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quicken78
03-13-2007, 03:28 PM
A friend of mine recently stated that flight times were reduced when flying with a tail wind, and increased when flying with a head wind. We spent about twenty minutes going over pseudo-physics, but couldn't convince each other that either of us was right.

From my limited understanding of Physics, I would assume that a plane has to travel faster with a tail wind. But I might be wrong and defer to the SD to provide a solid, physics-related answer.

FatBaldGuy
03-13-2007, 03:37 PM
You don't need advanced physics to explain this. Go out on the street during a heavy windstorm. Do you go faster when walking with the wind or against the wind?

Bill Door
03-13-2007, 03:39 PM
A friend of mine recently stated that flight times were reduced when flying with a tail wind, and increased when flying with a head wind. We spent about twenty minutes going over pseudo-physics, but couldn't convince each other that either of us was right.

From my limited understanding of Physics, I would assume that a plane has to travel faster with a tail wind. But I might be wrong and defer to the SD to provide a solid, physics-related answer.

It does travel faster with a tail wind. That's what reduces the flight times. A 2 hour flight might take 1:45 with a 60 knot tail wind.

FlyingCowOfDoom
03-13-2007, 03:40 PM
What if the plane is traveling in a tail wind on a treadmill?

--FCOD

Rick
03-13-2007, 03:42 PM
But is the plane on a treadmill?

OK assume same plane same throttle settings

Condition 1 No wind
Wind speed 0 MPH
Air speed 500 MPH
Speed over ground 500 MPH

Wind speed 100 MPH
Air speed 500 MPH
Speed over ground 500-100 = 400 MPH

Condition 3 tailwind
Wind speed 100 MPH
Air speed 500 MPH
Speed over ground 500 + 100 = 600 MPH

I should note that under condition 3 most air carriers will reduce throttle settings to save fuel, and arrive on time.

TNWPsycho7490
03-13-2007, 03:42 PM
With a head wind it would create more lift and make the aircraft lighter....creating the POSSIBILTY for it to go faster. But it would also create a lot more resistance, so it will go faster with a tail wind.

Kevbo
03-13-2007, 03:43 PM
Note that the difference in direction is non-linear with wind speed. Consider that a Cessna flying 100 kt into a 100 kt wind is stationary....flight time will be infinite, same airplane with a 100 kt tailwind is making 200kt groundspeed, so flight time would be halved over the winds-calm case. Thus a tailwind can help to a certain degree, but a very strong headwind can be a really serious problem.

nivlac
03-13-2007, 03:45 PM
Isn't this analogous to a boat traveling with the current vs. against the current. With a tailwind, the plane is traveling "with the current" and should fly faster which reduces the travel time. I don't see where the OP disagrees with his friend.

FlyingCowOfDoom
03-13-2007, 03:51 PM
Perhaps the OP is confused as to the definitions of tailwind and headwind?

Plane traveling this way ------------>

--FCOD

brewha
03-13-2007, 04:22 PM
With a head wind it would create more lift and make the aircraft lighter....creating the POSSIBILTY for it to go faster. But it would also create a lot more resistance, so it will go faster with a tail wind.

This is just wrong. An airplane has a top speed based on thrust and drag. The drag is determined by the size and shape of the plane - it doesn't change. The thrust is based on the type of the engine - it doesn't change. The plane will go pretty much at the same airspeed regardless of the wind speed or direction.

What can change is the groundspeed. A tail wind will cause a higher ground speed and a head wind a lower ground speed.

Thus a tailwind=get there quicker.

David Simmons
03-13-2007, 05:29 PM
From my limited understanding of Physics, I would assume that a plane has to travel faster with a tail wind. But I might be wrong and defer to the SD to provide a solid, physics-related answer.No, the plane doesn't need to go faster if there is a tail wind. I'm assuming you mean that it must go faster to provide the same lift as with no wind.

This can get a little tricky. Suppose an airplane with a takeoff airspeed (speed relative to the air mass) of 60 kt. is sitting on the runway facing a headwind of 20 kt. The airspeed indicator already reads 20 kt. so you only need to gain 40 kt in order to take off. On the other hand, if the same plane is on the runway with a 20 kt tail wind it must gain 80 kt in order to take off. However, in both cases the airspeed at takeoff will be 60 kt. This is why airplanes take off into the wind. You cover a lot less ground to gain 40 kt than to gain 80 kt and so you don't need as much runway for takeoff. It works the same way for landings. You can use a much shorter runway to land into the wind than if you land downwind.

