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## Turning upwind to down wind and acceleration

I just want to throw this topic out into the ether for discussion, but mainly for clarification.There are some who remember the old discussion about the turn to downwind and the loss of airspeed that resulted from the increasing tailwind component. This has thuroughly been discussed and debunked. However, there are still some who think that the aircraft feels an acceleration as the groundspeed increases during the turn to down wind.Example: Assume you are flying in a 50kt direct headwind at 5,000ft with a 100kt TAS. You currently have a 50kt ground speed- we can all agree on that. Now assume you make a 180 degree turn to downwind. Your new ground speed is now 150kts (100kt TAS + 50kt wind.) The aircraft accelerated 100kts over the ground, yet the TAS remained the same throughout the turn. There are some that think that that 100kt acceleration can be felt by the pilot and the aircraft physically experiences this acceleration. Obviously we are talking apples and oranges here, as the frames of reference are mutually exclusive. Fying in IMC with no reference to the ground, doing a 360 degree turn, you would not feel yourself going faster as you turned down wind and slower as you turned upwind (we are not talking about turns around a point, in which this would absolutely be the case)The only acceleration felt in the turn to down wind (or upwind) is the acceleration inherent in the turn itself, as acceleration is defined as a change in direction and/or speed. However, the forces felt are the same in a steady wind and a no wind (shearing wind is a different subject.)Does anybody disagree with was written here?

Disagree---there is the force of the moving air (wind) relative to the earth accelerating the aircraft relative to the earth if the aircraft is able to convert the kinetic energy of the air to motion relative to the earth. The human body cannot differentiate the difference in g force, but an Inertial Navigational System can.

I would have to agree with the reply.
If ground speed changes then your actual speed has changed and therefore you have accelerated. You are faster and you have more inertia. Just because TAS stays the same it does not matter, your speed has changed.

Someone needs to go back and read "Stick and Rudder", methinks.

No acceleration through the air turning downwind, although increase in ground speed (inertial reference, relative to the EARTH) with the tailwind.

delta_v is of course correct.
What's confusing about this is mentally we tend to remain "grounded" on solid earth even when we are flying along in a moving airmass, totally disconnected from the ground below. Forget about ground and groundspeed and imagine when you start a turn, off your wing at about the center of the turn radius is a special air molecule that is shining, shining so bright you can keep it in sight. As you make the turn you will see that shining air molecule staying right there around the center of your turn radius. There is no "acceleration", just you, the molecule and the whole air mass behaving as a unit, just the same as if it were dead calm.

The origin of this topic came from an email conversation between myself and on of the contributors to this magazine. Due to the fact that there is no acceleration when turning in a wind as experienced by the aircraft, I was confused about the internal calculations made by an INS. For example, when taking off into a headwind of say 20 knots and a climb speed of 100 knots, an on board INS would only measure an acceleration of 80kts. This is fine because the inertial force measured equals the actual ground speed so distance can be accurately calculated. When turning downwind, that 20kt headwind turns to a 20kt tail wind and the ground speed increases from 80 to 120kts, yet the only acceleration measured by the INS is the centripetal force inherent in any coordinated turn (and how an INS calculates heading changes). So now the aircraft is traveling with a ground speed of 120kts, but the INS only measured an acceleration of 80kts, and therefore still thinks that this is the speed of the aircraft. The only possible way to reconcile the speed difference is with some external input into the INS, either with an Air Data Computer giving it TAS, or with GPS or DME/DME. The discrepancy between the TAS and the measured acceleration would give a wind speed that could then be used to calculate ground speed, which is what inertial navigation systems are really for.
The argument arose because this gentleman was convinced that the acceleration from 80 to 120kts ground speed in the turn could be felt by the INS, and that no other input was required to operate correctly when flying in a moving air mass. I disagree.
GPS makes all of this a moot point, and this was more for my entertainment to find out how an INS dealt with winds on takeoff back when it was a sole source of navigation. (changing wind speed in flight CAN be felt as an acceleration on the aircraft however- something as little as 1 knot over the span of 1 hour- and is not relevant to this discussion)

Maddog, would you please drop me a line at gabrieljb AT gmail DOT com?

What you state about the aircraft and pilot not feeling anything different in a turn with no wind or constant wind is correct, however your explanation is wrong. I'd like to discuss it with you.