Question About Descending Turns

[Badmachine]

During a descending turn, if angle of bank (AoB) and angle of descent (AoD) remain constant or unchanging, will that therfore result in a constant or unchanging turn radius?

[rudderrudderrat]

Hi Badmachine,

In a straight line, once you are established in a steady climb, descent or level flight - you'll still need the same lift from the wings.

Your radius of turn only depends on your bank angle, TAS and hence the "delta g" you pull in order to hold the same climb, descent or level flight.

[p51guy]

I spent a day in my 757 measuring turn radius at FL350 vs 5,000 ft descending and verified that the lower you fly, the less the turning radius. I used a 25 degree bank throughout the maneuver at 220 K indicated airspeed. My GPS data is available if you want it. Both NASA and Boeing sent that info back so I have 2 copies.

[Badmachine]

Thanks for the replies.

Still looking for that elusive formula or descending turn calculator that would allow one to manipulate different descending turn variables.

[AerocatS2A]

Originally Posted by p51guy

I spent a day in my 757 measuring turn radius at FL350 vs 5,000 ft descending and verified that the lower you fly, the less the turning radius. I used a 25 degree bank throughout the maneuver at 220 K indicated airspeed.

No, you verified that the slower you fly, the less the turning radius, which we all know so what was the point? It seems like an expensive exercise.

[hetfield]

@p51guy

Please take your 757 and measure the miles travelled at 220 kt IAS at FL 350 versus 5.000`.

Thx

[aterpster]

During a descending turn, if angle of bank (AoB) and angle of descent (AoD) remain constant or unchanging, will that therfore result in a constant or unchanging turn radius?

Radius of turn is a function of bank angle and ground speed.

[rudderrudderrat]

Hi aterpster,

Radius of turn is a function of bank angle and ground speed.

True - but only to an observer on the ground.

[aterpster]

True - but only to an observer on the ground.

Or to the pilot looking at a nav map display, or to the flight control computer system.

The construction of radius-to-fix (RF) legs is all about radius of turn, ground speed, and aircraft bank angle limits.

[rudderrudderrat]

Hi aterpster,

All true again - I should have included "or to an observer using a ground based frame of reference."

How does the Nav computer calculate the RF (radius-to-fix) leg? Does it use TAS + Wind Vector or does it simply use the instantaneous ground speed?

[theshed]

deffo have to thing about TAS as well if you keep constant AOB and AOD then your speed will increase as you go.

[aterpster]

How does the Nav computer calculate the RF (radius-to-fix) leg? Does it use TAS + Wind Vector or does it simply use the instantaneous ground speed?

An RF leg that is part of a charted procedure (presently [mostly] only RNP AR, but will become more widespread than that) is fixed in the nav database with an arc center point and a radius value. The criteria used to design the RF legs in RNP AR have TAS values based on altitude and adverse wind assumptions agreed to by the FAA and industry. The idea is to not exceed a 25 degree bank under the worst circumstances.

As to RF legs computed "on the fly" by the avionics for a course change at a fly-by waypoint, I defer to the avionics gurus, and it could vary from vendor to vendor. Those "on the fly" RF legs certainly do vary with ground speed.

[Badmachine]

"Radius of turn is a function of bank angle and ground speed."

Could you elaborate? It seems that the "V" in the formula for radius of turn refers to airspeed instead of groundspeed (at least to my knowledge).

Thanks.

[theshed]

Well quite often you'll find formula's use an imaginary world. The difference being thought about here is the difference in still air or the real environment.

The theoretical radius is a function of TAS but the actual radius of a turn relative to a point on the ground would be GS.

Or am i barking up the wrong tree??

[rudderrudderrat]

Hi theshed,

You are barking up the correct tree - but I wouldn't call the pilot's perception an imaginary world. The radius of turn of the aircraft through the airmass depends on the TAS as Badmachine points out.

After the wind vector is added, we'd get the ground track, GS and ground radius.

[aterpster]

Indeed it is TAS in the air mass but ground speed for ground reference maneuvers, such as holding patterns, course changes, and RF legs.