Once you get into the air, as others have said. the only effect wind has is on navigation. It has no effect on flight characteristics.

Shagnasty
03-13-2007, 05:36 PM
Isn't this analogous to a boat traveling with the current vs. against the current. With a tailwind, the plane is traveling "with the current" and should fly faster which reduces the travel time. I don't see where the OP disagrees with his friend.

That is the most appropriate analogy. A common misconception is that planes are flying in space and merely acting on the air. That is not true. Planes fly IN the air just like boats travel in the water and airspeed is the only thing that matters.

This is where we get into airspeed versus ground speed. A Piper Cub can fly head on into a 60 mph headwind and be dead still or maybe even flying backwards in reference to the ground. The plane doesn't care and knows nothing about the ground. It just needs the appropriate airflow over its lift and control surfaces. Reverse the wind and that tiny plane would be going 120 mph over the ground but the flight is exactly the same from an aerodynamics point of view.

A 747 may cruise at about 600 mph. However, if you go into the cockpit at 35,000 feet, the air speed indicator won't read a speed anywhere near that fast. That is because the air is thinner at that altitude and the plane couldn't give a rat's ass about how fast the ground is passing below. The number or air molecules passing over it over second are all that counts (and it is the most important thing to the pilot too because the plane operates on airspeed and not ground speed).

Richard Pearse
03-13-2007, 07:38 PM
A 747 may cruise at about 600 mph. However, if you go into the cockpit at 35,000 feet, the air speed indicator won't read a speed anywhere near that fast. That is because the air is thinner at that altitude and the plane couldn't give a rat's ass about how fast the ground is passing below. The number or air molecules passing over it over second are all that counts (and it is the most important thing to the pilot too because the plane operates on airspeed and not ground speed).
Not to mention that the airspeed indicator under-reads significantly at altitude.

You may be at 25,000 with a true air speed of 250 kts, the air speed indicator might read 160 kts, and there may be 50 kts tailwind giving you a ground speed of 300 kts.

David Simmons
03-13-2007, 08:11 PM
Not to mention that the airspeed indicator under-reads significantly at altitude.

You may be at 25,000 with a true air speed of 250 kts, the air speed indicator might read 160 kts, and there may be 50 kts tailwind giving you a ground speed of 300 kts.And just to complet this, it's the indicated airspeed that is important for flight. The true airspeed is important for navigation.

If a plane stalls at 45 kt indicated at sea level it also stalles at 45 kt indicated at any altitude. As you go up for a constant true airspeed the indicated airspeed drops. Eventually the indicated speed equals the stalling speed and that's as high as you can go.

If you want to make any turns you have to fly at some lower altitude and have a reserve of indicated airspeed in level flight so that when you lose speed in the turn you won't stall..

Shagnasty
03-13-2007, 08:18 PM
Eventually the indicated speed equals the stalling speed and that's as high as you can go.

If you want to make any turns you have to fly at some lower altitude and have a reserve of indicated airspeed in level flight so that when you lose speed in the turn you won't stall..

Thanks David Simmons. I haven't finished my pilots license yet but reading about all things aviation including theory has been my passionate hobby since I was young. I have never heard the part above put quite that way but it makes perfect sense and made some things click beyond that simple example.

Richard Pearse
03-13-2007, 09:15 PM
If a plane stalls at 45 kt indicated at sea level it also stalls at 45 kt indicated at any altitude. As you go up for a constant true airspeed the indicated airspeed drops. Eventually the indicated speed equals the stalling speed and that's as high as you can go.

There is an exception to this. The air speed indicator over-reads slightly at high altitudes due to the less dense air being more easily compressed. So the indicated stall speed does increase a little.

The indicated stall speed also increases at high altitudes and true air speeds where developing shock-waves cause the airflow to depart from the upper surface of the wing sooner than normal. Shock-waves can develop well before the speed of sound.

Also, the stall speed increases in a turn so loss of speed is not required although it exacerbates the problem.

ETA: Just to clarify. The air speed indicator under-reads significantly compared to true air speed at altitude. It also over-reads slightly from what it would read in the same pressure at sea level.

03-13-2007, 09:26 PM
A friend of mine recently stated that flight times were reduced when flying with a tail wind ...
<snip>
From my limited understanding of Physics, I would assume that a plane has to travel faster with a tail windBut these two statements are the same? I don't see where you are disagreeing with each other.

