He's going deep into theory with his explanation. In reality, if two identical aircraft are gliding, the lighter aircraft will glide further.
US Airways plane goes down in Hudson River
- Thread starter .Griff.
- Start date
He's going deep into theory with his explanation. In reality, if two identical aircraft are gliding, the lighter aircraft will glide further.
I'll try and knock up a vector diagram to illustrate (picture tells a thousand words) but if the aircraft is descending there is a component of the weight providing the thrust. If the aircraft is heavier this component is larger which in turn increases the velocity of the aircraft. An increased airspeed produces more lift which supports the greater mass. Drag also increases (more lift = more drag) but it does so proportionally to the thrust so the ratio of thrust to drag remains the same. This is the important part, its the thrust:drag ratio that determines the glide performance. Lift/Drag = Glide Distance/Altitude.
I stressed the *distance* part of my original post because its fairly obvious that if you've got two aircraft covering the same horizontal distance but one is travelling faster than the other it will hit the ground sooner
Its not deep at all - on a practical level, you'd fly at Vmd. While the actual value of velocity for Vmd will change dependant on weight, the ratio of lift the drag remains the same and hence the glide distance
Disagreeing
Yep
Your instinct is to focus on the heavier aircraft but think about the lighter of the two, with all four engines out where is the thrust coming from?
Disagreeing
Yep
Your instinct is to focus on the heavier aircraft but think about the lighter of the two, with all four engines out where is the thrust coming from?
That goes against everything I was taught! Heavier aircraft requires more lift which equates to more drag. This requires a higher airspeed for a given AOA so the nose needs to be stuck down. The result being a steeper descent and depending on the aircraft in question, a higher rate of descent.
I'll have to dig out my old text books and have a look to make sure I'm not missing anything. All I can tell you is that air traffic controllers here are taught that for example a 747 fully laden going to LA with no thrust will hit the ground sooner and in less distance than an empty identical aircraft. If what we're being taught is wrong I'll have to bring it up at the next emergency training session.
I'm in agreement with Scuzi on this.
I do understand what you are saying MTA99 but it sounds like you are neglecting the increased drag caused by the increase in lift. The heavier plane is going to need to point it's nose further down in order to avoid stalling the wings.
I do understand what you are saying MTA99 but it sounds like you are neglecting the increased drag caused by the increase in lift. The heavier plane is going to need to point it's nose further down in order to avoid stalling the wings.
It doesn't go against everything you were taught, its just you're not considering that the greater weight provides a greater component of thrust. I agree that an aircraft with a greater mass needs a greater airspeed to generate the required lift at a given AoA which increases drag but I don't agree that you change the pitch attitude. The extra airspeed it supplied by the extra weight (and its component in the direction of flight). A heavier aircraft has a greater rate of descent (i've not questioned that) but you have to be careful not to confuse rate of descent with distance covered.
While the air traffic controllers might be expecting the plane to hit the ground in a shorter distance the pilots are expecting to go the same distance
Not at all, i've considered drag in all my posts
In fact my whole argument revolves around the fact that the lift:drag ratio stays the same. If the drag stayed the same but the thrust component of the weight increased the plane would accelerate indefinitely!! 
I don't think you understand what a stall is either. An aerofoil stalls at an angle of attack not at an airspeed
I don't think you understand what a stall is either. An aerofoil stalls at an angle of attack not at an airspeed
Yep, I'm confusing AoA with nose pitch. What I intended to say was that for a given nose pitch there will be an airspeed below which the wings will stall (Due to the increased AoA). So for a given nose pitch an aerofoil WILL stall at an airspeed, surely?
Anyhoo, I do see what you are saying with regards to the gliding, and it does make sense. I'll take your word for it
Last edited:
Soldato
- Joined
- 8 Mar 2006
- Posts
- 13,311
- Location
- Telemark
Scuzi is correct.
A 747 is A LOT better off empty if you're into gliding.
Some high performance gliders have an optimum weight, and will carry water basalt to make this weight, but these are gliders, not airliners.
The water balast us usually dropped before landing to glide slower.
A 747 is A LOT better off empty if you're into gliding.
Some high performance gliders have an optimum weight, and will carry water basalt to make this weight, but these are gliders, not airliners.
The water balast us usually dropped before landing to glide slower.
Yep, I'm confusing AoA with nose pitch. What I intended to say was that for a given nose pitch there will be an airspeed below which the wings will stall (Due to the increased AoA). So for a given nose pitch an aerofoil WILL stall at an airspeed, surely?
