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Old Apr 14, 2004, 09:39 AM
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Full Size Boeing 727 Pilots - Adverse Yaw Question

I'm close to a first flight on a GWS twin EDF-50 powered, 45"WS Boeing 727-100 model. My controls are ailerons and flying stab. I know I'm going to experience adverse yaw! I have made provisions for adding a rudder and if she flys well at 12.5oz, I can add the extra weight of the rudder servo and linkage.

My question is: If you could do an uncoordinated aileron only bank/turn, how bad was the adverse yaw at slow airspeeds? Was it possible to do an uncoordinated turn with ailerons only at all?

These little questions keep me awake at night. Thank you. David
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Old Apr 14, 2004, 10:04 AM
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That's something I've been concerned about as well with the 727 that I'm working on. To hopfully minimize adverse yaw (dutch roll?), I have my ailerons set up for differential throws.
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Old Apr 14, 2004, 10:12 AM
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If you rig your ailerons so that you have about twice as much up travel as down, you are highly unlikely to have any adverse yaw at all.
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Old Apr 14, 2004, 10:32 AM
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Most 'jet style' planes I have flown are so darned fast that a mm of throw is enough to roll them sideways in a split second. I wouldn't worry. I have dropped more wings in slow turns using rudder control than I have on ailerons.
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Old Apr 14, 2004, 11:34 AM
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Just configure your ailerons to move up only, and you'll be fine.
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Old Apr 14, 2004, 12:22 PM
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Vintage1: I ain't gona be flying fast!!!

Well my ailerons are larger than scale. ie: more area. Their hinged on the top surface for more up than down. At slow speeds, or any speed, there's no way to rid adverse yaw, in this configuarition, without a rudder. At higher speeds, you can get away with bank and yank. At slow speeds, w/o rudder, you need to keep a modest roll/bank angle, or you'll start to slide down.

Mostly interested in an actual 727 Pilots observations.
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Old Apr 14, 2004, 04:15 PM
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Quote:
Originally Posted by David A Ramsey
Vintage1: I ain't gona be flying fast!!!

Well my ailerons are larger than scale. ie: more area. Their hinged on the top surface for more up than down. At slow speeds, or any speed, there's no way to rid adverse yaw, in this configuarition, without a rudder. At higher speeds, you can get away with bank and yank. At slow speeds, w/o rudder, you need to keep a modest roll/bank angle, or you'll start to slide down.

Mostly interested in an actual 727 Pilots observations.
I must politely completely disagree with you. Adverse yaw is caused by the drag effect of a down aileron deflection being more powerful than the lifting caused by deflecting that same aileron downward. If you set up an aircraft to have much more up aileron throw than down, along the lines of twice as much, you can, in many cases, completely eliminate any visible adverse yaw. I have done this on many models.

While not a 727 pilot, I have flown a number of full scale aircraft (about 20 different types) Certain aircraft are really prone to adverse yaw, particularly the classic Piper Cub and training sailplanes like the 2-22 and 2-33. Other aircraft, with some differential added to the roll controls, do much better and need far less rudder to compensate for the minimal residual adverse yaw.

Comparing the flight charateristics of a full scale 727 and your lightly loaded model is not the most valid thing to do, considering the huge difference in wing loading and Reynolds numbers.

Most modern jetliners rely on spoilers for roll control and have, at most ,a very small aileron in play on the TE of the wing. Note that with spoilers for roll control, there is roll without adverse yaw., since no surface deflects downward

I had a scaled up 132" Klingberg wing with elevons only: no rudder control. A little time spent teaking the aileron differential gave perfect, no adverse yaw nicely coordinated turns.

If you make provisions in your aileron linkage for adding some differential beyond what is accomplished by top hinging, I predict with confidence that you will never miss having a rudder, unless you wish to do stall turns or spins with your 727 model..
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Old Apr 14, 2004, 05:02 PM
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Quote:
Originally Posted by Thomas B
I must politely completely disagree with you. Adverse yaw is caused by the drag effect of a down aileron deflection being more powerful than the lifting caused by deflecting that same aileron downward. If you set up an aircraft to have much more up aileron throw than down, along the lines of twice as much, you can, in many cases, completely eliminate any visible adverse yaw. I have done this on many models.

While not a 727 pilot, I have flown a number of full scale aircraft (about 20 different types) Certain aircraft are really prone to adverse yaw, particularly the classic Piper Cub and training sailplanes like the 2-22 and 2-33. Other aircraft, with some differential added to the roll controls, do much better and need far less rudder to compensate for the minimal residual adverse yaw.

Comparing the flight charateristics of a full scale 727 and your lightly loaded model is not the most valid thing to do, considering the huge difference in wing loading and Reynolds numbers.

Most modern jetliners rely on spoilers for roll control and have, at most ,a very small aileron in play on the TE of the wing. Note that with spoilers for roll control, there is roll without adverse yaw., since no surface deflects downward

I had a scaled up 132" Klingberg wing with elevons only: no rudder control. A little time spent teaking the aileron differential gave perfect, no adverse yaw nicely coordinated turns.