CookingWithGas
03-13-2007, 09:48 PM
N.B.: If you fly into a 30-knot headwind then return the same distance, now with a 30-knot tailwind, your overall flight time will be longer than if there were no wind. (Nobody asked, but this is a classic brain-teaser.)

David Simmons
03-13-2007, 09:57 PM
From my limited understanding of Physics, I would assume that a plane has to travel faster with a tail wind.But these two statements are the same? I don't see where you are disagreeing with each other.I interpreted the statement to mean that he thinks a plane must travel faster through the air with a tail wind in order to maintain lift.

Not so in the steady state. The airplane, as was pointed out, is immersed in the air and travels with it, like a balloon. Call this the plane's "balloon speed." If the wind suddenly changes direction it takes the plane a little while to respond and attain the new "balloon speed" and during that time the flight characteristics are changed. If this happens close to the ground, as in wind-shear incidents, the results can be disasterous.

Shagnasty. My example was greatly simplified, as was later pointed out by 1920's Style "Death Ray". However it was close enough for a general feel for why you can only go so high.

The maximum height can also be limited by the engine at a lower altitude. Engines don't get as much air at high altitudes as a low and so they eventually run out of enough power to maintain flying speed. However, even if there is plenty of power you still can't go above the point where your IAS is the stalling speed.

Xema
03-13-2007, 10:22 PM
As you go up for a constant true airspeed the indicated airspeed drops. Eventually the indicated speed equals the stalling speed and that's as high as you can go.
It seems you've omitted the part of the explanation that covers why there should be a limit on true airspeed.

The typical explanation offered here is that flutter will occur at a more or less constant true airspeed. At some reduced pressure, flutter speed and stall speed coincide, and you are in a "coffin corner" where the only option is to descend.

(There's some evidence that this view of flutter is oversimplified, but it's a common and generally safe approach.)

... so that when you lose speed in the turn you won't stall.There's no inherent reason why you need to lose speed in a turn. By contrast, it is hard to execute a turn without increasing the load factor, which in turn increases stall speed.

Richard Pearse
03-13-2007, 10:51 PM
There's no inherent reason why you need to lose speed in a turn. By contrast, it is hard to execute a turn without increasing the load factor, which in turn increases stall speed.

Increased load factor means increased drag which results in an increased thrust requirement to maintain the same speed. If thrust is not increased then the speed will decrease.

David Simmons
03-13-2007, 11:06 PM
There's no inherent reason why you need to lose speed in a turn. By contrast, it is hard to execute a turn without increasing the load factor, which in turn increases stall speed.True. When making a turn you can increase engine power, if you have any to use, so as not to lose speed. My statement was a tad careless.

(For the non flyers.) In order not to lose altitude in a turn the lift must be increased. This results from the fact that the lift force is perpendicular to the wing surface and when you bank to turn the lift vector is tilted. In order to maintain altitude the lift must increase so that its vertical component equals the weight of the plane. This means a higher wing angle of attack which causes increased induced drag and a lower airspeed unless you increase power.

:smack: I wrote this before 1920's Style "Death Ray" posted, previewed it and then forgot to post it. See what you've all got ahead of you when you get old? Do you still want to? :smack:

cornflakes
03-13-2007, 11:12 PM
A Piper Cub can fly head on into a 60 mph headwind and be dead still or maybe even flying backwards in reference to the ground. The plane doesn't care and knows nothing about the ground. It just needs the appropriate airflow over its lift and control surfaces.Silly piece of trivia - my father learned to fly small planes while stationed in the Pacific during WWII and flew Cessnas* on island hops. Dad claimed** that one of the islands that he flew into had winds strong enough that he would drop to stall speed and approach with a negative groundspeed.

*I think he said that they were Cessnas. He also called them "Maytag Messerschmitts", though that is at least now used to refer to the Ryan PT-22.

**...And are you going to call my dad a liar? :dubious:

David Simmons
03-13-2007, 11:23 PM
Silly piece of trivia - my father learned to fly small planes while stationed in the Pacific during WWII and flew Cessnas* on island hops. Dad claimed** that one of the islands that he flew into had winds strong enough that he would drop to stall speed and approach with a negative groundspeed.

*I think he said that they were Cessnas. He also called them "Maytag Messerschmitts", though that is at least now used to refer to the Ryan PT-22.

