Vmc and flaps. [Archive]

yxcapt

8th Mar 2002, 07:25

In a light twin, how do flaps affect Vmc?. .. .One thought: 23.149 calls for flaps in the take-off position. For most light twins that would be up. Extending the flaps would reduce Vmc becouse the pitch attitude has been reduced thus P-factor has been reduced (assuming both engines rotate clockwise, otherwise, Vmc would go up if counter-rotating)

Matthewjharvey

8th Mar 2002, 11:33

In many light twins the addition of flaps will increase Vmc. This is due to the accelerated slipstream on the side of the operating engine blowing over the flap which can effectively be thought of as an aileron. Increased airflow over that flap increases lift on that wing and increases the rolling moment towards the dead engine thus increasing Vmc. Vmc demos on a Seneca for example are (or should) be performed with 25 degrees of flap (and gear up). This is the 'worst case' scenario for that plane and is the configuration on which the published Vmc is based.

Zeke

8th Mar 2002, 13:22

Yxcapt,. .. .I would agree with The Greaser to an extent, and then disagree.. .. .Vmca is a somewhat artificial speed that is a benchmark for aircraft performance. Changing the aircraft configuration does not change Vmca (it is fixed) but will change the airspeed at which you may lose directional control, subtle difference but makes you a test pilot every time you have an engine failure that is not in the configuration that the aircraft was tested in, or at any pressure altitude above ISA sea level. . .. .If the aircraft in question has a T/O flap setting, and that flap setting is more of a lift producing flap, then I would expect an increase in the speed that you would lose directional control as The Greaser described.. .. .A manufacturer may specify a greater flap setting than takeoff for the intentional demonstration of Vmca in flight to provide a greater margin above the stall (Vsse), thus providing a better buffer over a single engine stall/spin scenario. This may not necessarily be the most critical configuration.. .. .On initial thought landing flap IMHO may have no effect on Vmca, it would reduce climb performance, and increases the stall margin on both wings, and as you have eluded to (generally) produces a nose down pitching moment, but does it always change the relative airflow and therefore reduce P factor ?. .. .I would have to think about it a little further to come up with an answer for aircraft that have slipstreams that go over the flaps to see if full flap may actually reduce Vmca by the increase in drag caused by the slipstream on the flap, and therefore reducing the thrust couple of asymmetric flight.. .. .It is a complex dynamic situation, most of what we are talking about assumes steady flight (equilibrium), what happens if the aircraft is accelerating/decelerating, where is the center of pressure for the lift and drag in relation to the longitudinal, lateral, and normal axis, at what position is the drag vector of the wind milling engine acting through, the vector for the thrust of the live engine ?. .. .When talking about Vmca/Vsse there is no hard and fast rule that will always work it is dependant of the aircraft configuration and powerplant. The underlying principles are common, but always revert to the manufactures procedures for your type.. .. .Z. .. .The Greaser . .. .“This is the 'worst case' scenario for that plane and is the configuration on which the published Vmc is based.”. I would disagree with this, it would be as per regs, takeoff flap. . .. .Do not misinterpreted the published intentional Vmca configuration and procedure (Vsse - Single Safe Engine speed) as being the basis for the published Vmca speed. Vmca should be for sea level ISA, but I could wager your manual says to use this configuration at least 5000ft AGL for adequate spin recovery.. .. .Z

yxcapt

8th Mar 2002, 20:57

Greaser & Z,. .. .I pose this thought. As long as the airplane is not in a slip (a yaw string used not the ball in the slip/skid indicator), wouldn't the lift on both sides be equal (we raise the dead and apply oppisite rudder to eliminate the side slip). The ailirons would maintain bank angle. Now keep the bank around 2-3 degrees. (or 5 degrees per CFR23.149) in to the operating engine.. .. .As airspeed decreases, you normally run out of rudder well before ailiron control. As you continue to bank into the operating engine you can continue to decrease the speed at which directionial control is lost at a rate of 3 knots per degree of bank.. .. .So I guess I'm not sold on the slip stream effect in this case, or am I missing something? I'm also not sold on the theory I presented in my first post, but it does make some sence.

