> On a powered craft you flare... trade some of your velocity for the extra lift you get.
You are correct that the flare reduces your velocity. You do NOT want "extra lift" when you're two feet off the tarmac! You want to set the plane down. Anyway, you're aware that the flare reduces airspeed below the approach speed. Now let's have a look at what the approach speed should be, before you slow with a roundout and flare.
Which says that to pass your test and get licensed you must: 5. Maintains a stabilized approach and recommended airspeed, or in its absence, not more than 1.3 VSO
Anything over 1.3VSO (stall speed) on approach is a fail. Maybe that's why you don't have your license? After the approach, which must be less than 1.3 X stall, you then slow in the roundout, and slow more in the flare. What do you think you get when you start with "less than 1.3â, then reduce it twice? You get about 1.0.
> you'll fnd yourself overshooting the field and landing in a hedge.
Sounds like you need to work on your roundout, and probably your approach speed. Again, your approach needs to be not much above stall, per FAA license requirement. Then you slow in the roundout to have a nice sink rate rather than descending by having your nose down. Finally, 1-2 feet above the deck, flare to scrub further speed as the main gear touch down. Hold the stick back to further reduce speed so you don't go off the end of the runway or have to stomp on the brakes - you don't have much traction at this point since you still have airspeed.
> Then why'd you post hosts file in YOUR subject line raymorris?
I don't know if maybe you didn't see the message I posted? I said "Let's see the quality of comments we get." As you're certainly aware, hosts files have been discussed here once or twice. We're seeing the quality of comments we get.
Btw, it's OKAY to not be a pilot, or an aeronautical engineer. Nobody knows everything. It's interesting to talk about things we don't know about because that's how we LEARN.
Or, we can pretend to be a pilot when it's plainly obvious we would have never passed ground school, and stubbornly refuse to learn anything. That's the way to guarantee everlasting ignorance.
You seem to think saying "hey that's cool, I didn't know that" means a huge failure on your part. It does not. The huge failure is the stubborn refusal to learn anything, claiming to be a pilot while also saying that approach velocity is the same thing as touchdown velocity. Which shows you probably haven't even flown an *RC model* aircraft. I have my FAA log book here. I've got my hours as pilot in command of an actual airplane, not pixels. That doesn't mean you have to. You probably know 1,000 times as much about sports as I do. That's okay.
As someone who actually HAS flown a aircraft, not a video game, if you think that the touchdown flare (nose up) is the same as the descent (nose down), please never get behind the controls of an aircraft that isn't pixels.
Funny you'd claim to be a pilot and in the same breath demonstrate that you don't know the difference between touchdown speed in a flare (nose up) and descent (nose down).
By the way, you found the maximum speed at altitude and the minimum speed at 3X air density, it might be fun to see if you can find or calculate the minimum speed at the same density / altitude.
I'd be willing to bet it's just about 1240 / 3 kts. That is, max speed / 3 (for the same altitude, since wind turbines don't fly).
Induced and parasitic drag (load on the aircraft) is proportional to air density. At that altitude, the air density is less than a third of what it is at sea level. That makes the structural load 1/3rd. You're accidentally comparing velocity at sea level with velocity at 1/3rd the density and thinking you're calculating the load factor at 10. To get the load factor you'd need to divide by the density difference, so 10X / 3 = 3.3X. Hey there's the same number AGAIN. That keeps coming up about the same for every aircraft we try, from a Cessna to a jet fighter and a 737 . Maybe that's not a coincidence.
> Airplanes can be designed for a wider range of speeds than you think they are capable of...and often are.
I'll believe you if you can give one single example. Go ahead, I'll wait.
Btw keep in mind that the forces (induced and parasitic drag) on a plane are proportional to air density. At 60,000 feet, there is 66% less air than at sea level. So the structural maximum speed at sea level will be about 1/3rd the maximum speed at 60,000 feet. Conversely, the minimum speed at altitude will be about three times the minimum speed at sea level. If you forget that and compare minimum at sea level vs maximum at altitude you'll accidentally get a factor that is three times too large. Wind turbines don't fly up to 60,000 feet.
Drag (and therefore the forces the structure must withstand) is proportional to air density. At 60,000 feet where the Concorde reaches top speed, the is 66% less air than at sea level. The same plane can go ~ three times as fast at that altitude than it can at sea level.
