TheStranger

Thanks. Good example with the stall conditions.


Oh, but as an instructor, you have to tell me why [/QUOTE]

How dare you! I'm unsure as to why, any help would be appreciated.

The "Complete Multiengine Pilot" says the gear has a stabilizing effect like the vertical stabilizer. Although, I don't know exactly how that would work, aerodynamically. Anyone?

My two guesses:

1. Extending the gear lowers the CG, which in turn would increase the keel effect, thus stability.

2. Extending the gear increases drag which decreases thrust, thus decreasing Vmc.

Cubdriver

See my post #12.

Keel effect

When [B] is stronger (see diagram), it adds a clockwise moment that aligns the aircraft with the slipstream which in turn improves (lowers) Vmc speed.

JamesNoBrakes

And then there's the issue that the nose-wheel is destabilizing for directional stability in the relative wind, so it ends up being a battle between the drag and station location of the nose wheel vs the mains. Yes, the mains might act a little like the vertical stab/rudder, as stated above, but the nosegear is going to act exactly opposite. Which one has a greater moment? Have we wind-tunnel tested this?

Or we could describe how the gear extension shifts the CG and affects the rudder effectiveness, and so on.

These effects are dependent on so many other factors that there is no hands-down effect of the landing gear, nor is there any huge effect to controllability. This is why it's left out of the any FAA document that tells you what you need to know or what you'll be tested on for the most part. You can figure many of these things out by reading Aerodynamics for Naval Aviators, but again, these are getting into areas that most people don't understand and even less people will be able to teach, not to mention how little they affect anything. Total waste of time IMO. This IS a good test of a flight instructor though, because getting into these kinds of idiosyncrasies and technicalities is exactly where an instructor should draw the line and not get into, because they are going to wrap themselves up and get confused quickly with a couple "student" questions from an examiner. At that point they aren't testing your knowledge persay, they are testing your ability to be a teacher. The student is going to have a hard enough time knowing and understanding what a critical engine is and what makes it "critical".

Understanding keel effect for lateral stability is a good idea. Again, maybe not something we'd teach to students every time, but it's a good idea to be pretty solid with the concept.

csucbrown

Keel effect is one thing, but what if the landing gear is in the accelerated slip-stream?

If the gear/flaps are down (and the gear is right underneath the prop, like a Brasilia or Saab), the gear/flaps will create more drag on the operating side, thus lowering vmc...right?

That's why i mentioned flaps up/gear up, for keel effect + asymmetric drag...or am I wrong? It's not in the PTS, but as a student if what I said is true, I would probably want to know it. Explaining that to a foreign national is a different story though, and I agree with you.

JamesNoBrakes

Maybe, or maybe it moved the CG rearward when you lowered the landing gear, which offsets a slight increase in drag due to the accelerated slipstream? The problem is we are talking about such highly variable situations that overall rules do not apply, and we were talking about separate factors, so gear for example, it's not going to have a huge effect. It might be a net decrease, but it's not significant. If it was found to be, I'm sure there'd be something in the POH about it and about what to do with gear up vs down.

Flaps? Again, which is stronger, the induced rolling force or the asym induced drag? Sure, adding flaps can affect Vmc, but if you are getting that much asym drag from adding flaps that it's significantly affecting Vmc, you are probably at such a crazy high power setting that your Vmc is right back up there due so much asym power, or that same super-powerful accelerated slipstream causing an extreme rolling moment. The POH sometimes limits flaps for SE approaches, but there are a few reasons they do this, not just directional control issues obviously.

The problem is you get pretty far out here saying some things that are true, but can be offset by other factors with no way to really prove which one "rules" and how "significant" it really is. If it lowers Vmc by .02 kts, who cares? Won't affect me. If I am at a low power setting on approach and I add flaps, is it going to significantly affect my control? If I have flaps in and have an engine failure it's going to be that much harder to accelerate to a speed where I have better control, but again, we start running around in circles.

