Airspeed vs groundspeed

[xcfufu]

Heya gents,
Just a quick question regarding my understanding on airspeed vs groundspeed.

The plane's stalling speed is 20kmh, so if i'm travelling at 21kmh, it's totally fine.
The wind is from north, at 10kmh. The airspeed now reads 21kmh, and ground speed is 11kmh, plane is flying fine.
Now the plane turns south, assuming the wind is still at 10kmh, the plane now needs to travel at 31kmh groundspeed to maintain proper flight? The airspeed will read 21kmh, am i right?

The above assumes all things being equal with plane perpendicular to the ground and no air density problem.

Would appreciate if any gent can advice.

Regards,
Charles

[remauled]

Hi, if you are taking the speed off the pitot tube, that measures true air speed, as long as the airspeed of the plane is above stall speed, plane flies.

If the speed is taken off the GPS/OSD, that measures the ground speed.

In that case, when flying upwind, you'll see Airspeed - windspeed = Groundspeed (GPS/OSD speed). When flying downwind, vice versa Airspeed + windspeed = Groundspeed (GPS/OSD speed). For all other aspect of flying, the windspeed will be a vector to groundspeed depending on the direction (vector).

[elmosan]

1. You are assuming that a plane's stall speed is a constant. It is not. It depends on your CG, flying weight for that flight, and its G-loading at that particular point in flight. It will never be exactly 21kmhr in all circumstances.

For example, you pull a hard G bank, thinking: Oh im at 25kmhr, should be safe!
Next thing you know, your plane wobbles and dips a wing. Hopefully you have altitude to recover. Same thing if you want to add 1 additional battery, or use a heavier battery for 'longer endurance'. If so, do adjust your minimum airspeed accordingly.

2. Flying at the minimum and relying on OSD airpseed is not recommended. It is hard to say about the reliability of the pitot. I have seen what a small speck of dirt in the pitot and APM auto mission mode can do to a plane. Its UGLY. I always have a safe margin above IAS(indicated airspeed) and cross reference with the groundspeed. The GPS groundspeed is an auxillary backup, as long as you factor in prevailing conditions.

Just my 2 cents worth.

Heya gents,
Just a quick question regarding my understanding on airspeed vs groundspeed.

The plane's stalling speed is 20kmh, so if i'm travelling at 21kmh, it's totally fine.
The wind is from north, at 10kmh. The airspeed now reads 21kmh, and ground speed is 11kmh, plane is flying fine.
Now the plane turns south, assuming the wind is still at 10kmh, the plane now needs to travel at 31kmh groundspeed to maintain proper flight? The airspeed will read 21kmh, am i right?

The above assumes all things being equal with plane perpendicular to the ground and no air density problem.

Would appreciate if any gent can advice.

Regards,
Charles

[viper1990]

Hi, i would like to contribute some inputs on this topic.

First off, we will be using IAS (indicated airspeed) to discuss airspeed as TAS (true airspeed) will be only differ from IAS at higher altitudes as we generally don't fly RC models higher than a few hundred feet in Singapore. Quoted from a website "IAS is also the most important airspeed number to a pilot. It is the speed he/she will reference for takeoff and landing and manoeuvring flight. Simply, it is the speed that defines if the airplane flies. If a few factors are held constant, an airplane in level flight will always stall at a constant indicated speed"

In my illustration below, When i fly Rc-planes, I like to imagine wind as a moving mass of air, like how ships move in the seas with current. To answer your question, if everything stays constant such as the wind speed, direction, plane mass , plane CG , small bank angle (you will need to bank to aircraft to turn from North to South, assuming the bank angle to turn the aircraft is kept small so it won't affect the stall speed drastically) than your plane will be fine and won't stall at all! Like what Elmosan has mentioned, the stall speed won't stay constant at all so its better if you always fly slightly faster at all cost.

Some extra pointers from my experience:
1) I always throttle and speed up my plane before turning downwind , assuming that the mass of air suddenly disappears (no wind) ,plane will stall as you have 11km/h airspeed around the airframe, if you are high up, you can always recover in time with sufficient altitude, if you flying low for an approach , uh oh...!

2) If, you are flying using OSD and flying long distances when you can't see the aircraft visually, i will always always rely on Groundspeed value. This is because airspeed might indicate 21km/h ,assuming strong headwinds are moving at 30km/h. Your plane will be flying fine but moving backwards at 9km/h! ,before you know it , your plane will get further and further away!

