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9 Answers

Flaps effect on Vmc

Asked by: 14222 views , , , ,
Aerodynamics

How do flaps affect directional control in a multi-engine airplane? Do they increase directional control (decrease Vmc) or vice versa? This is assuming all other factors are equal, unless of course flaps directly change some other factor... This has been an ongoing thing at my flight school, and there seems to be no consensus between any of the instructors. Everyone has their own explanations (some make sense, some don't), but ultimately this still leaves me at a loss of what to acknowledge and understand as far as flaps and Vmc.

The flight school I work at has a horrible mentality of teaching the absolute bare minimum. I literally asked around and the majority response I got was "your students don't need to know that, so don't worry about it." This kind of thinking really bothers me because it hinders any further learning or application-based knowledge... I don't necessarily plan on going into every little detail with my students, but I myself at least would like to know for my benefit, or even be able to answer any questions they may have about it.

Here's what I DO know: I've done an experiment in a Beech Duchess (counter rotating) where I maintained a constant airspeed at a constant altitude, gear up, both cowl flaps closed (up), placed one engine to idle, and the other to full power, maintaining heading. Afterwards I brought the flaps completely down from their upmost position, and what I've found as they were coming down is that I had to use more rudder input to maintain heading, and less once I fully retracted them. I pitched up/down accordingly to maintain a constant airspeed as I changed flap settings (still at same altitude). I tried this multiple times, each with the same result. I only had the opportunity to try it in a Duchess, not in any other aircraft.

One of the other instructors at my flight school said that my experiment was inadequate because I wasn't actually at Vmc to know how it affects directional control.... Am I not correct in understanding that if you require more input to maintain control at a higher speed, then your Vmc increases by definition? I've heard so many different explanations for flaps affecting Vmc, and most of them saying the opposite of what I actually experienced. Prior to that experiment, I thought the same thing myself, but now I'm just confused.... I'm very strong in math and aerodynamics, so I'd love to hear some lengthier explanations from experts too.

 

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9 Answers



  1. Mike Fasano on Jan 01, 2015

    When considering the effect of flaps on Vmc, you may be referring to the drag (or lack there of) flaps provide.

    Generally, flaps up increases Vmc while flaps down decreases Vmc as a result of drag counteracting thrust as the slipstream quality behind the operating engine is reduced which in turn reduces the engines efficiency and thrust. This will result in a lower Vmc number.

    There is also a train of thought that extended flaps have a stabilizing effect much like the keel effect experienced when gear is extended which helps limit the ability to generate adverse yaw.

    I also found the below on one of Embry Riddle’s sites concerning muli-engine operations:

    “When the flaps are down the wings create more lift than if the flaps were up. However, when lift is created, drag is also created (as lift increase, drag increases).

    The side with the operating engine is creating even more lift because of the accelerated air flowing over the wing. When the flaps are extended, the drag caused by the accelerated flow opposes the yaw caused by the inoperative engine allowing the pilot to use less rudder to maintain heading. Having more rudder available to the pilot lowers VMC.

    It should be noted more lift on the right wing will cause a roll to the left. If ailerons are used to counteract the rolling of the airplane, the drag from the adverse aileron yaw will actually increase the yaw towards the inoperative engine.”

    Hope this helps and will likely generate some debate.

    Regards,

    Mike

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  2. Dmitriy on Jan 01, 2015

    Mike,

    Thanks for the response. For the longest time during my training and my career as a CFI I’ve always been under the same impression, and it’s how I’ve actually taught it to my students (for the most part).

    However, when I went to go try it (as I’ve written above), I got completely different results… What would be the explanation for this? How would I explain it to my students who were there with me when I tried it?

    – Dmitriy

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  3. Mike Fasano on Jan 02, 2015

    Dmitriy,

    Couple of thoughts. With the engine idle, do you mean zero thrust? An idle engine may not adequately or accurately represent he conditions you are trying to create as the pitch angle of the props if left unchanged would exhibit different aerodynamic properties as when simulating zero thrust, full fowrard, feathered or in a cruise configuration.

