Effect of flaps on Vmca (on a twin turbojet aircraft).

[manucordier]

Hi,

I know that question has already been discussed here but I didn't find a very convincing and definitive answer. So let me summarize what has been said for a twin propeller engine and then let's talk about what happens on a jet engine with one reactor underneath each wing ...

What has already been said :

Is Vmca increased or decreased when flaps are down ?

Hypoteses :

On a twin engine propeller aircraft, if the right engine is dead, the left engine is blowing more air on the left wing while the right engine is shielding its wing. So the left wing is being lifted up towards the dead engine.

Now if you put your flaps down, the left wing is being even more lifted up because the left flaps is in the slipstream of the propeller while the right flap is still shielded by the dead engine and produces therefore less lift.

So you need to put more yoke to the left to counter that rolling moment towards the dead right engine. That left yoke increases the drag of the right wing so increasing the yawing moment towards the dead engine.

So this situation requires a higher Vmca to get a better counter yaw effect from the rudder.

Do you agree ? Anything to add or to rectify ?

And now,...

What happens on a twin turbojet ? How does the flaps influence Vmca ?

... ?

Thanks for your help.

[decurion]

I am not aware of any jet for which Vmca is a function of flaps setting. I have checked AFMs of several aircraft however the certified Vmca values apply for all flap settings.

[AdamFrisch]

The determination of Vmca is done gear up, flaps up because the gears and the flap have a "keeling" effect and will add stability. Even though what you say makes sense, it is generally accepted that gears and flaps down will lower the Vmca as it relieves the rudder and adds directional control.

Vmca is determined by flying:

Fully loaded
Full aft CG
Sea Level
Gear Up
Flap up

Remember, they're trying to reproduce the worst possible scenario in determining Vmca. Anything deviating from the above, will decrease Vmca, thereby adding safety.

[Mad (Flt) Scientist]

VMCA is most definitely not determined flaps up; it's determined in the takeoff configuration(s) applicable for the type, which usually included flaps deployed configurations, certainly for twins with underwing engines.

VMCA is also flown at light weight, not fully loaded. And it is flown above sea level, for safety reasons, although the engines will be boosted to create equivalent thrust levels at or above the sea level values.

Not underwing types, but I *am* aware of ywin engined jets with VMCA very much a function of flap position.

With regard to the OP, powered lift is certainly potentially a factor, but with the engines below the wings the powered lift effect should be less (the Antonov with the overwing engines is undoubtedly a different beats, as also the HondaJet I guess) so the degree to which the engines influence VMCA through powered lift should be small.

[172_driver]

So funny, if you ask 10 different people on effects on Vmc you are going to get 10 different answers. I am just one of those 10 that doesn't know any better than my neighbor. But, my take on it as follows:

Although I see the logical reasoning behind What has already been said: (Vmc increases with flaps extended), I was always taught that flaps decreases Vmc. If right engine fails, the aircraft will naturally yaw to the right. But the accelerated slipstream behind the left engine will impact the flaps at higher speed than on the right side, thus creating a counter-yaw moment. The theory is for a MEP (Multi-Engine Piston).

I understand that all aeroplanes are different, location of engines, design of flaps, it will influence Vmc differently. From now on I should probably drop all theories and simply state "I don't know, it depends...." on all such questions.

[manucordier]

Well guys, thanks a lot for your answers, I finally begin to think that there is no right answer to this question and that it depends on the aircraft type !

[john_tullamarine]

From now on I should probably drop all theories and simply state "I don't know, it depends...."

That's about the size of it. In the training world, folk tend to try to dumb things down to rules of thumb and the like .. and then either ignore or try to accommodate all the exceptions.

When it comes to Vmca and Vmcg, read the specific Type manual and proceed from there.

[decurion]

The air minimum control speed is influenced by a number of variables. There are basically only two aerodynamic variables related to the air minimum control speed which are influenced by flaps: the sideforce and yawing moment derivatives due to sideslip. It is not a clear cut how these derivatives change with different flaps settings and hence how they influence Vmca. When I am back in the office I will try to find some typical numbers obtained from flight tests which illustrate the influence of flaps on Vmca.

When adding propeller slipstreams to the problem, things become even more complex. See e.g. http://www.lr.tudelft.nl/fileadmin/Faculteit/LR/Organisatie/Afdelingen_en_Leerstoelen/Afdeling_ASDM/Design_of_Aircraft_and_Rotorcraft/Research/Publications/Preliminary_Design/doc/AIAA2007_1046_Final.pdf

[flyin4alivin]

"That's about the size of it" I'm with you John. Know your AFM, know your company POM and don't try to over analyze a challenging situation in the cockpit. Your training should keep you safe. :)

[Mad (Flt) Scientist]

@decurion

While those are perhaps the two derivatives most directly impacting the VMCA balance, it must be remembered that there is a great deal of potential for alpha to affect VMCA (through the effect it has on the various lateral-directional derivatives) and the kind of question that always seems to be posed about VMCA tends to refer to AIRSPEED at different flap settings - and the same airspeed will of course have different alpha for different flaps.

It probably would help the basic aerodynamic understanding if it were considered in terms of the effect of flap at fixed alpha, but that's of course a more theoretical approach.

[decurion]

The alpha effect is clearly a factor which makes it even more complex.

Regarding the sideforce and yawing moment derivatives due to sideslip in particular the first derivative can be influenced by alpha. However, the experimental data I have for many commercial transports doesn’t give a consistent picture regarding this influence. For some models the effect is only present at high AOA (more than 12-14) at which things starting to become less linear.

Coming back to Vmca again, it is the ratio of the sideforce and yawing moment derivatives due to sideslip that influences Vmca (CYB/CNB). I have checked this ratio for several aircraft comparing a clean configuration with a takeoff configuration. Although both the sideforce and yawing moment derivatives due to sideslip are influence by flaps, the ratio between the both was more or less the same (I looked at data for 8 transport aircraft) for both configurations.

Note: Adding flaps will increase drag which can infleunce the power setting and hence again the Vmca..