[PaulDT]

Guys, stall does not depend on either airspeed or ground speed, or even the attitude of the aircraft relative to the ground. Stall is related to the angle of attack of the wings against the relative wind. However, it is more commonly associated with air speed because each type of aircraft has a minimum airspeed to fly at the same level against the intended direction (relative wind). The wind that passes over and under the wings provide lift by creating lower pressure on top and higher pressure at the bottom. If this air passes with enough speed over and below the wings, the lift generated is enough to keep the plane from dropping altitude while maintaining a perfectly level flight. As this relative wind speed slows down, the plane loses lift while travelling level, hence the need to angle the wings more to generate additional lift and maintain the same level flight. That's why if you throttle down, you need to pitch up a bit to compensate to maintain the same altitude. But this pitching can only be done up to a point, where it reaches a critical angle against the relative direction of travel / relative wind. At this point, the wings lose lift and the plane goes into a stall.

[elmosan]

I understand that AoA is a more accurate measure of stall. But in the absence of a lift reserve meter or AoA dial in RC, the closest we have to rely on is the IAS meter and the ground speed indicators.

Guys, stall does not depend on either airspeed or ground speed, or even the attitude of the aircraft relative to the ground. Stall is related to the angle of attack of the wings against the relative wind. However, it is more commonly associated with air speed because each type of aircraft has a minimum airspeed to fly at the same level against the intended direction (relative wind). The wind that passes over and under the wings provide lift by creating lower pressure on top and higher pressure at the bottom. If this air passes with enough speed over and below the wings, the lift generated is enough to keep the plane from dropping altitude while maintaining a perfectly level flight. As this relative wind speed slows down, the plane loses lift while travelling level, hence the need to angle the wings more to generate additional lift and maintain the same level flight. That's why if you throttle down, you need to pitch up a bit to compensate to maintain the same altitude. But this pitching can only be done up to a point, where it reaches a critical angle against the relative direction of travel / relative wind. At this point, the wings lose lift and the plane goes into a stall.

[viper1990]

Guys, stall does not depend on either airspeed or ground speed, or even the attitude of the aircraft relative to the ground. Stall is related to the angle of attack of the wings against the relative wind. However, it is more commonly associated with air speed because each type of aircraft has a minimum airspeed to fly at the same level against the intended direction (relative wind). The wind that passes over and under the wings provide lift by creating lower pressure on top and higher pressure at the bottom. If this air passes with enough speed over and below the wings, the lift generated is enough to keep the plane from dropping altitude while maintaining a perfectly level flight. As this relative wind speed slows down, the plane loses lift while travelling level, hence the need to angle the wings more to generate additional lift and maintain the same level flight. That's why if you throttle down, you need to pitch up a bit to compensate to maintain the same altitude. But this pitching can only be done up to a point, where it reaches a critical angle against the relative direction of travel / relative wind. At this point, the wings lose lift and the plane goes into a stall.

Guys, stall does not depend on either airspeed or ground speed, or even the attitude of the aircraft relative to the ground. Stall is related to the angle of attack of the wings against the relative wind. However, it is more commonly associated with air speed because each type of aircraft has a minimum airspeed to fly at the same level against the intended direction (relative wind). The wind that passes over and under the wings provide lift by creating lower pressure on top and higher pressure at the bottom. If this air passes with enough speed over and below the wings, the lift generated is enough to keep the plane from dropping altitude while maintaining a perfectly level flight. As this relative wind speed slows down, the plane loses lift while travelling level, hence the need to angle the wings more to generate additional lift and maintain the same level flight. That's why if you throttle down, you need to pitch up a bit to compensate to maintain the same altitude. But this pitching can only be done up to a point, where it reaches a critical angle against the relative direction of travel / relative wind. At this point, the wings lose lift and the plane goes into a stall.

PaulDT is absolutely right. you explained the definition of stall perfectly. I have overlooked on the most basic knowledge of aerodynamics!

Here is what i got from the internet: "Stalls in fixed-wing flight are often experienced as a sudden reduction in lift as the pilot increases the wing's angle of attack and exceeds its critical angle of attack (which may be due to slowing down below stall speed in level flight)."

But my best guess is xcfufu is just asking a very general question, so lets try not to get into the topic so deeply to start a confusion! There are just too many variables to play with in Aerodynamics, but it would be good to appreciate every aspect of it to get a overall feel on the topic.