Phxdvt17

Gentleman/Ladies

I'm working to finish up my MEI and am having a hell of a time explaining how "trimmed for takeoff" effects Vmc speed. Can anyone help shed some light on this for me?

Thanks!

JamesNoBrakes

It doesn't, you are thinking in to it too much, as well as other instructors that sometimes try to "teach" stuff that simply isn't there.

The FAA sets up a condition for the aircraft to test Vmc and ensure the airplane is controllable.

That condition is takeoff configuration, because takeoff is the most critical time. The only thing that's different is that they assume the gear is up, because that's relatively easy to do, usually quick after takeoff and before the situation would be very critical (because you are still right next to the runway hopefully). So for takeoff you are going to have the airplane trimmed for takeoff, or hopefully pretty close. Not because the trim affects it, but because the "critical situation" is an engine failure right after takeoff. So setting up the takeoff configuration is the key.

Yes, we could get into a ridiculous discussion about whether trimmed away from control surface and acting like an servo-tab vs anti-servo tab, but that's NOT how you set up for takeoff, so it's irrelevant.

It's FAR 23.149, those are where the "factors" come from, but not all those "factors" are going to make a noticeable difference in control and some of them you won't have any control over, although they still might affect your ability to control the aircraft.

Confused yet?

I've heard it referred to time and time again as "9 factors-blah blah", well, there's only about 8 or so if you read the regulation, people usually make up "cowl flaps" and throw that in there, maybe it's assumed with the "takeoff config" thing, but it's not in there verbatim.

To prove some of these "factors" you'd have to throw your EXACT plane in a wind tunnel and find out whether the drag of the nose gear and it's moment overcomes the drag of the two main gear and their moments. Or whether the roll caused by the cowl flap "thickening" the camber/planform and flap is worse than the additional drag that is also produced.

The "airborne and out of ground effect" thing too...I mean we don't fly Ekranoplanes, so it's pretty much ridiculously not applicable. We "could" fly in ground effect with less power due to less induced drag, and get less overall yaw and roll, but seriously, it's ridiculously not applicable. The only part that is applicable is that the airplane is NOT in ground effect for this most critical situation, but it's not like we're worrying about how "ground effect" "affects" Vmc.

So the bottom line is that some of those things are a wash or "N/A" as far as applicability to effects on control. Hope this helps. Unfortunately, this gets to be a "rote-list" thing with many people (instructors, DPEs, etc) and not a real "understand" thing. The FAR lists the factors for CERTIFICATION, not real world control, although some of them do affect your control on any given day. The certification process is to test the aircraft for control in the most critical/worst case scenario. That's where the "factors" come from, they are "conditions" though...

Yoda2

With the prescribed loading/CG, the tailplane/elevator tab setting would be slightly forward in it's operational range. (To compensate for the aft CG on takeoff, the tabs slight displacement from an in trail position can be considered negligible) The rudder tab conversely is not. It's use would relieve some leg work, but should be adjusted to neutral for take off. DISPLACEMENT OF THE TAB WILL REDUCE RUDDER EFFECTIVENESS, if not in trail; thereby increasing Vmc. There would also be the trim drag penalty. Hope this helps. BTW, You should have known this for your initial multi. So much for quicki multi's. Good luck

JamesNoBrakes

You are kidding, right? I'm not saying that what you are saying above is "wrong", but it totally misses out on understanding the configuration aspect of 23.149, the concept of Vmc and the things that really affect it.

BTW, displacement of the rudder trim tab in the same direction of the rudder would INCREASE effectiveness by increasing camber/AOA, but you've got to push harder with your leg to compensate because now it's working like an anti-servo tab (if it isn't already one, many are). If you're not following, I'm saying having the trim tab trimmed opposite from the direction you are pushing on the rudder due to the engine failure.

Therein lines the problem.

What if your plane has enough rudder authority to allow for use of trim in the opposite direction and relieve rudder pressure whilst maintaining control? Many aircraft that have very powerful engines DO require some rudder trim to "help" ease the control force for SE control.