If you factor that in, looking at the Concorde's top speed at sea level, you'll find it's top sea level speed is about 3.3 times its minimum sea level speed. Similarly, its minimum speed at 60,000 feet is - one third of its maximum speed at that altitude. Again, this isn't a coincidence.
Wind turbines don't switch between sea level and 60,000 feet. As someone mentioned, wind turbines don't fly*.:)
* Though actually you can do the math as if they do - the airfoil bears a load - the generator, which is analogous to the load borne by the wings of an aircraft. It's all airfoils.
Btw another thing to consider is that drag (and therefore airframe loading) is proportional to air density. At 56,000 feet, the is 65% less air than at sea level. The same plane can go ~ three times as fast at that altitude than it can at sea level.
> The A380 lands at about 130 knots compared to a top speed of 550. That's already 4.2x.and they don't land at full stall, and that's a loaded plane
Actually the 130 knot touchdown speed is stalled. A stall means your wings non longer hold you up - which is pretty much the definition of landing. Approach speed I a tad higher, fast enough that with the nose down you don't stall. It would stall in level flight at approach speed.
> That's already 4.2x.
Actually about 4X since you used the touchdown speed, which is a stall, but certainly not 15X. It might seem like there isn't THAT much difference between 4 and 15, but remember the difference in loads is 4^3 vs 15^3. That's a huge difference.
> Yes, that was supposed to be swing wing instead of swept wing.
Ah yes. That's very, very different. A swing wing greatly changes the aspect ratio, becoming a very different wing.
> Take your F-14 as an example. Must be at least 10x difference.
The F-14 has swinging wings, full-length leading edge slats, trailing edge slots, and bleed vanes that can be opened and closed. These significantly change the camber of the wing, the aspect ratio, the total wing area... two totally different wings. Still, the F-14 was called the turkey due to its low-spwed handling. Coming in for a landing with its low-spwed wings, it flew like a turkey does. Very, very different from its high-speed configuration. With the totally different wing configurations, it is indeed about a 10X difference in speed. Not with the same wing, of course. You can't do Mach 1 with the slats open and you can't land with the bleeds open.
Actually if you click, you'll see I was replying to someone who said basically:
A given wind turbine design can easily function at both 10MPH and 150MPH, because airplanes do.
Well no, they don't. An airplane that flies at 10uV suffers structural failure at about 30-40uV, because the forces on the structure are so much higher - proportional to velocity cubed.
> aerofoils which are not supporting their own weight do not have a minimum speed under which they stall.
When flow separation (turbulent vs laminar flow) hits the calculated center of lift at about 25% of the chord from the LE, the airfoil is stalled. It has nothing to do with weight.
> So planes have to be built fairly lightly, but windmills can be much more solid.
I don't know if you've ever seen a wind turbine, but the blades are actually long skinny things, much like the wings on a glider, and for the same reason. We call that "high aspect ratio" when we're designing airfoils, and the aspect ratio needs to be high for a reason. It makes a HUGE difference in efficiency. Wind turbine blades need to have a much higher aspect ratio than airplane wings.
Long skinny things get broken easily, because of the leverage and certain mechanics of geometry that I don't a laymen's term for. The force of the wind pushing near the tips is far from the hub, having a lot of leverage that multiplies the force trying to break it off the hub. At the same time, the airfoil isn't very thick, and thin things break easily. I'm not sure how to explain why this is so, but you intuitively know that it's a lot easier to break a thin piece of wood than a thick one - even if the thick one is a hollow truss.
So you have a weak shape (long and skinny) and high loads due to high leverage between the turbine tips and the hub.
Quotes about actual cases of wind turbines getting destroyed by high winds, and then...
> I stand by my assertion.
Based on the fact that wind turbines are in fact destroyed by high winds, you're going to assert that wind turbines can't be destroyed by high winds? Okay.
> For grins, I tried calculating tangential velocity of propeller tips and I think it works out to 622 MPH. By your reasoning, you wouldn't even make it off the runway before the propeller self-destructed.
Awesome. Did you calculate the maximum speed, or the minimum speed at which the prop will do its job, pulling the airplane at minimum speed? Have a look at the factor between those two and I think you'll find a number that looks familiar.
