How flaps affect VMC?

bLizZuE

bLizZuE

Calling for engine starts en français
So I've never really delved into why flaps affect VMC, as it's just one of the configuration items much akin to trim setting and cowl flap setting. However a new student of mine asked, and I was taken aback because I never really thought about it.

So what say you, communal pot of knowledge? How do flaps affect VMC? For clarity and ease, lets use the Seminole.
 
My understanding is that extending the flaps behind the "good" engine increases drag and creates a stabilizing effect that opposes the yawing tendancy. Therefore Vmc decreases with the extention of flaps.
 
bLizZuE said:
So what say you, communal pot of knowledge? How do flaps affect VMC? For clarity and ease, lets use the Seminole.
Click to expand...

For the aircraft that publish a Vmc with flaps and without flaps, the data I've seen show a lower Vmc with flaps extended. Boundary Layer Research publishes some such data: http://www.blrvgs.com/. Here's an example for a Navajo: http://www.blrvgs.com/civilian/fixed_wing/popups/piper/navajo.html.

I don't know of any publicly available data that directly answers this question. However, NACA has published data that shows that flaps tend to increase the directional stability of aircraft, which is why they can sometimes work against you in crosswind landings. Anything that increases directional stability of an aircraft would lower Vmc.

Unfortunately, NACA reports often answer the question "what?" but ignore the "why?" My speculation is that the stability is provided by drag differential. When an aircraft yaws, the downwind flap is somewhat shielded by the fuselage, and the up wind wing has relatively greater drag, tending to yaw the aircraft back in line with the relative wind. For a ME, having the flap in the propwash of the working engine probably increases this tendency. In that way, it's the equivalent to a thrust reduction.

Another possibility is that flaps tend to divert the propwash towards the fuselage (says NACA data), which can increase the directional stability in some instances, such as with a non-critical engine or counter-rotating props. (Evidence in favor of this hypothesis would be a critical engine failure actually having a higher Vmc with flaps in. I haven't looked for the data.)
 
The school i went too loved this way of discribing it:
When you lower the flaps think about a parachute being deployed behind each flap. Thoes parachutes will help counter the yawing from the dead engine.
I would also think its alot to do with differental drag. Good ol Newton, For every action there is an equal and oposite reaction. The flap on the Seminole is right under the nacel and the prop wash will hit it directly. The increased airflow on the flap will help counter the yawing wich will lower Vmc and help you maintain directional control.
 
OK, my noobie question, never flown a Seminole or ME for this matter: One can control the flaps separately on each side? I tried researching and could not find anything that says this. If it is indeed both sides at one time.. I'm confused here???

This post BTW is ranked 18 on Google searching "Seminole flap control" after 10 minutes. Wow to the interwebs.
 
jonnydwolf said:
OK, my noobie question, never flown a Seminole or ME for this matter: One can control the flaps separately on each side? I tried researching and could not find anything that says this. If it is indeed both sides at one time.. I'm confused here???
Click to expand...

Not in any aircraft I am aware of. The seminole cannot separately control flaps, it is both of them or none of them.

They are considering that one engine is running and so one engine has prop wash over a flap while the other is not. Consider the left engine dead, right engine running. The left flap would have no prop wash since that engine is dead. The right flap would have prop wash from the engine on the right, running.
 
OK, so the whole Newton opposite but equal reaction applies here.

Come to think about it, during normal ops ensuring both flaps are extended the same amount might be something one does not want to worry about...
 
irittmer] Good ol Newton said:
OK, so the whole Newton opposite but equal reaction applies here.
Click to expand...

I ask both of you to read what tgray said, "For a ME, having the flap in the propwash of the working engine probably increases this tendency. In that way, it's the equivalent to a thrust reduction."

Newton is constantly regurgitated as if everything in the world relates to his three laws. In this case, I fail to see any correlation to the flap disrupting thrust to newtons law. Newton says that if object 'A' exerts 'F' force on object 'B' then object 'B' will exert '-F' force on object 'a'.

In this case a force is applied by the thrust and another object is shoved into the slipstream to, in essence, reduce the thrust (slipstream). The object put into the slipstream, the flap, is exerting a force on the slipstream and the slipstream is exerting a force on it. That is newtons third law, but it has nothing to do with the reduction of yaw. The reduction of yaw is because of the disturbance of the slipstream.


jonnydwolf said:
Come to think about it, during normal ops ensuring both flaps are extended the same amount might be something one does not want to worry about...
Click to expand...

Are you serious? Did you mean "does want to" or "does not want to"...:confused:
 
shdw said:
Are you serious? Did you mean "does want to" or "does not want to"...:confused:
Click to expand...