**...And are you going to call my dad a liar? :dubious:I took advanced flight training in Cessna AT-17's (http://aeroweb.brooklyn.cuny.edu/specs/cessna/at-17.htm) at Fort Sumner, NM. We were there in the early spring at a time when the winds could be pretty fierce. Several times the wind came up while planes were flying so they had those not flying go out to the head end of the runway. As planes landed several of us would grab the wing tips to steady them so they wouldn't turn over. I don't think we could have held them in a strong gust but as far as I know we had no landing accidents.

You could slow fly the AT-17 down to about 35-40 mph airspeed and in a strong headwind they descended like an elevator.

GusNSpot
03-13-2007, 11:58 PM
The "Sky King" airplane....

The main spar is so big and strong, it is airworthy with one half inch of dry rot on the bottom side.

PT-22 is a very sweet flying bird.

brewha
03-14-2007, 07:49 AM
I took advanced flight training in Cessna AT-17's (http://aeroweb.brooklyn.cuny.edu/specs/cessna/at-17.htm) at Fort Sumner, NM. We were there in the early spring at a time when the winds could be pretty fierce. Several times the wind came up while planes were flying so they had those not flying go out to the head end of the runway. As planes landed several of us would grab the wing tips to steady them so they wouldn't turn over. I don't think we could have held them in a strong gust but as far as I know we had no landing accidents.

You could slow fly the AT-17 down to about 35-40 mph airspeed and in a strong headwind they descended like an elevator.

Kind of a similar story. A friend of my dad's had a WWII P-51 Mustang at an airshow. The plane was worth close to \$1,000,000 IIRC and is normally kept in a hanger. Well, it was going to be sitting outside over the weekend because he had it on display at this airshow.

A thunderstorm came in and they were predicting wind gusts up to 70 MPH. Rather than risk his plane getting flipped over, he taxied it out on to the runway, and "flew" it through the storm. The wheels never left the ground, but he sat there for several hours with the tail up and the engine running to keep the plane pointed into the wind.

Richard Pearse
03-14-2007, 08:19 AM
I hope that story has lost something in translation, because that's probably not the best way to look after a \$1,000,000 aeroplane in a thunderstorm :eek:!

Shagnasty
03-14-2007, 08:31 AM
Kind of a similar story. A friend of my dad's had a WWII P-51 Mustang at an airshow. The plane was worth close to \$1,000,000 IIRC and is normally kept in a hanger. Well, it was going to be sitting outside over the weekend because he had it on display at this airshow.

A thunderstorm came in and they were predicting wind gusts up to 70 MPH. Rather than risk his plane getting flipped over, he taxied it out on to the runway, and "flew" it through the storm. The wheels never left the ground, but he sat there for several hours with the tail up and the engine running to keep the plane pointed into the wind.

While that is reasonably possible, I think that I would have flown that plane somewhere else if it were mine. I would bet staying on the ground was much more dangerous than just going someplace with more favorable conditions.

BTW, a P-51 Mustang in good, flyable condition is worth a good deal more than \$1,000,000 these days. They aren't making anymore original ones.

Richard Pearse
03-14-2007, 08:43 AM
BTW, a P-51 Mustang in good, flyable condition is worth a good deal more than \$1,000,000 these days. They aren't making anymore original ones.

There was a chap in New Zealand who bought an ex RNZAF mustang for 75 pounds in the lte 1950s. The wings were scrap but he aquired a new set of wings and an engine and had it made airworthy. He sold it after they started going for big money. It's now flying as part of Kermit Weeks' collection.

75 POUNDS!. I realise that was a reasonable amount of money back then but nothing like what they sell for now.

Xema
03-14-2007, 10:20 AM
Increased load factor means increased drag which results in an increased thrust requirement to maintain the same speed. If thrust is not increased then the speed will decrease.
This is true if you include the assumption of level flight at a constant airspeed, which is reasonable but fundamentally artificial. Sailplanes, for example, having no engine routinely violate this without difficulties.

Xema
03-14-2007, 10:28 AM
a P-51 Mustang in good, flyable condition is worth a good deal more than \$1,000,000 these days. They aren't making anymore original ones.
But note that a respectable (and increasing) number of Mustangs flying today have only a limited number of original parts.

I knew a guy who bought a nice Mustang for around \$2000 in the mid-1950s (which was about the going price at that time). He flew it for several years and then sold it for \$6000, congratulating himself on what a shrewd bargainer he was.