I didn't assert "things can't go fast". What I said is that because the load from wind is proportional to the velocity CUBED, a small multiple of wind speed (15X) will make a big difference in the power / loading (4,000 times more load). A prop designed to work well at velocity V is going to have some problems when loaded 4,000 times higher, at velocity 15V.
> comparing to aircraft stall speed is utterly irrelevant.
FYI, the wind turbine blades actually are airfoils. Just like airplane wings. In fact, technically they are wings. Like an airplane wing, they have a stall angle which implies a stall speed.
> There are prop planes much faster than the Cessna, like the TU 95.
And you'll find that the maximum speed of the TU-95 is about four times it's minimum speed. Not fifteen times. As a matter of fact, no matter WHICH plane to look at, you'll find the maximum speed speed is 2.5-5 times it's minimum speed. This isn't a coincidence. It all has to do with the loading - the forces being the cube of the velocity. The load difference between 3X and 15X is a factor of a hundred.
By the way, I didn't create the cube power law. I don't even like it, hence the title "the cube power law is a bitch". It's a pain in the ass when designing planes because the cube power bitch tries to rip the control surfaces and wings off. It did rip the nose off of one of my prototypes. So don't blame me if you find the cube power law inconvenient. I didn't create it, or even like it, I just have to know it.
The stall speed (minimum speed Vni) of the 172 is listed at 48 or 53 (flaps up or down). The Vr, minimum speed for level flight, is 55.
The green arc, extending to 129, is the normal operating range. 129 is Vno, the Maximum structural cruising speed.
The yellow arc is speeds that must only be hit in smooth air, and with great caution. "Maximum structur cruising speed"" means this in this range, above 129, turbulence can break the aircraft apart.
So the airspeed at which the aircraft may break is 2 1/2 times it's minimum speed. Hurricanes are 150MPH - a heck of a lot more than 2.5 times the 10mph sea breeze! (Hurricanes are turbulent, btw).
The red line is the Velocity Never Exceed, Vne. At 158 structural failure of the aircraft is to be expected.
So you want to make an analogy to prop-driven planes? They are destroyed at three times their minimum operating airspeed.
If you want to stick to the prop plane analogy, that suggests that a turbine designed for 10MPH would have structural failure at 30MPH. Still like that analogy?
> the Gulf Coast tends to see tropical systems of varying strength from time to time. > Unlikely wind turbines will be running during the storms
Worse than that, the power of wind is proportional to the the velocity CUBED. That means wind turbines are great where you have steady, sustained wind.
Suppose you build a turbine to start generating power with a 10mph wind. Obviously that has implications for the design, how sturdy vs "lightweight" you make it, if you want the power of a 10mph breeze to both spin it (overcoming inertia, friction, etc) and have extra power you can draw off as electricity.
When a 100mph tropical storm / hurricane comes to the coast, the turbine will have to withstand 1,000 times as power as it's designed to spin with. A stronger hurricane would require withstanding over 4,000 times as much power.
It's entirely likely they'd be not working because they'd be strewn across the beach in many pieces.
> Now, US is a merely a shadow of what it was doing in the 60s-80s.
Agreed.
> I don't think it's right to say that the US was always looking for big challenges. I think that's period is a relatively short period in US history.
Most people in the US either came here themselves from a another country, often not speaking the language, or their family did within the last 100 years. So on that alone you have culture of adventure, of not being timid. Many of those people arrived with nothing and now own successful businesses - again not by being timid.
The 1800s were the time when Americans ventured into the wild frontier to make whatever life they could make for themselves, from nothing but forest. So this spirit didn't start in 1940.
There are good things about people who see themselves as part of a much greater whole, people who do their part in the system. There are disadvantages to everyone being a "cowboy", doing their own thing.
I'm not looking down on either approach. Simply pointing out that they are two different cultural viewpoints.
Slashdot Mirror
> On a powered craft you flare ... trade some of your velocity for the extra lift you get.
You are correct that the flare reduces your velocity. You do NOT want "extra lift" when you're two feet off the tarmac! You want to set the plane down. Anyway, you're aware that the flare reduces airspeed below the approach speed. Now let's have a look at what the approach speed should be, before you slow with a roundout and flare.
Here's the FAA pilots license test requirements:
http://www.kingschools.com/pts...
