The dreaded down-wind turn, fact or fiction? - Page 10 - RC Groups
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Aug 31, 2012, 10:59 AM
Roll on Two !
Chophop's Avatar
I think we need to look at the whole picture, air and ground speed. Into the wind, the plane will EQUALIZE at the same airspeed as it does 180* downwind. But it's true velocity will be different. In order to change velocity, acceleration must happen. There will be a period of slip before the plane equalizes airspeed ant the new true velocity. Just as the motor rotor has to be accelerated to equalize with the rotating field.

Just how long does it take to equalize ? I'd say the standard rate of turn eliminates any need for concern. And a pilot should get his ticket pulled for flying into a box canyon at ceiling and max power. What an idiot.
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Aug 31, 2012, 09:39 PM
You can't take the sky from me
cfircav8r's Avatar
There is no acceleration needed. When you are flying in calm air and you change direction i believe you will agree that the change in direction of the inertia is accouted for in the loading increase in the turn. Now the wind is travelling 20 mph south, the plane is travelling 60 mph south. So you have 60 mph of inertia heading south from the airspeed and 20 mph of inertia from the wind. Now you turn 180 degrees. The 60 mph inertia from airspeed is accounted for in the turn, but what about the wind inertia? It is still travelling south, so why does the plane need to make up for that inertia when it has not changed direction? The problem comes about when you try to look at ground speed. If you are going to try to relate what is happening in the air compared to the ground when it is not in contact with the ground, then you will also have to look at the relationship to the sun, moon, galaxy and universe and the speeds we get into there are beoynd the ability of any aircraft ever made.
Aug 31, 2012, 10:06 PM
Roll on Two !
Chophop's Avatar
I don't want to argue it any more. The velocity of the airplane is being changed and it needs time to change. I wouldn't take some passengers and go up monkeying around at or below MCA. Probably won't kill 'em, the plane would likely drop a bit and you instinctively recover. But I can't risk it when the possibility exists.

Engineers and laymen see things in very different ways. I need to see airspeed and ground speed. The airplane equalizes at a certain airspeed. It's absolute velocity is not affecting airspeed at equalization but it will when the plane needs time to accelerate due to balance changes. A person would probably have to force this to happen, when in a slower turnaround, the acceleration process begins at the start of the turn.

Funny about that, some people get mad and think someone behind them is speeding up at them when changing lanes. If both are at the same speed and the turner makes a 20* angled turn, at 50 mph, the car behind would appear to speed up by 11 mph. A 45 degree turn angle would make the car behind, appear to speed up by 25 mph. Why do they always speed up at me when I change lanes.
Last edited by Chophop; Aug 31, 2012 at 10:37 PM.
Aug 31, 2012, 10:55 PM
Roll on Two !
Chophop's Avatar
Here's a nearly identical argument over a fatality the FAA blamed on a quick 180 causing a stall. Seems one expert understands dynamics and all else skip past it and only see it in the equalized state. This is good practice in deliberation. I did get a bit grumpy but who doesn't ?
Aug 31, 2012, 11:04 PM
You can't take the sky from me
cfircav8r's Avatar
Not an FAA report it was an Irish investigation agency and as I have said this permeates full scale as well as modelers.
Sep 01, 2012, 06:58 AM
Roll on Two !
Chophop's Avatar
Just testing. Maybe he should have used a four blade propeller.
Sep 01, 2012, 07:31 AM
You can't take the sky from me
cfircav8r's Avatar
My last point, as I am out of ways to explain me stance. The down wind turn myth is when you are flying down wind and make your turn to final, you loose airspeed because of the wind and stall/spin into the ground. In your explanation of inertia being the culprit this turn should see an increase in air speed initially until it stabilizes. The problem should be occurring on the turn from initial climb out to down wind, not from down wind to final as is the case. So inertia, acting as you have explained, can not be the cause.
Sep 01, 2012, 10:33 AM
3D? I only got two thumbs!
The book "Stick and Rudder" by Wolfgang Langewiesche has a whole chapter entitled "wind drift" to explain this. Might be worth a read.
Sep 01, 2012, 10:53 AM
Roll on Two !
Chophop's Avatar
This is an educational quote, not meant to be harsh but ,

QUOTE from post 1 :

My question is, why do so many people get into trouble when turning downwind? Whether or not there is any logical explanation, it is apparent that many crashes have been caused by not correctly performing that maneuver.

And if we do not discuss these things we do not learn.
Sep 01, 2012, 11:20 AM
Roll on Two !
Chophop's Avatar
Sometimes, well, often, due to sun or obstacles, I have to turn a downwind base leg and crosswind final. What happens is I drop some on the downwind turn, pick up too much speed , then overshoot or bounce around on touchdown. I haven't stalled on those downwind turns, I always assumed because I was fast enough to avoid it.
Sep 01, 2012, 12:12 PM
Wookster's Avatar
had a thought after a windy day flying the helicopter. On an RC plane its hard to understand whats happening on the downwind, but the heli gave me some enlightenment. The air really is like a plane of glass moving over a table. The thing I am noticing isn't so much a lack of airspeed due to the percieved groundspeed on the downwind leg, but more what happens on the turn even if you haven't reduce airspeed. Here's where the heli comes in.