Santo Rugger
03-14-2007, 11:25 AM
<snip>
The indicated stall speed also increases at high altitudes and true air speeds where developing shock-waves cause the airflow to depart from the upper surface of the wing sooner than normal. Shock-waves can develop well before the speed of sound.
<snip>
ETA: Just to clarify. The air speed indicator under-reads significantly compared to true air speed at altitude. It also over-reads slightly from what it would read in the same pressure at sea level.

Just to clarify faurther, are you saying shock waves can develop before the indicated speed of sound, compared to the actual speed of sound at that altitude?

I took advanced flight training in Cessna AT-17's at Fort Sumner, NM.

Ahh, good ol' Ft. Sumner, where the men are men, and the women are, too!

brewha
03-14-2007, 01:17 PM
I hope that story has lost something in translation, because that's probably not the best way to look after a \$1,000,000 aeroplane in a thunderstorm :eek:!

It certaintly isn't the best way to look after a Mustang. I'll have to ask my dad about the detail, but from what I remember, he was planning on flying somewhere away from the storm where he could get the plane in a hanger. By the time he got on the runway, the storm was already on top of him. He could either take off into a T-storm and try and outrun it, or he could sit at the end of the runway and ride it out. He chose the latter.

FWIW, there were several planes destroyed in that storm that were tied down.

This was the EAA airshow in Oshkosh WI. Somewhere between '78 and '82. I'll look into it.

brewha
03-14-2007, 01:25 PM
Ok, maybe it was 1973 (http://www.airventuremuseum.org/collection/aircraft/Ford%20Tri-Motor.asp).

While at the 1973 EAA Fly-In, a severe thunderstorm ripped the plane from its tie-downs, lifted it 50 feet into the air and smashed it to the ground on its back. EAA subsequently purchased the wreckage.

Johnny L.A.
03-14-2007, 01:30 PM
I knew a guy who bought a nice Mustang for around \$2000...
My dad once told me that he could have bought a Corsair after WWII for \$600, but this was before he got into flying and he didn't know what he'd do with it. (I think it was after WWII -- only there were Corsairs on his ship when he was in an AD-4N squadron during Korea, so they were still being used in active service.)

In the late-1970s or early-1980s he mentioned that he had a chance to buy a T-34 airframe for \$5,000. I told him he should buy one. Heck, but two!. He said it would take \$50,000 to get them airworthy (they needed engines and instrumentation, but dad had a very particular -- read: 'expensive' -- idea of 'airworthiness'), so he wouldn't buy one. I really wanted him to. I saw a T-34B sell at auction at the Santa Monica Airport for \$250,000 back in the '90s.

David Simmons
03-14-2007, 01:41 PM
Ahh, good ol' Ft. Sumner, where the men are men, and the women are, too!Carefull there. Pat Garrett and Billy the Kid might drop by. Garrett has been known to shoot from ambush and Billy just shoots at random.

chrisk
03-14-2007, 01:44 PM
All I know is, it's harder taking off with a tail wind. :D

Because whatever you use to pick up speed (wheels, legs, etc) will be limited to a top speed X, but the tailwind reduces your effective airspeed to X - T, thus reducing lift.

Turn around and take off into the headwind. :)

Shagnasty
03-14-2007, 01:45 PM
But note that a respectable (and increasing) number of Mustangs flying today have only a limited number of original parts.

It is the George Washington's axe phenomenon (This is George Washington's axe. The blade has only been replaced twice and the handle three times). Anyway, fighter plane airframes are extremely strong and durable. There should be a good deal of that left.

brewha
03-14-2007, 01:46 PM

My understanding was that you could buy a brand new - still in the crate - Mustang for about \$500. Several people killed themselves in them though. The Mustang has so much power, that the torque of the engine(or P-factor or ascending blade theory or whatever else you want to attribute to it) would cause the plane to flip over on take-off.

Pilots that were trained in a Mustang knew that you had to hold it on the ground until you reached a certain speed. The plane would fly at a speed lower than this, but the ailerons didn't have enough airflow over them to couteract the torque of the engine.

Subsequently, these people would taxi down to the end of the runway, firewall the throttle, lift off the ground, flip over, and crash.

brewha
03-14-2007, 01:54 PM
It is the George Washington's axe phenomenon (This is George Washington's axe. The blade has only been replaced twice and the handle three times). Anyway, fighter plane airframes are extremely strong and durable. There should be a good deal of that left.

I'll have to disagree with you on this one. WWII fighter planes were only designed to last a few months. Extremely strong and durable = un-needed weight.