Which says that to pass your test and get licensed you must:
5. Maintains a stabilized approach and recommended
airspeed, or in its absence, not more than 1.3 VSO
Anything over 1.3VSO (stall speed) on approach is a fail. Maybe that's why you don't have your license? After the approach, which must be less than 1.3 X stall, you then slow in the roundout, and slow more in the flare. What do you think you get when you start with "less than 1.3â, then reduce it twice? You get about 1.0.
> you'll fnd yourself overshooting the field and landing in a hedge.
Sounds like you need to work on your roundout, and probably your approach speed. Again, your approach needs to be not much above stall, per FAA license requirement. Then you slow in the roundout to have a nice sink rate rather than descending by having your nose down. Finally, 1-2 feet above the deck, flare to scrub further speed as the main gear touch down. Hold the stick back to further reduce speed so you don't go off the end of the runway or have to stomp on the brakes - you don't have much traction at this point since you still have airspeed.
> Then why'd you post hosts file in YOUR subject line raymorris?
I don't know if maybe you didn't see the message I posted? I said "Let's see the quality of comments we get." As you're certainly aware, hosts files have been discussed here once or twice. We're seeing the quality of comments we get.
Btw, it's OKAY to not be a pilot, or an aeronautical engineer. Nobody knows everything. It's interesting to talk about things we don't know about because that's how we LEARN.
Or, we can pretend to be a pilot when it's plainly obvious we would have never passed ground school, and stubbornly refuse to learn anything. That's the way to guarantee everlasting ignorance.
You seem to think saying "hey that's cool, I didn't know that" means a huge failure on your part. It does not. The huge failure is the stubborn refusal to learn anything, claiming to be a pilot while also saying that approach velocity is the same thing as touchdown velocity. Which shows you probably haven't even flown an *RC model* aircraft. I have my FAA log book here. I've got my hours as pilot in command of an actual airplane, not pixels. That doesn't mean you have to. You probably know 1,000 times as much about sports as I do. That's okay.
As someone who actually HAS flown a aircraft, not a video game, if you think that the touchdown flare (nose up) is the same as the descent (nose down), please never get behind the controls of an aircraft that isn't pixels.
Funny you'd claim to be a pilot and in the same breath demonstrate that you don't know the difference between touchdown speed in a flare (nose up) and descent (nose down).
By the way, you found the maximum speed at altitude and the minimum speed at 3X air density, it might be fun to see if you can find or calculate the minimum speed at the same density / altitude.
I'd be willing to bet it's just about 1240 / 3 kts. That is, max speed / 3 (for the same altitude, since wind turbines don't fly).
The SU-27 can do 1240 kts at 62,000 feet.
Induced and parasitic drag (load on the aircraft) is proportional to air density. At that altitude, the air density is less than a third of what it is at sea level. That makes the structural load 1/3rd. You're accidentally comparing velocity at sea level with velocity at 1/3rd the density and thinking you're calculating the load factor at 10. To get the load factor you'd need to divide by the density difference, so 10X / 3 = 3.3X. Hey there's the same number AGAIN. That keeps coming up about the same for every aircraft we try, from a Cessna to a jet fighter and a 737 . Maybe that's not a coincidence.
> Airplanes can be designed for a wider range of speeds than you think they are capable of...and often are.
I'll believe you if you can give one single example. Go ahead, I'll wait.
Btw keep in mind that the forces (induced and parasitic drag) on a plane are proportional to air density. At 60,000 feet, there is 66% less air than at sea level. So the structural maximum speed at sea level will be about 1/3rd the maximum speed at 60,000 feet. Conversely, the minimum speed at altitude will be about three times the minimum speed at sea level. If you forget that and compare minimum at sea level vs maximum at altitude you'll accidentally get a factor that is three times too large. Wind turbines don't fly up to 60,000 feet.
Btw you mentioned the Concorde.
Drag (and therefore the forces the structure must withstand) is proportional to air density. At 60,000 feet where the Concorde reaches top speed, the is 66% less air than at sea level. The same plane can go ~ three times as fast at that altitude than it can at sea level.
If you factor that in, looking at the Concorde's top speed at sea level, you'll find it's top sea level speed is about 3.3 times its minimum sea level speed. Similarly, its minimum speed at 60,000 feet is - one third of its maximum speed at that altitude. Again, this isn't a coincidence.