    Also, and this is merely a leap, but perhaps due to the counter rotating props on the Duchess and not having the traditional critical engine is causing your experiment to show the anomalies you are experiencing.

    Just some thoughts here. If conditions warrant, perhaps you could actually shut down the engine and feather the prop to see if conditions change, set up the engine for zero thrust if not already there or as you allude to, try this experiment in a conventional twin.

    Hope this helps or at least generate some additional debate.

    Mike

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  4. Dmitriy on Jan 04, 2015

    Mike,

    I have tried the same thing in the Duchess with the engine actually off and feathered this time with the same results as before. I needed significantly more application of rudder to maintain directional control as the flaps came down (all other things being equal).

    How would the counter rotating configuration affect directional stability via the flaps? That’s actually a possibility, but i don’t know where to start with explaining that one? How would that work? I unfortunately have no access to a conventional twin to try that same experiment… The only twins I have access to are the Duchess and the Seminole (and I have yet to try this experiment in the Seminole yet).

    Still searching for answers :/

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  5. Gary S. on Jan 08, 2015

    Dmitriy, I would look to the airplane flight manual, commercially published books on light twins, and search for a highly experienced light twin instructor for your answers.

    Don’t know what else to suggest. If you still can’t find them after an exhaustive search, maybe you should just accept the facts as you have found them and move on.

    Careful with feathering a good engine at Vmc. I wouldn’t do it.

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  6. Dmitriy on Jan 08, 2015

    Gary,

    I wasn’t at Vmc when doing my experiment, I was merely holding a safe constant airspeed so that it doesn’t skew my results (in my case it was closer to Vyse anyway). I simply said it affected my directional control adversely.

    Believe me I’m trying to find anyone who can shed some light haha. That’s why I’m here asking you guys. Also, I’m perfectly capable of moving on. However, that doesn’t mean I’m not going to exhaust every option I have. If anyone can help explain this to me, then I’m more than willing to listen. I tell my students and other instructors all the time: never stop learning, not matter how far you think you’ve gotten.

    – Dmitriy.

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  7. SurferPilot on Sep 25, 2017

    Hi Dimitry,
    I am not an expert, but maybe a thought:

    You had to pitch down with Flaps.

    Without counterrotating engines pitch up increases p-factor, which makes the critical engine more critical.
    Pitch down would reduces p-factor, or even reverse which engine is critical.
    So setting flaps and pitching down would help a lot.

    With counterrotating engines, the p-factor moment counteracts the moment from excentric trust.
    Pitching down reduces this beneficial moment and thus requires more rudder.

    Of course in addition to all other effects.

    What do you think?

    Best Regards
    Rob

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  8. VALKYRIE ONE on Oct 13, 2017

    Dmitriy,

    I would recommend that if you want to definitively lay this question to rest, you try your experiment as an altered Vmc demo.

    I haven’t tried it personally yet, though now I kind of want to. But essentially when we demonstrate Vmc in our aircraft for the purposes of the multi-engine training and checkride, we always simulate the conditions as specified in FAR 23.149, yes?

    So in order to determine, without a doubt, whether Vmc is increased or decreased with flap extension, perform a Vmc demo normally, note the speed at which directional control is lost, then perform another demo with flaps down. Remember that density altitude also has an effect on Vmc, so that’s why I say do a normal demo first. You aren’t like to really have Vmc where you expect it to be unless conditions are perfectly standard, so it’s important to establish a baseline at a particular altitude just prior to experimenting with any factor that affects directional control.

    I normally fly a Seneca which is also equipped with counter-rotating props, but I also have access to an Aztec. When maintenance gives it back [haha, don’t hold your breath 😉 ], I’d love to give this experiment a try and get back to you.

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  9. Dmitriy on Oct 14, 2017

    I posted this one a very long time ago.

    Since then I have performed said experiment many times and with pretty consistent results. I’ve also since found many different sources that say either/or effect.

    I’ve since stuck with the proof that I get myself. When I go to CAE again soo I’ll try it out in the Phenom sim too lol. Just for shits and giggles.

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