They were only built as strong as they had to be. The other problem is that aluminum does not have an infinite fatigue life. Over enough cycles, cracks form and pieces need to be replaced.

Chronos
03-14-2007, 02:02 PM
He could either take off into a T-storm and try and outrun it, or he could sit at the end of the runway and ride it out. He chose the latter.You say that as if outrunning a storm in an airplane would be difficult. Really, though, it'd be difficult to not outrun it. The storm has no engines, so it must be at rest relative to the ambient airspeed, while the plane is moving relative to the air.

Shagnasty
03-14-2007, 02:18 PM
They were only built as strong as they had to be. The other problem is that aluminum does not have an infinite fatigue life. Over enough cycles, cracks form and pieces need to be replaced.

Any fighter plane has to be extremely strong by definition. The airframe has to be able to withstand a high g-load and the severe punishment of a dog-fight. They are largely built out of non-corrosive airframe components as well. That is the difference between, say, cars and airplanes. A car from the 1950's is considered to be an antique and a museum piece. Airplanes from that era not to mention ones as far back as the 1930's (like the DC-3) are still used as working machines. Airframes have an indefinite life-span as long as they are maintained (not necessarily replaced peicemeal either).

Anyway, it isn't like the average owner of a P-51 Mustang is going to fly it enough to worry about metal fatigue. Metal fatigue may be a problem in airplanes that have been flown 10's of thousands of hours. Even 10,000 hours represents it being flown for an hour a day for 27 years and that just doesn't happen with an old warbird bought for fun and show.

Kevbo
03-14-2007, 03:18 PM
All I know is, it's harder taking off with a tail wind. :D

Because whatever you use to pick up speed (wheels, legs, etc) will be limited to a top speed X...

The problem isn't usually what speed the wheels can handle, but the finite length of the runway, and possibly obstructions beyond that which must be cleared (usually at least a barbed wire fence, sometimes power lines within a 1/2 mile, trees, etc) There is also normally a wind gradient, so you lose a few knots airspeed as you are climbing (trying to climb!) the first 10-30' or so.

Landing a hang-glider with a tailwind REALLY sucks!

And to answer the obvious question: (why would ya?) On fairly calm days, convection can cause a pretty stiff breeze to pop up with no warning, thus you can start a take-off into the wind, and have it turn into a tailwind just as you start to roll.

David Simmons
03-14-2007, 03:26 PM
I'll have to disagree with you on this one. WWII fighter planes were only designed to last a few months. Extremely strong and durable = un-needed weight.

They were only built as strong as they had to be. The other problem is that aluminum does not have an infinite fatigue life. Over enough cycles, cracks form and pieces need to be replaced.i would need to see a cite on this because I don't believe it.

In the first place, building a plane just strong enough to withstand the aerodynamic forces for some definite period of time would be diffcult, if not impossible. You don't know what those forces are without an extensive testing program and even that would be only statistical information.

In the second place, throw-away airplanes would creat a big logistical problem because of the continuous supply of planes that would have to be shipped overseas.

brewha
03-14-2007, 03:43 PM
You say that as if outrunning a storm in an airplane would be difficult. Really, though, it'd be difficult to not outrun it. The storm has no engines, so it must be at rest relative to the ambient airspeed, while the plane is moving relative to the air.

Let me rephrase that. Instead of :
He could either take off into a T-storm and try and outrun it, or he could sit at the end of the runway and ride it out. He chose the latter.

It should be:
He could either try to take off into a T-storm and outrun it, or he could sit at the end of the runway and ride it out. He chose the latter.

The thunderstorm was already on top of him before he took off.

As far as the durabiltiy of a WWII fighter plane, my cite is my dad's friend who owned a Mustang. He said that it costs him about \$800/hr to fly that plane. That is not the price of gas, it is the price to keep the thing flying.

If I take a step back, it doesn't make sense that it would be the airframe that would wear out that quickly.

What would make sense is that they would use a high horsepower engine that would eat itself up pretty quickly. An overhaul on a 12 cylinder engine can't be cheap. I don't if that is what he meant or not.

Shagnasty
03-14-2007, 04:36 PM
If I take a step back, it doesn't make sense that it would be the airframe that would wear out that quickly.

What would make sense is that they would use a high horsepower engine that would eat itself up pretty quickly. An overhaul on a 12 cylinder engine can't be cheap. I don't if that is what he meant or not.

