Flaps up, Gear Up

popaviator

popaviator

Well-Known Member
I was once again listening to another instructor explain the COMBATS acronym for factors affecting Vmc. I myself went to ATP and used SMACFUM. Now during his explination he said that the gear has a stabilizing effect, but the flaps adversely affect Vmc and should be left in the up position and never down b/c something along the lines of the camber and the opperative engine creating a problem on the side with the flap extended. Once again I'm scratching my head, I though flaps extended during Vmc gave the airplane a stabilizing effect. :confused:
 
The AFH doesn't say what effect they will have, but they note that it should be in the takeoff position.

I get what he is saying, though. It isn't the camber, it is the increased lift on the side of the operating engine, due to the accelerated slipstream and the chord line being changed from the flaps. The accelerated side is getting more lift and yawing towards the dead engine.

In theory, he may be correct, but in practice, not sure.

I'll just wait for tgrayson to chime in.
 
popaviator said:
Now during his explination he said that the gear has a stabilizing effect, but the flaps adversely affect Vmc
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Gear may or may not reduce Vmc, depending on the gear configuration and direction of sideslip. Flaps tend to be directionally stabilizing and you can find data showing that Vmc with flaps lowered is lower than flaps up.

Many, many people confuse performance issues with control issues. Flaps down will certainly reduce peformance even while possibly improving control. Regardless of what these things do for Vmc, gear and flaps should come up; performance more than control is generally your problem.
 
I agree with Tgray... having the flaps and gear down might stabilize the plane, however, the negative effects on performance will outweight any advantages in favor of VMC.
 
For optimal performance, clean it up, however, when your ready to bring it in to land, let it all hang out to be as stable as possible. Performance shouldn't matter that much when your going about 500 fpm descent, the seminoles did it with 1/2 - 3/4 throttle for 500 fpm dirty configuration. (I haven't flown it in 2 years please verify my numbers)
 
Flaps will increase the camber of the wing and lower the required angle of attack. A lower angle of attack will decrease the P-factor and accelerated slipstream effects. Extended flaps have a stabilizing effect that may reduce Vmc. Flaps will increase the total drag component in the aircraft. When an engine fails the increase in drag from the windmilling propeller will result in a smaller overall increase in aircraft drag with flaps deployed than with no flaps set. Finally, the increase in drag from the flaps will dampen the initial yawing effect created from asymmetrical thrust. Deploying flaps will lower the Vmc.
 
Yeah but how is performance effected with flaps down? Is it resulting in a benificial manner?

We know VMC may be lowered with flaps down, but why sacrifice the performance?
 
JustinA said:
Flaps will increase the camber of the wing and lower the required angle of attack. A lower angle of attack will decrease the P-factor and accelerated slipstream effects. Extended flaps have a stabilizing effect that may reduce Vmc. Flaps will increase the total drag component in the aircraft. When an engine fails the increase in drag from the windmilling propeller will result in a smaller overall increase in aircraft drag with flaps deployed than with no flaps set. Finally, the increase in drag from the flaps will dampen the initial yawing effect created from asymmetrical thrust. Deploying flaps will lower the Vmc.
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...the big thing that i find wrong with this is that even if the yawing moment is decreased initially, the rolling effect will increase due to the increased amount of lift on the operating side.The stabilizing effect with flaps would be minimal, if there was. That and a windmilling prop has adds a huge amount of drag whether flaps are in or out.

Drag < Performance. Taking away flaps will decrease the drag, increase performance. Using flaps while operating on a single engine will increase the rolling effect thus increasing the opportunity for Vmc (lose of directional control).

The landing gear decreases Vmc due to kheel effect. Depending on the landing gear design (the amount of surface area being put against the relative wind) will depend on how much it assists with Vmc.

Here's a link to my thread over Vmc with the factors that i presented on my checkride.
http://forums.jetcareers.com/cfi-corner/85286-m-e-checkride-soon.html
 
LS1 Life said:
...the big thing that i find wrong with this is that even if the yawing moment is decreased initially, the rolling effect will increase due to the increased amount of lift on the operating side.The stabilizing effect with flaps would be minimal, if there was. That and a windmilling prop has adds a huge amount of drag whether flaps are in or out.

Drag < Performance. Taking away flaps will decrease the drag, increase performance. Using flaps while operating on a single engine will increase the rolling effect thus increasing the opportunity for Vmc (lose of directional control).
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The windmilling prop in the seminole gives about 150 FPM descent, equivalent to about 5 degrees of flaps since 10 is about 300 FPM, 20 is about 500, and 30/40 is about 700/900. You can test all these if you think they are wrong, I have tested and confirmed in the seminole, arrow, and various Cessna aircraft.

How are flaps creating a rolling moment? Unless your referring to the prop wash over one flap vs. the other, which would be stabilizing. Think about having a right engine INOP with the left engine going trying to yaw you to the right. Lower the flaps gives the left wing with the thrust a little more lift than the right causing a right rolling moment which (introduce adverse yaw) would give a left yawing moment, exactly what you are trying for in the first place and a stabilizing effect.

I have never seen roll be a limiting factor in VMC so I would never worry about rolling moments in any VMC calculation. The seminole runs into VMC limit with full rudder, as does, from what I hear, most other light twin aircraft.