Wind turbines don't switch between sea level and 60,000 feet. As someone mentioned, wind turbines don't fly*. :)
* Though actually you can do the math as if they do - the airfoil bears a load - the generator, which is analogous to the load borne by the wings of an aircraft. It's all airfoils.
Btw another thing to consider is that drag (and therefore airframe loading) is proportional to air density. At 56,000 feet, the is 65% less air than at sea level. The same plane can go ~ three times as fast at that altitude than it can at sea level.
Wind turbines don't fly at 56,000 feet.
> The A380 lands at about 130 knots compared to a top speed of 550. That's already 4.2x.and they don't land at full stall, and that's a loaded plane
Actually the 130 knot touchdown speed is stalled. A stall means your wings non longer hold you up - which is pretty much the definition of landing. Approach speed I a tad higher, fast enough that with the nose down you don't stall. It would stall in level flight at approach speed.
> That's already 4.2x.
Actually about 4X since you used the touchdown speed, which is a stall, but certainly not 15X. It might seem like there isn't THAT much difference between 4 and 15, but remember the difference in loads is 4^3 vs 15^3. That's a huge difference.
> Yes, that was supposed to be swing wing instead of swept wing.
Ah yes. That's very, very different. A swing wing greatly changes the aspect ratio, becoming a very different wing.
> Take your F-14 as an example. Must be at least 10x difference.
The F-14 has swinging wings, full-length leading edge slats, trailing edge slots, and bleed vanes that can be opened and closed. These significantly change the camber of the wing, the aspect ratio, the total wing area ... two totally different wings. Still, the F-14 was called the turkey due to its low-spwed handling. Coming in for a landing with its low-spwed wings, it flew like a turkey does. Very, very different from its high-speed configuration. With the totally different wing configurations, it is indeed about a 10X difference in speed. Not with the same wing, of course. You can't do Mach 1 with the slats open and you can't land with the bleeds open.
Actually if you click, you'll see I was replying to someone who said basically:
A given wind turbine design can easily function at both 10MPH and 150MPH, because airplanes do.
Well no, they don't. An airplane that flies at 10uV suffers structural failure at about 30-40uV, because the forces on the structure are so much higher - proportional to velocity cubed.
> aerofoils which are not supporting their own weight do not have a minimum speed under which they stall.
When flow separation (turbulent vs laminar flow) hits the calculated center of lift at about 25% of the chord from the LE, the airfoil is stalled. It has nothing to do with weight.
> So planes have to be built fairly lightly, but windmills can be much more solid.
I don't know if you've ever seen a wind turbine, but the blades are actually long skinny things, much like the wings on a glider, and for the same reason. We call that "high aspect ratio" when we're designing airfoils, and the aspect ratio needs to be high for a reason. It makes a HUGE difference in efficiency. Wind turbine blades need to have a much higher aspect ratio than airplane wings.
Long skinny things get broken easily, because of the leverage and certain mechanics of geometry that I don't a laymen's term for. The force of the wind pushing near the tips is far from the hub, having a lot of leverage that multiplies the force trying to break it off the hub. At the same time, the airfoil isn't very thick, and thin things break easily. I'm not sure how to explain why this is so, but you intuitively know that it's a lot easier to break a thin piece of wood than a thick one - even if the thick one is a hollow truss.
So you have a weak shape (long and skinny) and high loads due to high leverage between the turbine tips and the hub.
> . Swept wing aircraft have far larger differences between their minimum and maximum speeds.
Stall speed of a (swept like the TU) Boeing 737: 140 knots
Maximum speed: 473 knots
Factor: 3.38
I said 2.5-5 and sure enough, it's right in the middle of the range at 3.38.
Quotes about actual cases of wind turbines getting destroyed by high winds, and then ...
> I stand by my assertion.
Based on the fact that wind turbines are in fact destroyed by high winds, you're going to assert that wind turbines can't be destroyed by high winds? Okay.
> For grins, I tried calculating tangential velocity of propeller tips and I think it works out to 622 MPH. By your reasoning, you wouldn't even make it off the runway before the propeller self-destructed.
Awesome. Did you calculate the maximum speed, or the minimum speed at which the prop will do its job, pulling the airplane at minimum speed? Have a look at the factor between those two and I think you'll find a number that looks familiar.