Right. They say if you want to know what hobby aircraft ownership is like, build a big fire in your backyard, throw all your money into it, and if you have fun, owning a hobby plane is for you. \$800 an hour doesn't seem unreasonable at all. The problem isn't the airframe. The engine has a TBO (Time between Overhaul) of about 600 hours with a mini-TBO at 300 hours. That is bad by today's standards and would cost tens of thousands of dollars each time. However, that isn't to say that you have to hand over \$800 each time you fly it for one our. That is an average over time.

A P-51 is a fairly simple machine designed with a strong airframe and easy to maintain controls and control surfaces. Any maintenance is very expensive but that doesn't mean that they are replacing parts left and right or that the aircraft is largely unoriginal.

David Simmons
03-14-2007, 06:19 PM
As far as the durabiltiy of a WWII fighter plane, my cite is my dad's friend who owned a Mustang. He said that it costs him about \$800/hr to fly that plane. That is not the price of gas, it is the price to keep the thing flying.

If I take a step back, it doesn't make sense that it would be the airframe that would wear out that quickly.

What would make sense is that they would use a high horsepower engine that would eat itself up pretty quickly. An overhaul on a 12 cylinder engine can't be cheap. I don't if that is what he meant or not.That sounds a lot better. Yes, the engines on all warplanes in WWII, not just fighters, got the max power out of the min weight so they were stressed right to the max. Engine rebuilding and switching was an ongoing operation.

As a side light. At the beginning of the war the army decided that they were going to make sure that planes had top-notch, reliable engines. So they replaced engines with a rebuilt after a certain number of hours. To their chagrin reliability went down. The old "infant mortality" gottem.

In any manufacturing, assembly process there will be a certain number of defective parts, mistakes in assembly, or even inadvertent damage in assembly. So when an engine is rebuilt, some of them will fail quickly. The army changed to the method of running the rebuilts in on a stand (which they should have done in the first place) and leaving them in until it showed the early signs of wearout, such as excessive oil consumption, failure to maintaim rpm on the runup magneto test, etc.

David Simmons
03-14-2007, 06:29 PM
And as to costing \$800/hr. I don't think that's out of line. Every plane we had was tended to by a whole crew of specialists. There was a crew chief, and someone for engines, airframe, controls, hydraulic, electrical, communications and maybe the fuel system.

Every morning the crew gave all systems a thorough checkout to make sure it was as ready to fly as could be.

Shagnasty
03-14-2007, 06:39 PM
And as to costing \$800/hr. I don't think that's out of line. Every plane we had was tended to by a whole crew of specialists. There was a crew chief, and someone for engines, airframe, controls, hydraulic, electrical, communications and maybe the fuel system.

Every morning the crew gave all systems a thorough checkout to make sure it was as ready to fly as could be.

I get a whole slew of aviation magazines and I still find myself gasping sometimes when I look at maintenance and ownership figures. The \$800 an hour figure isn't because a P-51 is an antique or especially troublesome because of age. I would expect something similar for a modern plane of roughly the same design. The P-51 consumes about 60 gallons per hour of fuel at cruise. Avgas costs about \$5 a gallon right now so there is \$300 an hour right there. However, people don't buy P-51's just to putt around in them. Fuel consumption goes way up as they show off and have fun. Nope, \$800 doesn't sound out of line at all especially once you think in terms of aviation economics. I have to pay \$100 per hour to fly around rather slowly in an entry-level two-seater.

Richard Pearse
03-14-2007, 07:18 PM
This is true if you include the assumption of level flight at a constant airspeed, which is reasonable but fundamentally artificial. Sailplanes, for example, having no engine routinely violate this without difficulties.

Sure, at the expense of altitude (or potential altitude if you happen to be in some lift at the time.) If you're allowed to descend in the turn then you don't need to increase the load factor either. The only reason the load factor increases is because it needs to to maintain level flight. When you turn you must give something up. It can be either speed, altitude, or fuel.

Just to clarify faurther, are you saying shock waves can develop before the indicated speed of sound, compared to the actual speed of sound at that altitude?
When talking about the speed of sound, the indicated air speed alone is no longer a factor. An aircraft (that goes fast enough) has a Mach meter which uses indicated air speed and local air pressure to compute the aircraft's speed as a fraction of the local speed of sound. This is quite accurate and so the indicated speed of sound is essentially the same as the actual speed of sound.