Building some knowledge of how some of these factors could apply is fine, where you pass the line IMO is when someone decides one of these things is an overall factor with what is really no real proof or testing. This is most commonly done IME when someone spouts off 8 or 9 "factors" of Vmc and how they all "affect" Vmc, which is not the point of that regulation. The "real" factors (things that can actually affect you and are significant) are in the PTS. A good example is the "windmilling prop" part. That's a worst-case scenario. The windmilling prop has to turn the cylinders, alternator, gear pumps, vacuum pumps, magnetos, and anything else connected to the engine. Takes a lot of energy and creates a lot of asym drag, so when they certify the aircraft they test it to know that it is controllable in this configuration/situation, but we may not have that prop setting and max power setup except for takeoff, so that would be our most critical situation. In other situations we'd have a lower Vmc, whether due to the proper rpm, or due to being at low power on approach with the props forward, so less asym thrust adding to that asym drag. So there is really not an "absolute" here.

I'll say it again, 23.149 is a great resource, helps learning about Vmc, and so on, but it's not a "list of the things that affect Vmc". What if it's the non-critical engine that failed? Is that a "factor" of Vmc in that list? What if you decided to hold in zero-yaw and crabbed down final, is that in the "list"? Can you do that? Would it be ok? In what situations? There may be multiple ways of thinking about these and multiple valid responses. The "list" idea or "factors" has been a bad thing for a while IMO.

TheStranger

Cubdriver,
Thanks for the post. But I don't understand how a sideslip would change the arm for the gear. Could you elaborate on that?

JNB,
Aerodynamics for Naval Aviators is an excellent resource, I have just recently started digging through it. However, I was not able to find anything in there in regards to stability effects of landing gear in the air. Maybe that was your point, but in what way were you encouraging that book?

Great counter examples with the gear & flaps to show how it could be argued either way. I understand your points that there are many variables to take into account in regards to gear/flaps, that they could be argued either way, and that the intention of 23.149 is to set up the most critical situation (not list all factors that affect Vmc.)

However, you lost me with your windmilling prop example:

Few windmilling prop questions:

1. I don't understand how this example supports your point, because the "windmilling prop" is not in the PTS but clearly affects Vmc. Just like the gear is not in the PTS but could affect Vmc adversely by being retracted (& setting up most critical situation). I'm confused there.

2. This topic, I'm sure, will be another tangent for us to discuss. Since this thread is labeled "Factors affecting Vmc," I hope you won't mind indulging me. I'm studying for my commercial multi and these discussions are very helpful.

The issue is regarding HOW the windmilling prop creates drag.

I've heard the argument you mentioned (cranking the engine, etc.). It makes sense that if the prop was driving the engine, it would need that energy from somewhere, which I assume would be from the forward velocity, thus increasing drag. Do you have a reference for this theory?

The Airplane Flying Handbook simply states that drag is increased for a windmilling prop. I assume they are getting at the fact that parasite drag from prop spinning rather than parasite drag from aircraft movement through the air. It would be higher for a windmilling prop than a stationary prop, which would have no spinning prop parasite drag. Although, an operating engine would have even more parasite drag since it is spinning faster and parasite drag increases with airspeed.

There is the "flat plate" theory, where the windmilling prop creates parasite drag like a disc into the wind. But this makes little sense to me because a stationary prop would be presenting the same blade surface area into the wind.

Lastly, I've seen the argument where the windmilling prop creates a negative AOA, resulting in reverse thrust. (see diagram on link). Airplane's forward velocity & prop velocity combine to create total velocity for prop. As rotational speed of prop decreases, the AOA decreases as well until it becomes a negative AOA, which will create negative thrust.
Link: http://flysafe.raa.asn.au/groundschool/windmilling4.jpg

Closing questions:

Where does it call for "windmilling prop?" I don't see that explicitly in 23.149.
When speaking of windmilling prop are we comparing it to an operating prop or a stationary prop?

Thanks.

Cubdriver

Yeah sure, here's another one. When the airplane sees an imbalance of moments from the opposing keels due to sideslip (ex. 2), it tries to correct by turning a little into the wind, a stabilizing effect. This improves vmc performance by making the airplane a little bit more stable directionally.