Unloaded turns

[tgrayson]

There has been some discussion of the value of "unloaded" steeply banked turns in order to get a "free lunch"...the rate of turn without the danger of the increased load factor. The technique is to roll into a steep turn without increasing the AoA.

I see this as having limited value for two reasons:

1. It's primarily the load factor which is responsible for the increased rate of turn in a bank, so you're giving that up, and
2. Without increasing the AoA, the airspeed of the aircraft will increase while the bank is held until the load factor ends up where it should have been based on your bank angle.

So is there any value at all in the technique? Let's run some numbers, using this diagram and formula as the basis of our calculations:

The first three entries in the table are "true banked turns" where the AoA is increased as the bank is entered to keep the airspeed the same. Here I assume a speed of 70 knots. (I use the term "true banked turn" because your turn rate is less than what you'd expect based on the bank angle, which is exactly the condition in a slipping turn.)

The entry highlighted in yellow is the unloaded turn. I show two values for some entries in the unloaded turn row, because the airspeed increases steadily during the maneuver.

Here are some interesting observations regarding the calculations:

• The smallest turn radius occurs with the 60° true banked turn, as you’d expect, but in this case, the aircraft stalls.
• The next best turn radius occurs with the 45° bank, with a 11 knot margin over the stall speed.
• The unloaded 60° banked turn has the third best performance, but the aircraft will steadily accelerate, generating an increasing load factor and increasing stall speed. Interestingly, the turn radius does not change as the load factor increases, because the increased velocity exactly compensates for it.

The unloaded turn doesn't get the expected performance because it only generates half of the centripetal force that a true banked 60° turn would. So in terms of safety and performance, you're better off maneuvering at the proper airspeed for the bank angle you've chosen. (IMO)

 

[Joe Gremlin]

Yep. Rudder, just do it. If you ain't steppin' enough then you're slippin'. Not that slippin' is a bad thing when you need it. But if you're trying to turn, you should just turn. If you're trying to slip, you should slip. And if you're trying to do both, you're probably getting ready to land a Stearman.

This is yet another area where there ain't no such thing as a free lunch. I wouldn't fly an unloaded turn unless I also wanted to lose altitude or airspeed or both.

 

[MikeD]

Agree with this, but have to ask: Where is the discussion of doing this? In primary training? Even without seeing the math involved (which hammers home the technicalities of this) unloaded turns to achieve less or minimized load factor doesn't pass the common sense test; what you save in loading, you lose in turn performance. As you mention with the "free lunch" description, everything does cost something.....nothing's for free; you'll pay for it somewhere performance-wise. There's no real positive cost-benefit tradeoff to unloaded turns, IMO.

It's the same with planes that don't naturally turn much when you roll into a bank, those you have to make turn. F-117/T-38 for example, if you roll into a 30 (40/50/60/etc) degree bank, the plane still flies essentially straight with little heading change, just in a 30 degree bank, or whatever bank you're at. Unless you load it up yourself with back pressure on the stick when rolling into a bank, it won't naturally turn well. Maneuvering-wise, you have to roll to place your lift vector where you want it, and pull the aircraft around the turn, generally speaking. And rudder isn't used (nor needed).

 

[tgrayson]

unloaded turns to achieve less or minimized load factor doesn't pass the common sense test;

Uhm, it may not pass *your* common sense test, but think the typical pilot doesn't understand the role of load factor in producing the turn; they view it as a side effect, rather than what you're trying to achieve.

As for where it's being discussed, I caught sniffs of it in our multiple recent discussions of related topics.

F-117/T-38 for example, if you roll into a 30 (40/50/60/etc) degree bank, the plane still flies essentially straight with little heading change, just in a 30 degree bank,

Sounds like low directional stability.

 

[MikeD]

Uhm, it may not pass *your* common sense test, but think the typical pilot doesn't understand the role of load factor in producing the turn; they view it as a side effect, rather than what you're trying to achieve.

As for where it's being discussed, I caught sniffs of it in our multiple recent discussions of related topics.

.

Good point. Reason I asked was I was wondering if this was some kind of new technique being taught in some circles, or whatnot. Good tech discussion highlighting the reasons behind what benefits one may think they're gaining from it, may not really be what they're getting.