I meant to say, in an effort to "retract"(no pun intended) the notion I put forth that there would actually be 2 separate flap controls, that there would be more things to worry about with 2 separate controls for the flaps. The pilot doesn't need to worry about controlling 2 separate flaps at once, this could be a huge problem if only one side of flaps went down because say of a faulty switch...

shdw said:
Newton is constantly regurgitated as if everything in the world relates to his three laws. In this case, I fail to see any correlation to the flap disrupting thrust to newtons law. Newton says that if object 'A' exerts 'F' force on object 'B' then object 'B' will exert '-F' force on object 'a'.
Click to expand...

My bad. I was thinking in terms of the vertical axis of the plane, where by putting the flaps down it causes drag on the good side (action) causes the bad engine side to pivot forward. I see now this isn't so. Thanks for the clarification.
 
This is how it was explained to me.
Flaps extended will reduce Vmc because with flaps extended we need less pitch attitude for a given airspeed. With less pitch attitude, P-Factor is reduced, which means we need less rudder to maintain control. Also, flaps add drag aft of CG which will help stabilize.
For my ME training I had to know all of the things that effect Vmc and why... it was then covered thoroughly on the oral.
 
z987k said:
With less pitch attitude, P-Factor is reduced
Click to expand...

Not exactly. Indirectly this is usually true, but the reason "less pitch attitude" isn't the why. With less pitch attitude you typically have a lower AOA, but not always. With less AOA you typically have a higher airspeed, again not always. A higher airspeed will reduce P-factor.

Two things affect P-factor: airspeed and engine power. You could probably argue that blade angle in a constant speed system or objects such as flaps that disrupt the slipstream also effect P-factor.

In most light piston twins we fly at Vyse for an engine failure, regardless of flap setting. Thus, P-factor will remain unchanged because our airspeed and power do not change.
 
z987k said:
This is how it was explained to me.
Flaps extended will reduce Vmc because with flaps extended we need less pitch attitude for a given airspeed. With less pitch attitude, P-Factor is reduced, which means we need less rudder to maintain control. Also, flaps add drag aft of CG which will help stabilize.
For my ME training I had to know all of the things that effect Vmc and why... it was then covered thoroughly on the oral.
Click to expand...


What you really mean is that flaps, for a given airspeed, will produce a lower AoA, which will reduce P-factor. Yes, this is true, and is a possible mechanism too. I don't have a feel for the magnitude of the difference, though and I haven't seen any experimental data. I suspect that those who claim this is THE mechanism haven't seen any data either.

As for the drag behind the CG, it 1) doesn't matter whether or not the drag item is located behind the CG, all that matters is that the drag vector passes behind the CG, and 2) even that doesn't matter unless you can explain why there is differential drag of the flaps on the working vs failed engine.

Without real test data on a variety of airplanes of differing configurations, I think that it's intellectually dishonest to tell a student definitively why this effect occurs or even if it occurs across all multiengine airplanes. And MEI instructor gouge that's been passed around from instructor to instructor doesn't count as a source of authoritative or reliable information. The fact that an instructor can pass an oral means nothing about the accuracy of their information.
 
My understanding as I was taught/teach and how I presented to a DPE as an FAA inspector observed my MEI checkride:

Vmca is a speed determined in a given configuration. This configuration includes the prop on the dead engine windmilling. A windmilling propeller presents a "disc" of drag if you will in front of the nacelle/wing. If the flap is positioned behind the inoperative prop "disc" then no appreciable increase in drag on that side will be felt with flaps deployed. The only flap that is capable of producing an increase in drag when deployed is the flap behind the operating engine. As already stated this flap "deploys a drag parachute" behind the operative prop/engine thus limiting its ability to create adverse yaw.

Experts correct me as necessary.
 
tgrayson said:
What you really mean is that flaps, for a given airspeed, will produce a lower AoA, which will reduce P-factor. Yes, this is true, and is a possible mechanism too. I don't have a feel for the magnitude of the difference, though and I haven't seen any experimental data. I suspect that those who claim this is THE mechanism haven't seen any data either.

As for the drag behind the CG, it 1) doesn't matter whether or not the drag item is located behind the CG, all that matters is that the drag vector passes behind the CG, and 2) even that doesn't matter unless you can explain why there is differential drag of the flaps on the working vs failed engine.

Without real test data on a variety of airplanes of differing configurations, I think that it's intellectually dishonest to tell a student definitively why this effect occurs or even if it occurs across all multiengine airplanes. And MEI instructor gouge that's been passed around from instructor to instructor doesn't count as a source of authoritative or reliable information. The fact that an instructor can pass an oral means nothing about the accuracy of their information.
Click to expand...

Thanks to t for actually posting real data instead of something that was regurgitated from an MEI course. Somewhere along the way someone said "This is what is required for part 23, so OBVIOUSLY these things increase Vmc" when there was not proof for all the items. Since then the information has grown like a bad weed that can't be killed and has to be spit up on every MEL ride.
 
You must log in or register to reply here.
Back
Top