Departure with a crosswind needs less runway than a departure with no wind?

LeSving wrote:

Newton’s third law is about forces between objects, it’s not really about accelerating a fluid or any dynamics at all.

Do you mean that Newton’s third law only applies to rigid objects and not fluids? Really!?

Airborne_Again wrote:

Do you mean that Newton’s third law only applies to rigid objects and not fluids? Really!?

Perhaps another way to look at what LeSving is saying, is to ask what is the mass of your fluid, or where does the air end, or where is the boundary? You won’t be able to answer those questions because the air is a continuum.

It is also true that newtons laws do break down when describing motion at the molecular level.

However lift should be be same, for any given weight, ignoring any contribution lift might have towards accelerating upward.

Let’s try to get back to the essentials and forget the nitty (molecular) details where possible.

There still is the belief that wind increases the airspeed of flying aircraft moving from flight perpendicular to the wind to a flight against the wind, while it doesn’t increase the airspeed at all. The climb might be better, but that is in relationship to the ground and not as such within the air(mass) that the aircraft is flying through.

AeroPlus wrote:

Let’s try to get back to the essentials

So you have two identical TB-20s (one with 20kts crosswind, and the other nil wind, and there is magically no wind shear).

You accelerate to an arbitrary but identical IAS (say 70knts) and begin to lift off, at this point both aircraft have considerable lift capability in excess of aircraft weight.

Lets first consider Peter’s scenario and assume that the crosswind causes increased dynamic pressure to be measured at the pitot tube such that 67knts GS results in 70knots IAS for the crosswind TB-20, while the nil wind TB-20 will have 70knts GS and IAS. Remembering the crosswind TB-20 should accelerate slower because of the added drag.

Once airborne, if you already counted the increased pressure at the pitot tube you can’t double count it now as the aircraft yaws in response to the crosswind.

You now need to accelerate to the correct IAS and arrive at the screen height (i.e. 50ft), at lets say 80knots, so 10knots more for both aircraft. The crosswind TB-20 has a lower ground speed at the screen height and the angle of climb will be greater to reach that point.

The other scenario as discussed earlier is to imagine the ASI is immune to any such errors and effectively only measures the dynamic pressure rise due to aircraft’s forward movement through the air.

In which case both aircraft should leave the ground at 70Kts IAS and GS, with the crosswind TB-20 still having lower acceleration while on the ground, but getting a small increase in IAS after lift off due to inertia.

In the end I think it’s all a wash, and as said previously the pilot made his original calculations correctly.

Hey Pedro, is this discussion for real, or are you trying to wind-up (pun intended) a poor old Scottish farmer?

Is there really anyone who “calculates” their take-off run?

And if so, do they believe that the magic number which emerges from their “calculation” (i.e. taking a figure based on tests on a somewhat similar airplane flown by a somewhat different pilot and applying half a dozen fudge-factors before dolloping a 30% Jesus-factor on top)… where was I… ah yes, do they actually believe that figure is any more useful than the aforesaid POSF licking his finger and prodding the ground with his zimmer frame?

If we watch a STOL competition for a few minutes – yes, I agree, it’s about as exciting as watching paint dry, but we only need to watch a few take-offs – we see that the SAME airplane with the SAME pilot can do take-offs which vary by 10 percent, or more.

We also see that the ground run of similar airplanes with different pilots can vary by 100%.

So in answer to your OP, a bit of cross wind is probably about as significant as whether the crew ate a good curry the night before (given that methane is lighter than air).

AeroPlus wrote:

Interesting how easily you dismiss this book as the Stick and Rudder book written by Wolfgang Langewiesche is still considered relevant

I thought I hadn’t read it. Googled it, and downloaded an electronic copy, only to find I already had it. I have only read parts of it, at least that I can remember. IMO he should have compressed it into 1/10 of the size, and it still would be lots of words and repetitions. But he do no such thing as explaining lift. On the contrary, he focus on what a pilot needs to know about lift to become a better pilot, considering he/she knows very little up front, and is not an “engineer” as he describes. What he does is to take common engineering/flight aspects, and explaining them in in such a way that the reader with no relevant background is able to get a mental image of the mechanics of how an airplane flies. He probably does that well (although using lots and lots of words IMO). For a PPL pilot who started with aviation at the age of 30+ and with no other related background, this book probably is very good ? For all others, it’s mostly a whole bunch of words. The rudder chapter should be completely revised though, it’s just nonsense, proven wrong in the last half a century.

@LeSving: I do agree that the author needs a lot of words and repeats himself indeed. For myself, I remember that it helped me a lot to better understand the principles of flight. He does have a chapter on wind where he explicitly explains in a lot of words and with quite a bit of repetition that wind has no effect on the airspreed.

Jacko wrote:

Is there really anyone who “calculates” their take-off run?

I don’t in the Super Cub, except in the crudest terms, but I surely do in a bigger ME type which is likely to be runway limited in some situations. It may also be limited by OEI climb performance to do the IFR departure.

It may well be that in your aircraft if you can land you can definitely take off, which is broadly true for a Super Cub too, but many types are much more performance limited.

Peter wrote:

This is also why the average GS calculated over any large number of flights is always less than the TAS – because the “average wind” (the wind vector calculated over some large number of flights) is an exact crosswind except for the impossible scenario of always flying in calm air, yet even an exact crosswind gives you a headwind when you are flying.

No. You are thinking of average over distance, but the right average is over time. That’s the crucial difference, and the reason your “average wind” (over time!) will have headwind, not only crosswind. The two reason the average GS is less than the TAS is:

1. Crosswind (with respect to track) costs you GS, too, since you have to turn into the wind to maintain track, which essentially transforms crosswind into headwind :)
2. More importantly, as per Amdahl’s law, the headwind affects you more than the tailwind. For a TAS of 100kts, and a wind of 50kts:

• a headwind will multiply your enroute time by a factor of 2. That is, one 1h for a 100nmi trip.
• a tailwind will only cut your enroute time by a factor of 1.5 (that is 3/2), that is multiplies it by 2/3. That is, 20min for a 100nmi trip.

So, in total you’ll have spent 2h40min for 200nmi. Any crosswind only further reduces your GS.

Another way to see it, the average is to be computed over time. You’ll have 50kts headwind for 2h and 50kts tailwind for 40min. So the average is (-50*120 + 50*40)/(120+40) = -25. Meaning your average wind is 25kts headwind, your average speed (over time) is 75kts, and you spend 200/75=2h40 min for 200nmi.