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I just started training and this may be a stupid question. But everyone keeps saying never land with a tailwind. Can somone explain aerodynamically why this is bad?

I am sure I will find out in training but I wanna know now.

 

Settling with power (aka vortex ring state) is one good reason. Time to hit the books dude.

 

I'll try to answer (even though I've only done the reading). The blades send vortices off the tips when the helicopter is generating lift. If you try to fly in these vortices, the angle of attack/relative wind for the rotor system is affected. Add in the effect of rate of descent flow through the rotor system during your approach, and you end up with stalled sections of the blades and less lift than you'd like to have for the power you're using. Somebody's going to ding me for saying the rotor is stalled--it's not completely stalled, but part of it is, and part of it is not generating lift because of excessive induced flow. Pulling more collective makes the situation worse. During descents, you need to maintain forward speed to fly out of your vortices or descend fast or slow enough to just avoid them. With a tailwind and insufficient forward speed, the vortices can get blown back into the rotor system (similar concept to wake turbulence avoidance), and you're one step closer to making a really quick descent.

 

There are some other good reasons related to the effects of a tailwind on the tail rotor and the helicopter itself (weathercocking tendency and tail rotor VRS can occur under some tailwind condtions), but they aren't necessarily related to approaches.

 

This isn't a great answer, and most of it's probably Greek to you if you haven't been studying, so the best thing to do is to start studying. The rotorcraft handbook is online for free (look up at the top of the forums page); Fatal Traps and a book by a guy named Wagtendonk can be ordered from Amazon--I like thier VRS explanations better. Fatal Traps has a picture of a VRS accident: the smoke plume illustrates the downwind condition in that case. This page also has a pretty good illustration of some of the forces at play--between this and the rotorcraft handbook you'll get a good idea of what's going on.

 

--chris

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Plenty of good reasons:

1. It reduces your exposure to the H-V curve

2. It allows you to keep translational lift much longer

3. It gives you a slower groundspeed for the same airspeed, reducing the area you need to slow down

4. It avoids needing buckets of power to go from translational speed to zero speed to having a wind up your clacker (which needs heaps of power and lots of back stick)

5. if you are in a B206, it avoids the potential for running out of pedal, erroneously referred to as LTE.

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Even if you made it through all of the above...it's a bit harder to control the hover with a tail wind. But as you can see...that will be the least of your porblems.

 

My instructor was introducing Settling with Power to me the other day. We couldn't even induce it upwind because it was a quite windy day. However, once we turned downwind at 1000-AGL, began to hover and descend just faster than 300ft per minute...we were in it (Settling in our own downwash while trying to climb). It was shaky and began to be more "unsettling" (no pun intended) wtih every inch of power he tried to pull in. After a few seconds of continued descent while trying to pull power (collective) we flew out of it. In that case, flying out of it was forward and right cyclic to gain speed and fly out of our own downwash (because the tailwind was about 17mph). We got out of it immediately !!!

 

Quite a rush to experience it for the first time...

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Making an approach with a tailwind is a situation incurs more risk, but is not one that has to be avoided at all costs. It can be done safely if you understand the additional risks and make allowances for them. If you plan do fly helicopters professionally you will on occasion be presented with situations (usually obstacles surrounding a LZ)where landing with a tailwind might be your safest course of action.

 

It is something you should discuss with your instructor and should practice before the completion of your flight training to demonstrate the factors involved and the precautions you need to take when making the desicion to land downwind.

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Sometimes there is just no other choice than to land or take off with a tailwind. It's not the first choice, but if it's the only choice, you may have to do it, and you need to know the dangers and how to do it safely. Therefore, everyone should get some practice doing it. It can take gobs of power, depending on the conditions and how you make the transition to forward flight, but it can be done if you use finesse and care. Like all helicopter flying, you have to pay close attention all the time. Accidents usually happen when the pilot isn't paying full attention to the task at hand.

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Indeed, landing with a tailwind is tricky, but can be done. Another thing to look out for is when you have a crosswind from the left at a low speed. This is when you get into LTE, loss of tailrotor effect. The ship yaws violently to the right and you must input left rudder... opps, pedal (old airplane habit) to avoid spinning into the ground.

 

Try landing with a tailwind of about five knots and then land in the headwind. you'll see and feel the difference.

 

Later

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If you're heavy, you want the wind from the left, because that requires less power. Just don't let it get behind you. The thing to remember is to keep up on the pedals, and don't let a rate of turn develop. As long as you keep the nose straight, it's not usually a problem - LTE occurs when the pilot lets a big rate of turn develop, and then it's too late. A steady hand and nervous feet are what you need.

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