If you make provisions in your aileron linkage for adding some differential beyond what is accomplished by top hinging, I predict with confidence that you will never miss having a rudder, unless you wish to do stall turns or spins with your 727 model..
Appreciate your thoughts. I haven't forgoton about the drag effect of the down aileron. For my model I have limitations in setting up the ailerons and I'm expecting adverse yaw. I'm interested in a 727 pilots point of view on using just ailerons at slow speeds. Not looking to make a comparison, or how to reduce adverse yaw, just curious how the real 727 might behave.
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Old Apr 14, 2004, 05:11 PM
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Interesting points.

On an actual 727, during a turn with autopilot engaged, there is an "up" elevator input to compensate for loss of lift during the turn, this is referred to as "versine".

The 727 utilizes the outboard ailerons only when the flaps are extended. This is progressive. The further the trailing edge flaps are extended, outboard aileron travel also increases.

The major design reason for not using outboard ailerons during flaps up flight (high speed flight) is due to wing twisting. The outboard aileron deflected during high speed flight would have the tendency to twist the wing; the aileron thus acting as a control tab and potentially causing a control reversal.

This is also why roll spoilers are used. The spoilers, mounted toward the more rigid center wing does not have the effect of twisting the wing.

Inboard ailerons are fulltime though.
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Old Apr 14, 2004, 05:22 PM
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Interesting information, FDXmech.

I got interested and found this highly informative page on the internet that describes the 727 flight control system. It goes into some detail about the roll controls and how the spoilers are indeed the primary roll control. I predict that adverse yaw is not a huge issue with this aircraft, due in part to the spoliers for roll control.

http://www.boeing-727.com/Data/syste...ofltctrls.html
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Old Apr 14, 2004, 06:54 PM
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Keep in mind adverse yaw, as commonly found in many airplane designs, is a bit different than swept wing "dutch roll" as the initial batch of swept wing jets taught us. Dutch roll is an uncoordinated roll and pitch oscillation that will present itself in straight and level flight. Jets and other high performance straight wing aircraft will often have "yaw dampers" that activate rudder control and work in combination with the autopilot for pitch and yaw dampening.

I believe that in a properly set up 727 model, as long as you have a good CG placement, dutch roll will not be such a concern. If you keep your adverse yaw under control with a slight amount of aileron differential then you have gone far enough. I really don't think you will need a micro gyro - and that's good because that would certainly not be in thew weight budget.

Another interesting bit of Boeing aerodynamic trivia notes:

The original "inboard airlieron" configurations have been abandoned in the new generation narrow body airliners in favor of plain ailerons and continued use of flight spoiler roll control assistance.

Leading edge flaps or Kreuger flaps have been long abandoned for full span, slotted leading edge slats.

The original, complex triple slotted fowler flaps common on the B-727 and B-737 have been phased out in favor of simpler and more efficient double slotted fowler flaps in the new generation 737's.

In some ways the complex aerodynamic flight control setups of the 1960's B-727 and 1st generation B-737 design have been simplified over time in favor of simpler flight control systems, better airfoils and more efficient fairings.
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Old Apr 14, 2004, 07:31 PM
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The original "inboard airlieron" configurations have been abandoned in the new generation narrow body airliners in favor of plain ailerons and continued use of flight spoiler roll control assistance.

The 737 only had outboard ailerons and spoilers. A stout wing as compared to the 727 resistant to twisting.

Interestingly, the original A300 incorporated inboard - outboard ailerons + spoilers.
The newer A300-600 retained the inboard ailerons/spoilers but did away with the outboard ailerons.

Though the yaw damper is an autopilot subsystem, it nevertheless is full time whether the A/P is on or off (unless of course it's turned off).

Besides yaw damping, the yaw damper provides turn coordination.
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Old Apr 15, 2004, 08:13 AM
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Found the above fasinating.
FDXmech, again, thank you. And eddie P, thanks. Thomas B, thanks for the link, will have a look.
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Old Apr 16, 2004, 12:21 PM
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Quote:
Originally Posted by FDXmech
[i]The 737 only had outboard ailerons and spoilers. A stout wing as compared to the 727 resistant to twisting.

Interestingly, the original A300 incorporated inboard - outboard ailerons + spoilers.
The newer A300-600 retained the inboard ailerons/spoilers but did away with the outboard ailerons.
No, the 737 never had an inboard aileron. The point about the 737 New Generation airliners is that they have much simpler fowler flap and leading edge device designs that are more efficient than the older more complex systems due to better aerodynamics.

The point about inboard ailerons is still valid as I was referring to Boeing narrow body airliners. The A-300-600 is still based on a 1970's airframe, though it has several improvements over the original (sort of like the 737 line), it's still a wide body and at times the larger airframes will still need a little help not initiating wing twist with outboard ailerons. So I still stand by that original point
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Old Apr 16, 2004, 07:36 PM
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Gws Edf 50 727

Thought you folks who contributed to this thread might like to know my 12.5 oz. Boeing 727 enjoyed it's flirst flight this evening. very little adverse yaw.

David
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