Tailwind on Takeoff

[awacs94]

Hello,

Can anyone explain why on takeoff a tailwind increases takeoff distance. I know it relates to ground speed but am slightly confused at to exactly the factors that contribute to the longer takeoff roll.

Thanks.

[JEP]

Originally Posted by awacs94

Hello,

Can anyone explain why on takeoff a tailwind increases takeoff distance. I know it relates to ground speed but am slightly confused at to exactly the factors that contribute to the longer takeoff roll.

Thanks.

Well for starters, you don't have the added benefit of the wind helping your wings with lift.

I will leave the more technical answers for others....

[CamYZ125]

Essentially, it takes longer for an airplane to achieve the required airspeed for lift off if there is a tailwind. Remember, it's all about the amount of airflow over a wing (forward to aft).

Let's say we have an airplane that can takeoff at 50 knots. In a no-wind condition, we have to accelerate to 50 knots to takeoff.

Now let's add a 40 knot headwind, straight down the runway. The same airplane now only needs to accelerate 10 knots before reaching it's takeoff speed of 50 knots (remember, it has the 40 knots of wind, plus the 10 that we gained from moving down the runway).

Now let's change that headwind to a tailwind. If we've got 40 knots coming from behind us, we're going to need to travel down the runway at 40 knots to essentially achieve 0 knots airspeed over the wings. Then we have to accelerate another 50 knots until we've got enough airspeed to lift off.

Hopefully that makes some sense.

[Realms09]

Main idea:
While it is true that the tailwind "pushes" you for part of the takeoff roll, this is only the part of the roll in which you have negative airspeed. Once you accelerate forward to zero airspeed, the negative drag goes to zero and the "pushing" wind goes away. From this point forward the takeoff is just like a zero-wind takeoff, but you've wasted space accelerating to get to that point.

Other things to think about:
As has been mentioned in some other threads, parts of the propeller are typically stalled at zero airspeed, at least in a constant pitch propeller. A tailwind would only make this situation worse, further reducing the practical power the engine is delivering.

Rolling friction coefficient is more or less constant with ground velocity. I am not sure what sort of lift coefficient a wing develops when flown backwards, but this could change the force with which the plane pushes into the ground, which could in turn change the amount of friction force retarding the aircraft during the initial part of the roll.

[frog_flyer]

Let's say you need an airspeed of 55 knots to get off the ground.

If you have a 5 knot tailwind, your airspeed is -5k when you're not moving. (think if the wind was calm, to have the same effect you'd be going backwards at 5 knots)

So in order to get that 55 knots of air speed, your ground speed is going to be 60knots.

Hence, longer takeoff roll.

[awacs94]

Thanks guys.

[CamYZ125]

Originally Posted by awacs94

Thanks guys.

That's what we're here for.

[Flysher]

Originally Posted by CamYZ125

Essentially, it takes longer for an airplane to achieve the required airspeed for lift off if there is a tailwind. Remember, it's all about the amount of airflow over a wing (forward to aft).

Let's say we have an airplane that can takeoff at 50 knots. In a no-wind condition, we have to accelerate to 50 knots to takeoff.

Now let's add a 40 knot headwind, straight down the runway. The same airplane now only needs to accelerate 10 knots before reaching it's takeoff speed of 50 knots (remember, it has the 40 knots of wind, plus the 10 that we gained from moving down the runway).

Now let's change that headwind to a tailwind. If we've got 40 knots coming from behind us, we're going to need to travel down the runway at 40 knots to essentially achieve 0 knots airspeed over the wings. Then we have to accelerate another 50 knots until we've got enough airspeed to lift off.

Hopefully that makes some sense.

I sat here thinking about this, and having never taken off with a 40 knot tailwind i cant quite get it straight in my head. Are you saying that in order to takeoff in this case you would have to accelerate to 40 knots groundspeed before the airspeed indicator started registering? Because I imagine myself standing out on the wing during the takeoff roll, and while I would certainly feel wind from behind I would think that the instant the plane started to move forward there would be SOME relative wind from the front, thus adding to the indicated airspeed.

[SteveC]

Originally Posted by jfleisher

I sat here thinking about this, and having never taken off with a 40 knot tailwind i cant quite get it straight in my head. Are you saying that in order to takeoff in this case you would have to accelerate to 40 knots groundspeed before the airspeed indicator started registering? Because I imagine myself standing out on the wing during the takeoff roll, and while I would certainly feel wind from behind I would think that the instant the plane started to move forward there would be SOME relative wind from the front, thus adding to the indicated airspeed.

Picture yourself standing in a river. The river is flowing at 10 mph, pushing at your back. Now, instead of standing still you start walking downstream at 5 mph. Are you going to feel water pushing at your back or your front?

[Flysher]

well given your example I'm going to say you will feel it at your back but not your front.. I dunno lemme run over to the creek and see

[seagull]

Originally Posted by jfleisher

I while I would certainly feel wind from behind I would think that the instant the plane started to move forward there would be SOME relative wind from the front, thus adding to the indicated airspeed.

You feel wind from the front and back at the same time? Where does the wind from the front come from if you're not moving faster than the air?

[SteveC]

Now let's add a treadmill.....

[Flysher]

Originally Posted by seagull

You feel wind from the front and back at the same time? Where does the wind from the front come from if you're not moving faster than the air?

Sorry guys I was really tired last night, it makes sense now that I got sleep.

[Flysher]

Originally Posted by SteveC

Now let's add a treadmill.....

Hey i DO get that one... it will take off just fine

[casey]

edit: i should read the thread before making treadmill jokes.... beaten.