I was looking again the the FR24 plots from my flights over the weekend and noticed something interesting: on a low level (3000ft) flight in moderate turbulence the ground speed variations are huge and you can clearly see the point where I entered IMC:
On flight (at FL100, well above the clouds), the groundspeed was much more stable – right up to the point where I encountered light turbulence from the hills in northern england:
On both of these flights, IAS didn’t vary by more than 10kt (I usually fly usually around 117KTAS). This implies that wind direction and intensity changes in turbulence can dominate over the airspeed change due to rising/sinking air.
Has anyone else noticed this?
Can often see it at high altitude at work. We have an indication of wind direction and strength and often, especially over terrain like the alps and more often the Pyrenees you can see quite a variation with a relatively small variation in IAS.
I have not noticed such rapid variations, apart from obvious cases of headwind/tailwind when crossing mountainous terrain when you can go from 70kt to 150kt in tens of seconds.
FR24 GS values could be way out because unless you are radiating the GPS GS (does it get radiated over either Enhanced Mode S or ADS-B? – a TB10 will have neither except Enhanced Mode S could be the result of a “non EU” installation) it will be deriving it from either the Mode C/S radar-derived position or from the way the GPS position varies, and both of those will have the problem with a small change in the data causing big changes in the derived speed (the classic jitter in the absolute value of the small difference between two large numbers).
But then I spend very little time in any significant turbulence. I can imagine the IAS could drop a lot in heavy turbulence.
Could it not just mean that the airspeed sensor is more damped than the groundspeed sensor?
Seems to me the slower the plane the greater the effect of turbulence will be. Although Im surprised that it seems so large a value. You say IAS did not change? Id be curious to see if you didnt maintain Altitude but allowed the plane to seek its own level what the outcome might be.
It is also a function of inertia. In a large wind shift, a light aircraft has little inertia and will very quickly adapt to the speed of the new airmass, with minor fluctuations in IAS but larger changes in ground speed.
In a larger aircraft, you have stacks of inertia and your 65 ton airliner will continue at the same speed relative to the ground for much longer, meaning your IAS will either plummet or rocket, and the GPWS to start screaming “WINDSHEAR” at you.
Isn’t it actually a function of wing-loading?
The X15 might be even less sensitive to wind shifts, even though at any given speed it will have much less inertia than your 65 ton airliner.
Two different things being talked about here now really that wing loading is mentioned.
Wing loading will affect how turbulence feels, how rough it is and so on, but this is different to what is being said about inertia having a role in speed fluctuations.
Is it? Acceleration = force/mass. As you enter a headwind, the forces it exerts on the aircraft will be in proportion to the wing and surface area so if you doubled both, you should expect the aircraft’s behavior to stay roughly constant.
In practice, larger aircraft will have a higher volume:surface area ratio, and so a higher wing-loading in general.
Looking up a few aircraft, it seems the A320 has a higher wing loading (110 lb/ft^2) than an F104 starfighter (105 lb/ft^2), so there will be a strong association between the mass of an aircraft and its sensitivity to changes in windspeed/direction, but it still seems to me that it should be down to the ratio between force/mass rather than simply the mass of an aircraft.
Another example: the U2 has a wing loading of about 40lb/ft at takeoff, and I’d be willing to bet it would be more susceptible to windspeed changes than the lighter F104.
It is a fairly complex topic, as wing loading, aircraft mass, gust direction and cross sectional area presented to the gust all combine to produce the aircraft’s gust response.
There is also a difference between instantaneous response, which is mainly a function of wing loading, and longer term response, which is more to do with mass and inertia.
Talking about wing loading of a large transport category aircraft is of course very different from a fighter or SEP as the weight range is much greater. The 737 has a max landing mass of 65 tonnes, and I have flown it at around 47-48 tonnes on the line. It is very unpleasant to handle in gusty conditions and the only two factors that have changed are mass and wing loading, which act in concert to make the aircraft more susceptible to weather conditions.