I didn't assert "things can't go fast". What I said is that because the load from wind is proportional to the velocity CUBED, a small multiple of wind speed (15X) will make a big difference in the power / loading (4,000 times more load). A prop designed to work well at velocity V is going to have some problems when loaded 4,000 times higher, at velocity 15V.
> comparing to aircraft stall speed is utterly irrelevant.
FYI, the wind turbine blades actually are airfoils. Just like airplane wings. In fact, technically they are wings. Like an airplane wing, they have a stall angle which implies a stall speed.
> There are prop planes much faster than the Cessna, like the TU 95.
And you'll find that the maximum speed of the TU-95 is about four times it's minimum speed. Not fifteen times. As a matter of fact, no matter WHICH plane to look at, you'll find the maximum speed speed is 2.5-5 times it's minimum speed. This isn't a coincidence. It all has to do with the loading - the forces being the cube of the velocity. The load difference between 3X and 15X is a factor of a hundred.
By the way, I didn't create the cube power law. I don't even like it, hence the title "the cube power law is a bitch". It's a pain in the ass when designing planes because the cube power bitch tries to rip the control surfaces and wings off. It did rip the nose off of one of my prototypes. So don't blame me if you find the cube power law inconvenient. I didn't create it, or even like it, I just have to know it.
Here's the airspeed indicator of a very popular plane, one I studied thoroughly, the Cessna 172:
https://fsxtimes.files.wordpre...
The stall speed (minimum speed Vni) of the 172 is listed at 48 or 53 (flaps up or down). The Vr, minimum speed for level flight, is 55.
The green arc, extending to 129, is the normal operating range. 129 is Vno, the Maximum structural cruising speed.
The yellow arc is speeds that must only be hit in smooth air, and with great caution. "Maximum structur cruising speed"" means this in this range, above 129, turbulence can break the aircraft apart.
So the airspeed at which the aircraft may break is 2 1/2 times it's minimum speed. Hurricanes are 150MPH - a heck of a lot more than 2.5 times the 10mph sea breeze! (Hurricanes are turbulent, btw).
The red line is the Velocity Never Exceed, Vne. At 158 structural failure of the aircraft is to be expected.
So you want to make an analogy to prop-driven planes? They are destroyed at three times their minimum operating airspeed.
If you want to stick to the prop plane analogy, that suggests that a turbine designed for 10MPH would have structural failure at 30MPH. Still like that analogy?
> the Gulf Coast tends to see tropical systems of varying strength from time to time.
> Unlikely wind turbines will be running during the storms
Worse than that, the power of wind is proportional to the the velocity CUBED. That means wind turbines are great where you have steady, sustained wind.
Suppose you build a turbine to start generating power with a 10mph wind. Obviously that has implications for the design, how sturdy vs "lightweight" you make it, if you want the power of a 10mph breeze to both spin it (overcoming inertia, friction, etc) and have extra power you can draw off as electricity.
When a 100mph tropical storm / hurricane comes to the coast, the turbine will have to withstand 1,000 times as power as it's designed to spin with. A stronger hurricane would require withstanding over 4,000 times as much power.
It's entirely likely they'd be not working because they'd be strewn across the beach in many pieces.
Let's put that to the test. Let's see the quality of comments we get.
> Now, US is a merely a shadow of what it was doing in the 60s-80s.
Agreed.
> I don't think it's right to say that the US was always looking for big challenges. I think that's period is a relatively short period in US history.
Most people in the US either came here themselves from a another country, often not speaking the language, or their family did within the last 100 years. So on that alone you have culture of adventure, of not being timid. Many of those people arrived with nothing and now own successful businesses - again not by being timid.
The 1800s were the time when Americans ventured into the wild frontier to make whatever life they could make for themselves, from nothing but forest. So this spirit didn't start in 1940.
> but from what I can tell they've been at the prototype stage for decades.
Yeah they've been a hot new thing for at least 50 or 60 years.
Maybe one day they will actually have practical application.
There are good things about people who see themselves as part of a much greater whole, people who do their part in the system. There are disadvantages to everyone being a "cowboy", doing their own thing.
I'm not looking down on either approach. Simply pointing out that they are two different cultural viewpoints.