Zeke

10th Mar 2002, 20:41

yxcapt,. .. . <blockquote>quote:<hr />As long as the airplane is not in a slip (a yaw string used not the ball in the slip/skid indicator), wouldn't the lift on both sides be equal<hr /></blockquote>No, that only indicates that the sum of the rolling moments is equal to zero, the sum of the rolling moments due to sideslip, yaw, aileron and rudder. Aileron deflection alone should indicate to you that the lift distribution on either wing is different.. .. . <blockquote>quote:<hr />As airspeed decreases, you normally run out of rudder well before ailiron control. As you continue to bank into the operating engine you can continue to decrease the speed at which directionial control is lost at a rate of 3 knots per degree of bank.<hr /></blockquote>No, as you increase the angle of bank the vertical component of lift is decreased and therefore you will need to increase the AoA to get the same lift, before long you will stall/spin.. .. . <blockquote>quote:<hr />So I guess I'm not sold on the slip stream effect in this case<hr /></blockquote>The airflow behind the windmilling prop is very disturbed, behind the good engine an artifically increased IAS from slipstream.. .. . <blockquote>quote:<hr />I'm also not sold on the theory I presented in my first post, but it does make some sence.<hr /></blockquote>I am not also, but not for the reason you presented, a high relaive airflow angle makes the P factor (Centre of Thrust Position) critical. An analyis on the relative airflow angle, flap setting and IAS OEI would be needed.. .. .Z

yxcapt

13th Mar 2002, 02:49

Z-. .. .Two thoughts on your second point.. .. .The amount of force the rudder produces depends on the airspeed squared, and on the rudder's angle of attack. Therefore as you decelerate you will need progressively more rudder deflection in order to maintain zero slip (and thus directional controll). . .. .As you increase bank angle you increase the horizontal componate of lift. The horizontal componate of lift reduces the effort required from the rudder. This can be easily demonstrated with 15 to 20 degrees of bank into the operating engine. The airplane may be descending at this point, but I'm not concerned with the ability to maintain altitude at this point. The FAA estimates the 3 knot decrease in Vmc per 1 degreee angle of bank. . .. .You can continue to add the bank up until either the wing or rudder stalls. The short ride to the earth will be quite exciting.. .. .Excepts in a few rare situations, I have always run out of rudder before the ailirons working around single engine Vmc.. .. .Since the lift is greater on the one side, drag must be greater as well. Wouldn't this off set some of the turning tendiancies?. .. .When all is said and done, in a light twin, would a pilot even notice the differance in airspeed? If you were flying a P-3 maybe but an old Senaca 1 I doubt it.

big pistons forever

13th Mar 2002, 05:09

This thread kind of reminds me of the debate on how many angels will fit in the head of a pin, interesting but ultimately irrelavent. Since virtually no light twin will have a positive rate of climb at VMC you are going to pull the other engine as the only time you should be at VMC is either on the TO roll or in the landing flare. The critical issue is not letting the aircraft develope significant yaw, which can easily turn into Stall-Spin-Die. When I teach the Multi endorsement I thoughly explore this region of flight by having the student only use one half rudder ( And at a really high altitude ). I want them to get lots of practice recognizing when a reduction of power on the good engine is the only way to prevent a departure from controlled flight.. . . . [ 13 March 2002, 01:10: Message edited by: big pistons forever ]

yxcapt

13th Mar 2002, 08:32

Big piston,. .. .Can a pilot fly successfully without knowing this? Sure, but I want to know the how and the why. When a question is raised, I don't always except the first answer especially when it conflicts previous information. Oh well.. .. .FYI, reducing power on the operating engine is only one way to prevent departure from controlled flight. Two others,. .. .1. Reducing Pitch and . .. .2. Having the knowledge to prevent this situation in the first place.. .. .There has been a push in this area (FAA types)to recover from a Vmc situation by just reducing the pitch attitude. I haven't bought into this argument. I beleave you must use pitch and power together to save your tail. It's a long story, but at least the meetings are interesting.

big pistons forever

14th Mar 2002, 04:35

yxcapt. .. .You are right on both counts. Personally I am not a big believer on the reduce pitch theory as pitch attitude is not producing the yaw at VMC ,all the power on one side is. Therefore it seems reasonable to eliminate the destabilizing force first. Anyway you hit the nail on the head,. .The trick is to avoid the situation in the first place.