Windy day, flying circuits and doing low passes. Wind is coming straight down the runway, but is fairly smooth. on the upwind turns the heli sounds normal, flies normal, and turns its slow 180* to head downwind at alarming speeds. Then on the downwind turn, I'm pulling back hard and adding lots of collective and I can hear the blades ripping at the air. Then she straightens out and slowly passes down the runway fighting the wind. Nothing crazy, but she always turns like this. easy upwind, clawing downwind. Got me thinking about this conversation, and that got me thinking about aerobatics.

instead of the pattern lets take this debate vertical. Lets do a loop on a windy day.

if we just pull back and don't correct we get a strange out of round loop. In order to make it look good from the ground we have to pull up lightly at first, then bring her around hard as it goes inverted. Even if you maintain the same well above stall airspeed for the whole loop, when the wind is up, it takes more elevator to finish that loop cleanly.

I think the problem people get into isn't that they slow down on the downwind, as much as they don't realize whats going to happen on that turn. No the plane does not care what you doing as far as the downwind or upwind turn, it makes no difference. Until that is you add in one factor thats unique to the downwind turn. A fixed point on the ground you are aiming for (the runway). As the RC pilot you are going to make the downwind turn aiming for this fixed point, but the wind is pushing you away from the runway, you are now farther out than you want to be. So you tighten the turn to bring it around quicker. It's the same thing as the loop, just on its side Well if you are flying near the stall speed for landing and you have a heavy bird, you may have just caused a problem. Loading G's onto the wing and tip stalling it out of the sky. You could be coming in the same way and same speed you have for thousands of landings, but this one windy day you had to tighten that turn and crashed your bird.

I also think thats why this debt never dies out. It's true that the plane does not care if its an upwind turn, or a downwind turn when you are in the air, but at the same time it's true that on a downwind turn you will have a tendancy to increase the rate of turn to keep the bird from drifting too far downwind on the approach. Not sure if that translates at all in full scale, but in RC the turns are faster, and the effect of wind is greater due to lower airspeeds.
Sep 01, 2012, 03:11 PM
An itch?. Scratch build.
eflightray's Avatar

The down wind turn.

Perhaps a slightly different way of thinking. A very simple way of thinking because that's how I think.

A plane flies because the lift is greater than its weight.

A plane's wing loading is weight/area, which that area could also be called its 'lift area'.

So, a plane is flying level, then banks. The banked plane now has a smaller 'wing span', (as seen by gravity), so the 'lift area' is also reduced. Just as a plane in knife edge flight has a very small lift area, (still the same wing area, yes, but not lift).

The way a pilot tries to retain his wings lift when banked, (and not fall out the sky), is by using the elevator to pull the nose round so that the plane is now in a slip/skid whatever you want to call it, and still using its maximum wing lift area against the air.

Applying aileron to bank and elevator to turn, plus a change in direction, is going to increase the drag, slowing the plane down.

Now if the plane was originally flying level not much above the stall speed, any reduction in air speed is not a good thing.

So if a pilot was in a plane, he would be trying his darn hardest to not let the air speed get to near that stall speed.

As far as I understand it, a stall, especially banked, is a good way to fall out of the sky and possibly into a spin. So when flying a model plane I try to keep the speed up in a landing circuit, stretching it out further down wind, then use the longer final approach to reduce speed for the touch down.

I've added a simple drawing to explain my simple thinking regarding 'lift area'.
Sep 02, 2012, 10:24 AM
four candles
I've been following this with interest..i'm a newby with no previous experience of flying, full scale or RC, with a particular interest gliding. I have a Radian which by it's very nature flys fairly close to stall speed most of the therefore I have to fly upwind/downwind turns differently and have to be very careful on the downwind turn, but I keep reading that it's all in my head... is it..?
Sep 02, 2012, 03:03 PM
An itch?. Scratch build.
eflightray's Avatar
Well as your eye's are in your head, then I guess you could say "that it's all in my head" , it a problem with perception between air speed and ground speed.

Just don't fly on a windy day to start with, try to pick a calm day and you should have no upwind and down wind turn problems.

As you gain experience of flying then you can try slightly windier days.

By the way, your Radian doesn't have to be flying close to the stall, a little down trim will cure that, (just a click or two on the trim). Flying too close to the stall is what this thread is all about. People do it, slow down a bit more, then blame the resulting crash on everything else except themselves.

Hope you enjoy the Radian.
Sep 02, 2012, 03:55 PM
four candles
i've been flying the Radian for 18mths now so I have a little experiance, what i'm saying is because I fly close to stall I have to treat upwind/downwind turns differently, I don't crash, i don't have a problem with them, i'm not trying to lay the blame on anything or anyone.. but they are different..

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