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Old 28th Dec 2010, 20:51
  #1 (permalink)  
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VMO/MMO are the maximum indicated speeds.
Boeing 737's Mach/airspeed warning systems are based on VMO/MMO.

My question here is, what about the ground speed? If we're having a strong tailwind during cruise at high altitude wouldn't lead to overspeed event without any overspeed warning and protection?
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Old 28th Dec 2010, 21:00
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I've got to ask, because of this post and your previous posts. Are you a professional pilot or a flight simmer. If a professional then I'm very worried!

In answer to your question. Groundspeed will never affect an aircraft as it is the relative effect of the airflow over the wing that is the only thing that counts when discussing aerodynamics. Remove groundspeed completely from the equation.
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Old 28th Dec 2010, 21:02
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Overspeed is only in relation to the airflow around your ownship, or indicated speeds.

The ground doesn't feature.

EDIT: Damn! Too slow!
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Old 28th Dec 2010, 22:36
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737 FCTM says "During cruise at hight altitude, wind speed or direction changes may lead to overspeed events."

cortilla, I am a flight simmer.. and I was thinking about it like: when a strong tailwind exist, wouldn't this "push" the aircraft forward (to the air) hence the 'air flow over the wing' increases/become faster?

Last edited by ElNull; 28th Dec 2010 at 22:55.
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Old 29th Dec 2010, 00:06
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No it won't. The aircraft is unaware of movement of the air mass in which it flies. Think of a boat on water. On a still lake its water speed will be x knots, which will also be its speed past the ground at the shore/ground). Same boat on large river. No matter which way it points - upstream, downstream or across - its water speed will still be x. It's speed past the ground, however, will vary. Pointed upstream (ie into the prevailing wind for an aircraft) the speed past the ground will be slower. Pointed downstream (wind behind you if an aircraft) the ground speed will be faster. Across the flow then ground speed will be a vector sum of the water (air) speed and the current (wind) speed.

What the advice you read is on about is that high winds can be association with *sudden* changes in wind direction. In this case the change in wind speed occurs faster than the inertia characteristics of the aircraft allow it to accelerate or decelerate so there can be an instantaneous speed excursion. Left to its own devices the aircraft would eventually stabilise back at it original airspeed (but with a now different ground speed).
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Old 29th Dec 2010, 00:08
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Elnull ,, the ground speed is the speed of the aircraft relative to the ground it has nothing to do with VMO/MMO, ground speed can be used for some calculations , as our friend cortilla told you (Groundspeed will never affect an aircraft as it is the relative effect of the airflow over the wing) which is correct, plus somthn you need to know VMO/MMO is indicated airspeed all the time cant be TAS or GS or CAS.
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Old 29th Dec 2010, 01:58
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thanks you all for the answers, all is clear now!
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Old 29th Dec 2010, 07:36
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Let's make this very simple. Imagine a fly in a car. As the flies around inside the car, it will have an airspeed. If you now start to drive the car, will you change the fly's airspeed? No. Will the fly even know you have started to move? Probably not. Always the fly's airspeed is that which it makes through the air inside the car and it's groundspeed it that which it makes across the ground over which the car takes it.

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Old 10th Jan 2011, 00:21
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But would it`s ground speed change if it tired of flying and landed on the discarded chocolate bar on the back seat ?
l mean would the mass of the car be increased by the weight of the fly causing a reduction in speed ?
Or was the airborne fly already contributing to the mass of the car ?

The next question would seem to be is balanced flight truly balanced ?

Just a thought.

Also, whilst the car is moving if the fly flew from front to back it`s ground speed would be less than if it flew from back to front l would think.

l also think l`ve got a headache.

Last edited by overun; 10th Jan 2011 at 06:48. Reason: another thought.
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Old 10th Jan 2011, 10:32
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l also think l`ve got a headache.
Before you reach for the headache pills there is a true story about a truck trying to get up a hill on the Hume Highway near Gundagai in New South Wales. Every now and again the driver would stop (much to annoyance of a queue of cars stuck behind him) and hit the canvas cover of the truck with a baseball bat. He would then resume driving. Repeat ad nauseum. Eventually an irate motorist demanded to know what he was up to.

The driver replied "I have got ten thousand budgerigars( small parrots) in the back of this truck and unless I keep them flying, I'll never get up these hills".

My turn for a headache...
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