V speed questions

BobDDuck

BobDDuck

Island Bus Driver
I am in the process of memorizing Vspeeds for the PA-44 and had a question. Actually I had a few of them.

1)Is Vxse always going to be the same speed as Vx (and likewise for Vyse and Vy)?

2)Is Vfe for all settings of flaps or just for the minimum setting?

3)Vlo (down) and Vle are the same. Is this always the same, or can you ever go faster once the gear is down and locked?

4)Va decreases as weight decreases. Why isn't this the opposite. When there is more weight (ie more inertia) I would think a control deflection would do more damage to the control surface. Or is this from the point of view of the airframe and not the control? ie. it takes more force (airspeed acting on control surface) to upset the airframe from stable flight?

Thanks for any help on these.

Ethan
 
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1)Is Vxse always going to be the same speed as Vx (and likewise for Vyse and Vy)?

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Nope. I'd say the Seminole is probably the exception, rather than the rule. Its the only multi that I've flown thats like that.

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2)Is Vfe for all settings of flaps or just for the minimum setting?

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No, its usually the max speed with flaps beyond the approach setting. Often you can extend approach flaps above the white arc, but the top of the white arc is the "hard" (any flaps beyond approach) Vfe speed. Not sure if the Seminole is this way or not, however...its been awhile since I've been in one.

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3)Vlo (down) and Vle are the same. Is this always the same, or can you ever go faster once the gear is down and locked?

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Not always the same. Sometimes you can exceed Vlo once the gear is down and locked. In the Bonanza for instance, Vlo is 145, and Vle is 154. In the King Air, every gear speed is 153 (easy to remember)...so it varies.

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4)Va decreases as weight decreases. Why isn't this the opposite. When there is more weight (ie more inertia) I would think a control deflection would do more damage to the control surface. Or is this from the point of view of the airframe and not the control? ie. it takes more force (airspeed acting on control surface) to upset the airframe from stable flight?

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There was recently a lengthy discussion on this in tech talk. Check that thread for some good info.
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Va answer...

From Gleim’s Pilot Handbook:
“Design maneuvering speed is the maximum speed at which an airplane can be stalled without exceeding its structural or load limits. Theoretically, at speeds below Va, a damaging positive flight load cannot be produced, and the airplane stalls before the load becomes excessive.”

When we talk about placing loads on the aircraft structure in flight, we are referring to loads due to accelerations of the aircraft. For the 60-degree steep turn, the load factor is 2; in a turn, we are accelerating around a circle. Or how about the chop stemming from convective thermals on a sunny afternoon? Here, the airplane accelerates in a vertical direction.

Now, from Newton’s second law, we have the famous result F = ma, i.e. force produces an acceleration inversely proportional to the mass of a body. Consider two objects with the first being less massive than the second. If forces of equal magnitudes are applied to each of these objects, the first will experience a greater acceleration than the second, since the first object is less massive.

Thus, Va decreases with the decrease in weight of the aircraft. At lower weights, it is easier to accelerate and place excessive loads on the aircraft; it is harder when the aircraft weighs more.

I think what we see with this background is that Va is not applied to control surfaces but rather to aircraft structures, such as fuselage, wings, etc. Deflecting controls rapidly at speeds above Va may not damage the control surfaces but is likely to damage the aforementioned structures.
 
I'm beginning to see why the Seminole is such a nice training platform. Thanks for the clarification. Now if I can just memorize all of them
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Ethan
 
I'm not sure if anyone caught it, but the AOPA Magazine from April had an article in it about the new Seminoles called "Lord of the Light Twins". It was a pretty good article. Our CFI Examiner up here in Prescott pointed out a few glaring mistakes in it, but nonetheless it was a good article. Our school up here (North-Aire) flies a Turbo Seminole and a regular PA-44. It and the Duchess are probably the few airplanes where Vxse and Vx (82 I believe) and Vyse and Vy (88/blue line) are the same. With an aircraft like that, as far as the aerodyamics are concerned it's approximately the speed for L/D max single engine. Just remember though that Vxse takes you closer to stall/Vmc (in a PA44) than you need to be. VFe for the Seminole is 112 and you can dump 40 degrees of flaps in at that speed, which is nice if you enjoy going fast up until the last possible second. Vle is 140 on the PA44, and it can be extended at that speed but not retracted. The way the nose gear retracts, the air loads on the nose gear would be too great on the system so it's restricted to 109 or something close to that, but once it's down you can do 140 unless you want to take gear doors off.

As for the Va thing, depends on how much you want to get into it. Just remember that at a higher weight, the aircraft consistently flies closer to the stalling angle of attack therefore requiring less control input to stall it therefore allowing a higher speed to do deformation damage to the airframe.
 
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Just remember though that Vxse takes you closer to stall/Vmc (in a PA44) than you need to be.

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If Vmc is way down at 56, is the 82kts for Vx really that close to Vx and Vxse?

Ethan
 
I knew I was gonna hear about that. The thing I see (in my eyes and I'm sure someone will disagree with me) is that the number one thing you need to be concerned with when an engine fails is keeping the airplane under control. The Seminole just happens to be a much happier airplane at 88-95 kts single engine than it is at 80-88kts. You'll see. Personally I think a good rule when it comes to flying a twin is blue line or better unless I absolutely have to. The Seminole will stall and then lose directional control so the descent from a perfectly controllable airplane to one that's not is fairly docile whereas a Baron or a Cessna 340 will actually agressively roll at Vmc...above the stall speed. And that my friends is why twins have a higher fatality rate than singles...but never let anyone tell you I was Mr. Doom and Gloom when it came to a light twin
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I'll take your word for it. Once I have some time in the airplane I may actually be able to comment on it. But until then... sounds like it makes sense.

Ethan
 
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