Constant radius turns around a ground feature

[2FLY]

Looking for some help from other instructors with the following question?

"If flying a constant radius turn (constant IAS) around a ground feature with a constant wind where will be the Maximum and Minimum angle of bank?"

According to Wagtendonk, considering ground speed and drift as variables, Angle of Bank is greatest immeadiately downwind and least immeadiately into wind to produce the required CPF to maintain the desired radius.

However in practise (I have asked my peers and who tend to agree) the max/min A of B appears to be further toward crosswind than these points...:confused:
Why? presumably the above theory is not incorrect rather that there are more variables at play..?? any bright ideas?? I have read Cyclic and Collective to no avail and can't get my hands on a copy of Prouty at present.

Not sure if this is the place for such debate but doubtless I will soon find out... any imput would be appreciated.

One idea ready for rubbishing....

1: As the aircraft turns crosswind the rotor disc is tilted relative to the wind, this increases induced flow when downwind of the ground feature (disc tilted into wind) and decreases induced flow when up wind of the ground feature (disc tilted away from wind) with subsequent increase/decrease in TRT and CPF production.
To compensate the pilot increases/decreases angle of bank to maintain constant radius (whilst adjusting collective to maintain height)

[Arm out the window]

Good luck on your quest, but I'm afraid my 'usefulness v. neural activity expenditure' light came on when I started considering this one!:)

[Oogle]

Sorry, too much.

Why are you worried about this? If you are flying around a ground feature wouldn't your eyes be outside most of the time rather than thinking about angles of attack with crosswind and downwind?

Enjoy the flying and try not to hit the "ground feature" rather than trying to get your head around this complex question.:{

[rotorfossil]

Rather than worrying about a constant radius turn, it's more important to worry about not losing airspeed as you turn downwind due to the perceived increase in groundspeed. This has led to several people losing airspeed/ heading control/uncontrolled gyrations/loss of height/ vortex ring, all sorts of possibilities. Often as a result of orbiting friends house, trying to take photos etc and ending in a big hole in the ground.
Flying a constant radius turn at lowish height on a windy day with the constant variations of angle of bank and ground speed is a serious exercise, but basically involves rolling off the bank as you turn into wind and tightening it as you face downwind which is why it is prone to error.

[Eldon]

Hello All,

Just a thought but perhaps by 'downwind' he means flying the crosswind portion of the orbit on the downwind side of the spot :} A picture in this case would be worth 1,000 words. At any point when you were flying directly with or against the wind there would be no wind correction angle ( extra angle of bank)...only the bank angle required to keep the orbit going round. On the crosswind leg on the upwind side of the orbit your bank angle would decrease slightly because the wind is drifting you towards the spot...then opposite effect on the dowind side of the spot flying the crosswind leg.

Just a thought from a former CFI now flying lots of orbits around powerlines.

[Flingwing207]

It's a bit non-intuitave, this bank-angle vs where on the turn thing, but a little time spent with some graph paper will make it clearer. First draw (or imagine) a birds-eye view of an helicopter orbiting a windsock with the wind coming from the south - you are keeping a zero airspeed station over the center of the orbit's circle while the orbiting helicopter keeps a constant angle of bank. The path of the helicopter would look like a perfect circle while the windsock would seem to travel in a staight line to the south. (Imagine the orbit starting at the 12:00 position with the helicopter facing E - a CW orbit).

Now draw/picture you are holding a zero groundspeed station over the windsock while the orbiting helo does the same constant-bank circle. Now the path of the orbiting helicopter would look like the number 6, with the windsock sitting near the base of the 6.
Keep this in mind...

Now draw/envision the same helicopter doing "s" turns along an E-W road, with the same wind out of the south. Image the helicopter at its direct downwind (going N), crosswind (E), upwind (S) and crosswind (E) points, while holding a constant radius on either side of the road. Draw the required wind correction at each of these points, and figure the bank angles required to hit these WCA's. Finally transpose these diagrams to the original circle.
Or just take Shawn's word for it, 'cause he's right! :ok:

[[email protected]]

When you are into wind or downwind you will have no drift and so a still air AoB for the desired radius would be correct (but you will need to spend longer into wind than downwind if you keep the same IAS). When you are crosswind you will have to have a minimum AoB when you are being blown towards the centre and maximum AoB when you are being blown away from the centre.

So the answer is constantly varying AoB to compensate for the drift and achieve the desired ground track - but why would you want to do it?

[uncle ian]

Crab is, of course, absolutely correct but in practice it's not something to consciously consider. It's quite hard to do in anything over 10kts and the practical application (that I'm familiar with) is aerial camera work where directors look for it frequently.

[oldbeefer]

For once I agree with Crab! Better to judge the constant distance from the objective visually, using whatever bank is required. The VITAL thing is to monitor IAS when turning downwind and avoid a descent which, as has been mentioned before, can cause probs if the IAS is allowed to become too low. So, fly the AOB to maintain distance from the object and WATCH IAS!

[havoc]

I know this is not the answer to the orginal question, but this procedure was used in mountain flying to determine the predominate wind prior to going into an LZ.

We had to use a constant altitude, airspeed, bank angle, and ground reference point to start. Of course you had to have a large enough are for the turn or you used a different procedure (time between two points).

The end result was to identify the drift to determine the winds. I cannot recall any discussion about the aerodynamics other than retreating blade stall at the higher altitudes that we operated at 10-14000 ft (Rocky Mountains).

Maybe one of the operators that normally fly in the mountains may have an answer for you?

[delta3]

Sorry guys, I read a lot of answers but I may seam to disagree (unless I didnot read carefully enough)

Bank angle is a combination of two things

- a simple relation between ground speed and radius
- a drift effect because of side component

Take the following 4 cases.

1. upwind
- no side effect
- slower GS so smaller AoB

2. downwind
- no side effect
- higher GS so higher AoB

3. cross wind
- GS-IAS, so no effect
- side wind will make that your HDG will lag the track, so that you will have to make a yaw of more than 90° in the next 90° of ground track : so in the first 90° we turned inertially only for instance 80°
- so banking will build up

4. base
- GS-IAS no effect
- side wind effect will require you to turn heading by more then 90° in the first 90° of ground track, so in first 90° we will turn inertially 100°
- so banking decreases


So I would say banking is greatest downwind and least in up wind

Now turning perfect circles (drift offset) is still different from keeping the centre point at the same spot and is more difficult because in that case all 4 quadrants have different aspects.

A good way to test is try this in 40 knts, and look at the GPS track you manage. Getting circles is not easy...


My 2 cents worth