What I was saying is that shockwaves develop well before the aircraft reaches the speed of sound. This is because the wing works by accelerating air over the upper surface. This means that there are local sections of airflow that are travelling a lot faster relative to the aircraft than the aircraft is through the whole air mass.

The air travelling over the upper surface of the wing will reach the speed of sound at somewhere between Mach 0.7 and Mach 0.8 (or 70% to 80% of the speed of sound.) This speed is when shockwaves start to form and is the start of the speed range known as "transonic."

A certain amount of aircraft design involves delaying or minimising the formation of these shockwaves. A swept wing is one design feature that achieves this. A swept wing doesn't accelerate air as much as a straight wing. Other methods include flattening the top of the wing which also reduces the acceleration of air.

chrisk
03-14-2007, 07:33 PM
And to answer the obvious question: (why would ya?)

Well, I'd imagine for a beginner, or someone who's never really thought about it...

1) Taking off is hard
2) A tail wind 'helps' you fly.
3) Thus, I want to take off with a tail wind.

:D

Chronos
03-15-2007, 01:35 AM
Quoth Scalar Weapon:This is because the wing works by accelerating air over the upper surface.No, a wing works by deflecting air downwards. Many wing designs do incidentally also accelerate air over the upper surface, but this is not inherent to the way the wing actually works. I would have thought that a seasoned SDMB veteran like yourself would have seen one of the previous threads on this question.

Richard Pearse
03-15-2007, 02:32 AM
Quoth Scalar Weapon:No, a wing works by deflecting air downwards. Many wing designs do incidentally also accelerate air over the upper surface, but this is not inherent to the way the wing actually works. I would have thought that a seasoned SDMB veteran like yourself would have seen one of the previous threads on this question.
I'm familiar with discussions on the topic. I'm pretty sure that regardless of what is responsible for the lift, all wings do accelerate air over the top surface, even if it is a flat plate inclined at an angle. Otherwise they'd be able to design wings that didn't develop shockwaves until Mach 1.

Xema
03-15-2007, 07:58 AM
If you're allowed to descend in the turn then you don't need to increase the load factor either. The only reason the load factor increases is because it needs to to maintain level flight.
Since both an aircraft in level flight and one descending at a constant rate must develop lift equal to their weight, you can limit the increased load factor in a turn only by accelerating downward (possible, but rarely practical).

Richard Pearse
03-15-2007, 08:12 AM
True, the increased load factor is required to maintain equilibrium, not necessarily level flight.

GusNSpot
03-15-2007, 10:14 AM
Too tight a turn to final has killed many an airplane.

In light aircraft, an extremely tight turn can be made if you are able to give up altitude. By maintaining an above average speed to use after leveling out to stop the rate of decent without stalling enables one to make some really quick and useful approaches. ::Do NOT try this without knowing your aircraft really, really well and doing a lot of practice at altitude.::

:: Our insurance required 3000 hours not just in type but in that particular aircraft. Got a heck of a deal since we both qualified for that. (Owner and myself) Took it to 5000 hours after he quit flying and I had the time. (He would do anything to save a buck.):::

The heavier the aircraft, the more attention must be paid to sink rates. Ask the big Iron guys, they can tell you all about momentum and resistance to changing directions.

Cardinal
03-16-2007, 12:24 PM
With a head wind it would create more lift and make the aircraft lighter....creating the POSSIBILTY for it to go faster. But it would also create a lot more resistance, so it will go faster with a tail wind.It may push it up from the ground more, but it doesn't change the mass. I think you're kind of confusing "weight" with "mass".

Xema
03-16-2007, 03:39 PM
It may push it up from the ground more, but it doesn't change the mass.
Doesn't do either. As noted above, once off the ground, a steady headwind or tailwind has no effect on a plane's flying qualities.

Shagnasty
03-16-2007, 06:27 PM
Doesn't do either. As noted above, once off the ground, a steady headwind or tailwind has no effect on a plane's flying qualities.

Correct. Once again, this is like the theory of relativity where reference points can change depending on what is referencing it. In a plane's case, the reference point is primarily the air and not the ground. Sustained air movement in any direction doesn't affect the plane's flying abilities at all. The plane is immersed in the air and that is all it knows. That is why planes can fly in hurricanes rather safely. The plane doesn't know it is in a hurricane. A sustained 150 mph wind is the same as no wind because the wind is measured in relation to the ground and the plane always sees it as 0 mph. Only the airspeed in relation to that counts.