Negative groundspeed??
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Negative groundspeed??
Hello guys,
This question has been hurting my brain for a while
so if anyone can answer this or rather help me to understand it I would be much appreciated.
I understand the relationship between IAS, TAS, GS, Wind component.
At sea level, IAS is equal to TAS (Forgetting any pressure or instrument errors in the ASI)
Also TAS + or - Head/Tail wind = Groundspeed
If we sit on the runway in our aircraft with a 50 knot headwind then the IAS will read 50 knots even though we are stationary.
If airborne we have an aircraft speed of 45knots with a headwind of 50 knots then 45kts-50kts=-5 i.e a negative groundspeed, moving backwards with reference to the ground. There are some cool videos of this on youtube.
My question is, how could we get to a TAS 45kts in a 50 kt headwind?
Assuming ISA conditions surely the IAS would only ever be a minimum of 50 knots just like on the runway.
I'm sure this is a simple problem but I think I may be overthinking it!?
Your help would be appreciated.
This question has been hurting my brain for a while
so if anyone can answer this or rather help me to understand it I would be much appreciated.I understand the relationship between IAS, TAS, GS, Wind component.
At sea level, IAS is equal to TAS (Forgetting any pressure or instrument errors in the ASI)
Also TAS + or - Head/Tail wind = Groundspeed
If we sit on the runway in our aircraft with a 50 knot headwind then the IAS will read 50 knots even though we are stationary.
If airborne we have an aircraft speed of 45knots with a headwind of 50 knots then 45kts-50kts=-5 i.e a negative groundspeed, moving backwards with reference to the ground. There are some cool videos of this on youtube.
My question is, how could we get to a TAS 45kts in a 50 kt headwind?
Assuming ISA conditions surely the IAS would only ever be a minimum of 50 knots just like on the runway.
I'm sure this is a simple problem but I think I may be overthinking it!?
Your help would be appreciated.
ISA conditions are an irrelevant distraction.
I'm sure this is a simple problem but I think I may be overthinking it!?
Your help would be appreciated.
Your help would be appreciated.
BUT
Be very careful about applying this in practice! In a 50kt wind the windspeed 0.001" above the runway is zero. The windspeed 1" above the runway is about 2kts. It doesn't get to the full windspeed until some distance above the ground - this is called the "wind shear effect". If you mess around at 30 feet flying "backwards" in a strong wind you can easily let the height drop a little, and then your airspeed will drop dramatically while the aeroplane accelerates. This has killed many an unwary pilot; it's one of two cases* where wind direction and speed actually affect the handling of the aeroplane. In strong winds stay well clear of the ground until you need to land, and you are then best advised to fly down the approach significantly faster than normal approach speed, leveling and bleading off the excess speed a couple of feet above the ground to negate the risk of falling out of the sky in windshear.
€0.05 supplied,
* the other beeing when doing steeply-banked turns close to the ground in strong winds in an aeroplane with greater tnan typical aspect ratio. This usually means gliders doing thgeir final turn onto a landing heading. Once pointing back into the wind in the very steep bank the wing nearest the ground may be in air with significantly lower wind speed, making it very reluctant to pick up that wing. This has killed a few people and broken a lot of gliders.
Quote from PilotJames:
"My question is, how could we get to a TAS 45kts in a 50 kt headwind?
Assuming ISA conditions surely the IAS would only ever be a minimum of 50 knots just like on the runway."
Well, assuming - to take a random example - the a/c was a glider capable of flight at an IAS of 45 kt, parked nose-down into-wind on the runway and somehow chocked, its IAS/TAS would be 50 kt and it could simply be rotated into flight. As it became airborne the IAS would immediately start to decay, and it would therefore start moving backwards in exactly the way you describe.
(Not a good idea, because a backwards landing would inevitably follow almost immediately!)
Moving on to the case of a powered aeroplane, the engine thrust would obviously allow the IAS to be controlled once airborne, and with it the ground-speed.
Does that help?
PS:
I presumed your question was purely theoretical, but PDR1 is quite right to point out the hazards of wind shear near the ground. At a given and time and place, this is even more relevant for small aeroplanes than large, partly because the lift-generating wing is nearer the ground on landing.
"My question is, how could we get to a TAS 45kts in a 50 kt headwind?
Assuming ISA conditions surely the IAS would only ever be a minimum of 50 knots just like on the runway."
Well, assuming - to take a random example - the a/c was a glider capable of flight at an IAS of 45 kt, parked nose-down into-wind on the runway and somehow chocked, its IAS/TAS would be 50 kt and it could simply be rotated into flight. As it became airborne the IAS would immediately start to decay, and it would therefore start moving backwards in exactly the way you describe.
(Not a good idea, because a backwards landing would inevitably follow almost immediately!)
Moving on to the case of a powered aeroplane, the engine thrust would obviously allow the IAS to be controlled once airborne, and with it the ground-speed.
Does that help?
PS:
I presumed your question was purely theoretical, but PDR1 is quite right to point out the hazards of wind shear near the ground. At a given and time and place, this is even more relevant for small aeroplanes than large, partly because the lift-generating wing is nearer the ground on landing.
Last edited by Chris Scott; 12th Dec 2016 at 15:03. Reason: PS added
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Here's how you can get negative groundspeed....
Well, assuming - to take a random example - the a/c was a glider capable of flight at an IAS of 45 kt, parked nose-down into-wind on the runway and somehow chocked, its IAS/TAS would be 50 kt and it could simply be rotated into flight. As it became airborne the IAS would immediately start to decay, and it would therefore start moving backwards in exactly the way you describe.
(Not a good idea, because a backwards landing would inevitably follow almost immediately!)
Moving on to the case of a powered aeroplane, the engine thrust would obviously allow the IAS to be controlled once airborne, and with it the ground-speed.
(Not a good idea, because a backwards landing would inevitably follow almost immediately!)
Moving on to the case of a powered aeroplane, the engine thrust would obviously allow the IAS to be controlled once airborne, and with it the ground-speed.