Anyhoo, I do see what you are saying with regards to the gliding, and it does make sense. I'll take your word for it
You're looking at stalling back to front. Lets imagine we have an aircraft that weighs 1,000kgs. To fly we need to generate 10,000N of lift (1,000kgs x 10). In the cruise this is easy, plenty of air rushing over the wings so we don't need much in the way of AoA. As we slow down the amount of air rushing over the wings is significantly reduced so we need a greater AoA to achieve the 10,000N lift, if we go really slow we need a very high AoA to achieve the 10,000N lift. We can't go on increasing the AoA and expect to still be flying (think about very high AoA like 60-70°
) so there's a limit to the AoA and this angle is the critical angle. The wing will stall at *any* speed at this angle. The stalling *speed* of the aircraft is the airspeed at which if we're at the critical angle exactly 10,000N are produced.You're right gliders are not airliners but they follow the same rules for flight. The glider has no means of creating thrust other than the thrust component of the weight. If the glider is required to achieve a minimum airspeed to maintain flight then ballast could be added to increase that thrust component above a minimum value. I know a couple of glider pilots so i'll have a word to check that's the case.
Hopefully this pic will help explain the effect of weight on the glide angle.
Green arrow is weight, blue is lift, red is the total reaction (lift + drag) and white is the thrust component of the weigh (glide angle in this example is 30°).
Hopefully you can see that the ratio of sizes of the forces between the two aircraft are the same. If you increase the weight the thrust component goes up, this must be balanced by an increase in drag. The drag contributes to the total reaction so that must increase. To complete the vectors the lift must increase to contribute to the increased total reaction (increased airspeed = increased lift). You can check it with trigonometry, the numbers still work out
Since the two aircraft are on the same line the glide angle is the same and therefore if they both start at the same altitude they'll both hit the ground on the same spot - its just the heavier one will get there sooner.
I must admit I can't see his reasoning either to be honest scuzi but then I am certainly no maths expert at trying to work it out..
All I can see is that you would need to go faster to produce the same lift as the lighter aircraft, the only issue I can see with that is the fact the only way you can go faster is to "nose down" to give you more airspeed ... and then you lose the lift you were trying to create in the first place.
Where am I going wrong with that... I guess it's in the drag thingy but that's where my brain begins to melt
I must admit I can't see his reasoning either to be honest scuzi but then I am certainly no maths expert at trying to work it out..
All I can see is that you would need to go faster to produce the same lift as the lighter aircraft, the only issue I can see with that is the fact the only way you can go faster is to "nose down" to give you more airspeed ... and then you lose the lift you were trying to create in the first place.
Where am I going wrong with that... I guess it's in the drag thingy but that's where my brain begins to melt
See the pic above. The nose attitude stays the same but as the weight increases magically the thrust increases
You're looking at stalling back to front. Lets imagine we have an aircraft that weighs 1,000kgs. To fly we need to generate 10,000N of lift (1,000kgs x 10). In the cruise this is easy, plenty of air rushing over the wings so we don't need much in the way of AoA. As we slow down the amount of air rushing over the wings is significantly reduced so we need a greater AoA to achieve the 10,000N lift, if we go really slow we need a very high AoA to achieve the 10,000N lift. We can't go on increasing the AoA and expect to still be flying (think about very high AoA like 60-70°
Yep. I agree with all of that. You know what I was talking about though, right?
As far as the gliding theory goes, I reckon you've sold me on it, makes perfect sense.
Can I assume from your location that you attend a well known european flight school?
Lift = CL 0.5 Rho V^2 S
Where CL is the coeff of lift (a function of the AoA and shape of the wing), Rho is the density of the air, V is the airspeed and S is the wing area
Total drag = Cdt 0.5 Rho V^2 S
Where Cdt is the coeff of total drag and all other terms are the same.
Yep. I agree with all of that. You know what I was talking about though, right?
Errr no
There's a speed above which a given AoA will stall but you'll not achieve that during a glide Might do
Just spoken to an jumbo (cargo) airline pilot (Ex RAF Bosses mate)
He's not sure either LOL.
Comments he did make were "depends on the aircraft"
Some aircraft are designed to fly fast, and heavier = faster.
Comments he did make were to say that he is taught to dump fuel if the need arises to get over a mountain, whether that was a separate issue or what I'm not sure.
Before he hung up (Late at night where he is) he said what the eternal question should be is how do flies land on a ceiling
He's not sure either LOL.
Comments he did make were "depends on the aircraft"
Some aircraft are designed to fly fast, and heavier = faster.
Comments he did make were to say that he is taught to dump fuel if the need arises to get over a mountain, whether that was a separate issue or what I'm not sure.
Before he hung up (Late at night where he is) he said what the eternal question should be is how do flies land on a ceiling