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Old Dec 12, 2013, 04:03 AM
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knife edge flight dynamics

How does a plane in a knife edge attitude without SFG fly? is it the fuselage creating lift? combination of thrust and rudder?
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Old Dec 12, 2013, 07:30 AM
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The question is "which plane?"
A aircraft designed for knife edged flight does this as easily as if it were flying in level attitude
Basically you need enough lateral area (the fuselage ) to produce lift at some speed
Forget about airfoil -
any shape which can generate enough pressure difference. will work. The rudder size ? not a big deal - just enough to hold the fuselage at the lifting attitude.
We have flown a number of models which fly as easily on the side as upright
A side benefit is that NO bank is needed for a turn.
Think about it ---
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Old Dec 12, 2013, 07:47 AM
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The "rules" governing knife-edge flight are no different from those governing wings-level flight. Create enough "upward" pressure differential over enough surface (including the "thrusting" surfaces) and you can achieve level flight.

Is the fuselage creating lift? It's very likely creating an upward force. Whether you bookkeep that force as "lift" or "side force" is a semantic decision.

Is thrust creating lift? Conventionally, thrust and lift are bookkept separately. In most knife-edge flight, there will be an upward component to the thrust. A common misconception is that thrust acts opposite the drag. This is nearly true at small angles of attack (and sideslip), but definitely not true at large angles of attack (or sideslip). If you see a jet flying level at say, 25 degrees angle of attack, the drag is acting parallel to the horizon, but the thrust is inclined 25 degrees above the horizon.

The behavior of the rudder in knife-edge flight is analogous to the behavior of the elevator in wings-level flight. The rudder deflection largely determines the airplane's sideslip angle. In knife-edge flight, the amount of sideslip will largely determine the amount of upward aerodynamic force acting on the airplane. Whether the rudder itself is pushing upward or downward in knife-edge depends on CG placement (it's likely pushing downward).
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Old Dec 12, 2013, 09:10 AM
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how about jets? like f-16 where there is virtually no side area..
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Old Dec 12, 2013, 09:16 AM
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Originally Posted by su33seaflanker View Post
how about jets? like f-16 where there is virtually no side area..
Everything they told you holds for an F-16 as well. Generally, the less area, the faster it must go.
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Old Dec 12, 2013, 10:14 AM
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I don't believe an F-16 can do a sustained knife-edge. Since I have a Falcon 4 simulator, I just tried it: 5000 ft altitude, full afterburner, 800kts airspeed, nose 10 above horizon, roll 90 (altitude 6000ft when reaching knife-edge), full top rudder, elevator neutral. After 15 seconds, the nose was 10 below horizon, altitude 4500ft and dropping at a steady rate. Yes, it was only a simulator, but it is supposed to have pretty accurate flight model.
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Old Dec 12, 2013, 10:38 AM
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That's probably accurate - a modern fighter is designed for advanced combat maneuvers, and knife-edge isn't one of them. In fact, flight with a large amount of yaw, that doesn't cause much direction change, is one such maneuver, as it aids with pointing weapons at a target.
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Old Dec 12, 2013, 11:09 AM
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Modern fighters can be susceptible to departures at large sideslip angles. A previous version of the F-18A-D flight control software didn't significantly restrict rudder deflection near zero AOA. Knife-edge inputs would often result in rapid sideslip buildup and a really violent departure. The current software restricts rudder deflection near zero AOA to prevent this. Even though fighter airframes may be capable of generating "level-flight" side forces, the flight control system often prevent the pilot from commanding sufficient sideslip.
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Old Dec 12, 2013, 11:45 AM
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If you look around at the evolution in the bigger pattern models from those of the 60's and 70's you'll find that broom pole like fuselages were the norm because there was no requirement for Knife Edge (KE) flying.

Along came the late 70's or into the 80's and some bright lad built a model with a deeper fuselage and added KE to their "free style" repertoire. Since that time the fuselages have been designed and modified to allow better and better knife edge flying. So the side area and lift from the fuselage is certainly recognized as part of the lift created in knife edge flying.

But with the high yaw angles there is also going to be some lift created by the power plant. The amount is simply the geometric vector that is vertically oriented.

The fuselage may not look much like a wing but with the big side area presented by models that are designed to fly well in KE and the speed at which the KE passes are generally made the fuselage allows enough lift to be generated. If it didn't then the models would not be able to actually climb away in KE. Looking back on the early KE designs with less side area this was often not possible.

Then we have the flat foamie 3D lightweight models so popular these days. In this case the big side area, flat plate shape and super light weight allows for unlimited KE flying even to the point of doing KE loops and figure 8's.

Adding side force generators is like adding more side area. And such things can certainly help by adding to the side lift available.

Or big rudders with lots of throw can force and hold models with less side area to very high yaw angles. That allows the prop to provide more of the total lift needed for KE flying. We see examples of this with scale like models such as Extras.
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Old Dec 12, 2013, 12:29 PM
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Quote:
Originally Posted by ShoeDLG View Post
Modern fighters can be susceptible to departures at large sideslip angles. A previous version of the F-18A-D flight control software didn't significantly restrict rudder deflection near zero AOA. Knife-edge inputs would often result in rapid sideslip buildup and a really violent departure. The current software restricts rudder deflection near zero AOA to prevent this. Even though fighter airframes may be capable of generating "level-flight" side forces, the flight control system often prevent the pilot from commanding sufficient sideslip.
Very true. Modern fighter airframes are inherently unstable, and require flight control software to keep them in check. When a pilot 'discovers' a new way to depart, the software gets updated, or other, mechanical means are added to the airframe to control the airflow that led to the departure; hence, the enhanced forward strakes on the engine inlets of the F-18A-D models, to control airflow at the tails that led to fatigue and control problems at high AOA.
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Old Dec 12, 2013, 01:24 PM
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F16 in knife edge here

F18 in knife edge here
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Old Dec 12, 2013, 01:38 PM
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Originally Posted by hoppy View Post
F16 in knife edge here

F18 in knife edge here
F-16: didn't show transition from level flight, not quite wings vertical, some AoA (turning), decending, lasted about 6 seconds.

F-18: started from climbing/nose high attitude, began to sink halfway through flyby, rolled upright after about 6 seconds.

Yep. That's what they are capable of.
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Old Dec 12, 2013, 06:45 PM
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Yep; a model helicopter can do a knife-edge for seconds, too.
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Old Dec 12, 2013, 08:32 PM
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this knife edge is quite long..
F-16 Knife Edge Pass @ airshow (0 min 49 sec)
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Old Dec 12, 2013, 09:22 PM
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I counted no more than about 12 seconds, during which time it appeared to lose altitude. I've seen aircraft designed for sport flying on knife edge at a steady, very low altitude, for a longer time; granted, they were moving a lot slower, so the distance covered was probably less. That F-16 was moving fast, and was still losing altitude. Show us one where it goes more than the length of a full runway, at a steady 100 feet, and I'll be impressed.
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