Again it is all situational, on takeoff clean it up. On landing with the runway made why the heck wouldn't you use the flaps? Conversely on takeoff why would you ever leave flaps in? IMO don't worry so much about these little tiny effects and just clean the aircraft up for max performance if you need it and if you don't then set it up however the hell you want realizing that you are always balancing between stability and control.
 
tgrayson said:
FAA's view regarding gear and Vmc
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I love how they mention gear design on 3 separate occasions, the first 2 times to tell us that design will effect aerodynamics (like we didn't already know that). Then finally they bring up 337 to relate retraction for gear doors causing more drag which in no way helps explain Vmc towards stabilizing or not.

Not once did the author explain how one type vs. another is stabilizing vs. not. Got to love the FAA, yet another completely ambiguous answer that dodges the why behind the question and gives a half assed explanation with an example that has absolutely no relationship to the topic at hand.

Tgray I am disappointed, your citing an FAA document for aerodynamics, what happened to you?
 
shdw said:
Tgray I am disappointed, your citing an FAA document for aerodynamics, what happened to you?
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It's most effective to counter an appeal to authority with another one. ;) People are quick to say gear is stabilizing, without any data or authoritative source that says so, but simply because that's part of the CFI lore.

The FAA says "it depends", which should be apparent based on first principles.
 
tgrayson said:
It's most effective to counter an appeal to authority with another one. ;) People are quick to say gear is stabilizing, without any data or authoritative source that says so, but simply because that's part of the CFI lore.

The FAA says "it depends", which should be apparent based on first principles.
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What do the engineers say? If it depends, then what does it depend on? I mean in your opinion or from what you have read.

I would think there would be little to no stabilizing effect from the mains since they are on the CG and the arm on the longitudinal axis would have to be forward or aft of the CG to give any directional help. Which leaves the nose gear that has a forward arm and I would think be the cause for any stabilizing effect.

That would mean the arm of the nose gear and most importantly its surface area/shape perpendicular to the relative wind would determine its effectiveness. The closer the nose gear resembles a wing the more effective it would be as a stabilizer. It might also help if that nose gear was steering as left rudder would deflect the tire left.

Thats just my crazy ideas though so...
 
shdw said:
Which leaves the nose gear that has a forward arm and I would think be the cause for any stabilizing effect.
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Consider when you're in a 5 degree bank towards the good engine....the nosegear will be destabilizing. The mains *may* be stabilizing if they're behind the CG. As you pointed out, they're probably pretty close, but there are two of them.

What about if you apply rudder but don't bank at all? Then you're sideslipping the other way and the nose gear is now stabilizing, but the mains are not.

Hence the "it depends" answer.
 
tgrayson said:
Consider when you're in a 5 degree bank towards the good engine....the nosegear will be destabilizing. The mains *may* be stabilizing if they're behind the CG. As you pointed out, they're probably pretty close, but there are two of them.

What about if you apply rudder but don't bank at all? Then you're sideslipping the other way and the nose gear is now stabilizing, but the mains are not.

Hence the "it depends" answer.
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Edit: I don't understand how it is destabilizing in a bank? If it were a rudder hanging down in front (in place of the nose gear), then regardless of bank angle pivoting it to the left (just like a nose wheel, steerable, would do) would generate a left yaw. This to me would act exactly the same as the nose gear. Common sense airflow just doesn't seem to support this idea, is my common sense flawed here?
 
shdw said:
Edit: I don't understand how it is destabilizing in a bank? If it were a rudder hanging down in front (in place of the nose gear), then regardless of bank angle pivoting it to the left (just like a nose wheel, steerable, would do) would generate a left yaw. This to me would act exactly the same as the nose gear. Common sense airflow just doesn't seem to support this idea, is my common sense flawed here?
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I doubt pivoting the nose gear left or right is going to have much influence on which way the force is developed. It's not an airfoil. Probably best to think of it as a drag producer and the drag is opposite the relative wind.

If you're banked into the good engine a full 5 degrees, the yawing moment by the nosewheel would be towards the dead engine, which would be bad. If you aren't banked at all, you're sideslipping in the opposite direction, and the yawing moment would be towards the good engine, which would be helpful.
 
shdw said:
The windmilling prop in the seminole gives about 150 FPM descent, equivalent to about 5 degrees of flaps since 10 is about 300 FPM, 20 is about 500, and 30/40 is about 700/900. You can test all these if you think they are wrong, I have tested and confirmed in the seminole, arrow, and various Cessna aircraft.
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The arrow??? I will assume thats a typo. Cause I would like to see an Arrow with a windmilling prop that can hold 150fpm descent. Maybe 1050fpm :D.
 
wjmiller3 said:
The arrow??? I will assume thats a typo. Cause I would like to see an Arrow with a windmilling prop that can hold 150fpm descent. Maybe 1050fpm :D.
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You are misinterpreting what I mean. I am saying that if you were flying along at 90 knots with a prop windmilling in a Seminole and likely many other small twins (as prop diameter would determine drag so = diameter will give about equal drag) and you are climbing at say 150 FPM if you feather the prop you will now climb at 300 FPM.

Conversely if your flying any small aircraft inside flap range speed at level flight if you leave the power settings alone, lower the flaps, and keep the speed up by pitching the nose down you will see the noted FPM descents from my previous post. On another note, the gear is approximately 500 FPM, a perfect descent for those ILS approaches, cross the FAF, drop the gear and you can fly a pretty sweet approach.

Tgray - I see what you mean, cause the tire would be almost like a sail in the RW pushing the nose around toward the dead engine. Thanks
 
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