/direction can. The problem is you're flying a heavy object through a relatively light one.

Momentum has nothing to do with airspeed. That's why, when experiencing windshear, you can see dramatic changes in airspeed. I've seen +/- 10 kts in a light aircraft occurring quicker than closing hte throttle would have done.

We had only descended about 200 feet when the airspeed went crazy. First, the speed increased by 30 knots, almost reaching the maximum speed for the flaps. Then, just as quickly, it reversed and dropped rapidly into the red, low-speed tape on the primary flight display.

The Danger of Windshear

mikewint · Feb 21, 2020

DerAdlerIstGelandet said:
Mike, I don't need airspeed and ground speed explained to me.

Chris beyond any shadow of a doubt I know that and it was not intended for you but to the posters who seem confused by the concept of independent speeds. I thought maybe taking the concept out of the air and putting into a more familiar context might help with the understanding.
Much like that airliner and the +800mph speed which passed almost unnoticed with little comment

gumbyk · Feb 21, 2020

mikewint said:
Chris beyond any shadow of a doubt I know that and it was not intended for you but to the posters who seem confused by the concept of independent speeds. I thought maybe taking the concept out of the air and putting into a more familiar context might help with the understanding.
Much like that airliner and the +800mph speed which passed almost unnoticed with little comment

Mike, I'm a pilot and ex instructor too, so I understand the concept of airspeed vs. groundspeed. I also understand that momentum isn't governed by your airspeed, so if the wind speed/direction abruptly changes, that aircraft is slower to respond.

mikewint · Feb 21, 2020

gumbyk said:
momentum isn't governed by your airspeed,

Was not trying to patronize anyone just trying to move an air problem experienced by a select few into a more common medium.
Just as you mentioned momentum in your post. Momentum is a tricky concept because it is a frame of reference measurement since all velocities are relative to the observer making the measurement. Relative to the air the plane has one velocity, relative to the ground a second velocity and I can step off the Earth and include the latitudes rotational velocity eastward. Holding a ball motionless in my hand it has zero momenta. Then I step out of that frame and see that I am in a boat traveling down a river at 10kph. But I can also step onto the river's bank and see the ball moving down river at 15kps. Momenta is relative to the observer but is conserved within any particular frame

tyrodtom · Feb 21, 2020

gumbyk said:
After a time it does, but your groundspeed doesn't instantly change, while the wind speed/direction can. The problem is you're flying a heavy object through a relatively light one.

Momentum has nothing to do with airspeed. That's why, when experiencing windshear, you can see dramatic changes in airspeed. I've seen +/- 10 kts in a light aircraft occurring quicker than closing hte throttle would have done.

The Danger of Windshear

I guess I've got to agree with that.
In my own limited flying ( 150 hrs.) I've been lucky and haven't experienced the radical changes in wind direction, but I experienced pretty sharp changes in air speed during wind gusts..

But 50 years ago as a crew chief of a OH-6 I experienced some pretty radical winds , and turbulence when a typhoon caught us by surprise , it advanced a lot faster than predicted.

I've had to step away from private flying, too expensive, and I doubt I can pass the physical anymore.

Helicopter Main Rotor Blade in slow-mo

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