iatorres2002

Can somebody please help me solve this mistery? I'm wondering what went wrong during a tailwind takeoff I performed this morning.

I had always known I was not supposed to take off with a tailwind, but since this was the only way I was going to be able to takeoff at the flying field I was at, I decided to take off with a tailwind thinking it was not going to be a problem.

The plane picked up speed amazingly fast during the take off run (due to the tailwind), I rotated it after about four seconds and it started to climb, when it was about 10 feet above the ground I noticed it did not want to climb anymore; as a matter of fact, it was descending! so I gave it full aft elevator to make it go up and the plane went down as if my elevator was reversed.

I checked my elevator after the crash and it was fine. If I pulled the stick back on the remote control, the elevator moved up and if I pushed the stick forward, it moved down. I know my elevator servo was not reversed.

What went wrong? Could it be the taildwind reversed the effect of the elevator or was that a take off stall? Can a take off stall occur at what seemed to be a high ground speed?

crashntrash

If a gust caught the plane, it doesn't matter how high it is. If the AIRspeed of the plane isn't sufficient to to produce adequate lift RELATIVE to the air's motion and speed, the plane will stall (that's what airspeed is). Ground speed means absolutely nothing to the lift production of a wing. If you were close the stall speed already and then gave the plane up elevator, you accidentally contributed to the stall. If you were at full power, then the stall was a power-on stall and those are more violent than power-off stalls.

If you're positive that the servo was not reversed (and it certainly sounds like it was not), then I can't think of any other reason for the crash outside of a stall.

How fast was the wind blowing? Was it gusting? What kind of plane were you flying? Planes designed to go fast or with symmetrical wings are not going to be very forgiving in this kind of situation.

I hope you can rebuild it.

MinnFlyer

My Feedback: (4)

Crashntrash is exactly right. Ground speed means nothing.

Let's say your plane has a stall speed of 30 mph - that is, it needs to be moving THROUGH THE AIR at 30 mph in order to keep from stalling.

If you have a 15mph tail wind, you start your takeoff roll and the wind will help to push the plane along - giving you a faster GROUND speed.

Now lets say to took off at 46mph (Ground Speed) 46 minus the 15mph tail wind is only 31mph or 1mph faster than your stall speed.

Now if you climb too quickly, you will loose speed and drop below stall speed - or - if the wind picks up with a small gust, that will do the same thing.

Either way, I totally agree that the reason you crashed is that you stalled - IE, even though your plane was moving fast in your eyes, it was not moving fast enough THROUGH THE AIR to keep flying

Mechanos

I agree with the stall theories present already. What I don't understand is your statement, "since this was the only way I was going to be able to takeoff at the flying field I was at". If you why couldn't you have turned around and taken off into the wind?

Stew99

Most likely it was stalled and if so pulling full up elevator is the worst thing to do. Contrary to intuition you have push in down elevator to get the nose down and try to get the wing flying again. The 3D type planes can usually blast their way out with full throttle.

Bax

My Feedback: (11)

Except when you're trying to get somewhere, have limited space, or are about to hit something, groundspeed is irrelevant to an airplane. Airplanes fly only in relation to the air, as far as how they work. A downwind takeoff is no more dangerous than an upwind takeoff if you have the room to accelerate to the proper airspeed and climb out at the reduced climb gradient. The wind coming from the rear will not 'blow' the tail around unless you're flying in some seriously-gusty wind...and then it's likely not wise to attempt flying at all.

With models, because we don't have real-time airspeed indication, we usually rely on the visual cue of its speed to determine when to initiate the takeoff rotation. If the model's taking off downwind, the model will be moving a lot faster than normal, so we have the illusion that it's moving fast enough to fly, even if it really isn't.

The model left the ground to early, flew out of ground effect and stalled. There was nothing wrong with the model or the equipment.

(edit for typos)

Geistware

My Feedback: (16)

When you have a plane that is loosing lift, you want to nose it DOWN not up.
Eveyones statement on stall is accurate.

BMatthews

In additon to the excellent answers already there may be a further complication to add to what happened as well. If the wind was strong then you have a significant ground shear effect where the windspeed is lower at ground level than it is 3 feet up and 10 feet up it's even stronger. So if you take off at ground level and assume a climb that does not result in any increase in speed then as you move up into the stronger wind your airspeed can easily drop and the stall results. This will be particularly bad with a heavy and low powered model that is slow to accelerate under the best of conditions and doesn't accelerate worth a hoot while trying to climb. Adding in the high drag from a near stall condition during a minimum airspeed takeoff and you have a wonderful recipe for the crash you had.

Next time taxi down to the other end and take off into the wind.

crashntrash

iatorres2002,

I'm sorry you crashed, but I'm really glad you started this post. It's been a good reminder to me about some principles by which I fly. I was trained by a full-scale, commercial rated, IFR pilot. In other words, he knows his stuff. Because of that training, I'm a stickler for certain rules regarding safety and flying (yes, in that order). One of them is to ALWAYS take off and land into the wind, or at least a crosswind when I can VERY carefully climb out turning into the wind. Most times, a plane with a long tail moment, like a trainer, will automatically "weather-vane" into the wind anyway. Bax did a good job of explaining Center of Pressure in another post that describes this principle of airplane movement. Just search on Center of Pressure and you'll no doubt find it. Thanks Bax, that was a good one!

An understanding of how planes move through the ocean of dynamic air in which they are moving is very important to safe and successful flying.

Best wishes on a quick rebuild. Let us know how it goes.

iatorres2002

Thank you guys for all the responses. I now now fully understand the difference between an upwind takeoff and a downwind take off. I had to learn about downwind take offs the hard way but it has been an experience I most definitely needed, as I also fly full scale airplanes. Just imagine getting that experience in a real airplane! I wouldn't have lived to tell about it. Thank you guys.