Which is really better?

Are they applicable at all and does the FAA consider either of THESE situations in 23.149?

If you can keep up with such a conversation, great, but it's not really relevant to ME control IMO and experience.

I will agree though that this shouldn't be the first time the OP has heard of 23.149

Might want to check out "Flying Light Twins Safely" (has been updated recently) and "Always Leave Yourself an Out"

The most recent version of FLTS does a real good job of outlining what's important. Notice it doesn't discuss "trim" affecting Vmc.

Yoda2

Nope, not kidding. I practially did a masters thesis when studying for my MEI; it's a 3" thick binder and chock full. He also didn't seem to ask a direct question regarding certification or was not interested in certification or any other Vmc criteria. Seems he just wanted an answer to his question of how trim affects Vmc, and it very much can. Alot of folks are dead over a knot or two... This should get interesting! Gotta go to bed now. Hope this threads up tomorrow.

JamesNoBrakes

They aren't dead over knot or two IMO, they are dead due to lack of correlation between pitch, airspeed, control placements, and power. They don't understand what the airplane is doing outside and how that relates to the control positions inside. I'd say trim never comes into it significantly. If if did, the FAA and manufacturer would say something like "Warning! Use of trim can adversely affect Vmc and Aircraft Control! Do not use trim!!" (lol).

It's trying to make something out of nothing. It's like asking "what affects takeoff distance" and having to talk about how round the tires are, their coefficient of friction depending on the temperature, how the cabin air vent(s) might be open and increasing drag due to ram pressure as the air tends to "pile up" outside an open vent, etc. Discussing those things indicates a lack of understanding about the real subjects and the time should be spent ensuring those are understood and delving into more significant topics. Lord knows there's never enough time to cover the important stuff and spend time really assessing and discussing it. The more a student got experience DISCUSSING and EXPLAINING the relevant factors was usually better, because they could usually use the practice at not getting tripped up while explaining critical engine asymmetrical p-factor, accelerated slipstream, how torque effects, etc.

The point I was trying to make initially is this "8 (9) factors of Vmc" thing came directly from 23.149 and it's been a rote-answer thing for so long that people don't even know what they are talking about anymore, students, DPEs, and everyone in between. They don't know how the "factors" apply or what they are about (worst-case scenario conditions and setup for an engine failure, not "things that affect Vmc").

To the list of things that really affect control and may change on any given day/conditions/flight that you can extrapolate from above I'd add:

Control placement and zeroSS vs zeroYaw, which engine failed and how fast you are going.

The PTS really tells the tale. I'll include my own "drill down" for what I'd probably ask during a checkride:


(and yes, I have nothing better to do tonight).

Yoda2

JNB, I want to keep this in good spirits though I refuse to address some of your points as they are incorrect. You are not alone in your thinking. Many pilots do not really understand some of this and end up mixing apples and oranges or get wrapped around the axle with the ME deal. This is why the person asked the simple and common question in the first place. I do understand the subject and follow your line of thinking, but have been through this so many times it gets tiring and I don't have the time or energy now, I am not being evasive. If the guy can make sense of my answer he will be fine, or at least will give the correct answer.

JamesNoBrakes

I guess I'm saying that I do not see any allowance in the PTS to ask "what are 8 or 9 factors of Vmc", because it's not covered under anything in the PTS, and it's not a real thing. Those certification factors are not meant to be "things that affect Vmc", they are meant to be conditions to set up for the worst case scenario. Since you are testing it in takeoff and that's the worst case, you are setting up for takeoff. Not every one of those things is meant to "affect" Vmc. I didn't even mention AoA as a "factor of Vmc", but it is too, even though it's not in the "list", due to changes in P-factor at different AoAs, but that's my point, it's the condition of takeoff that's being set up in the FAR and that's simply what those "factors" are geared towards- certification testing conditions for Vmc. There's especially nowhere in the PTS where I could justify ever asking about "trim affecting Vmc". If that's going on, I'd say report the checkrides to the responsible FSDO as not abiding by PTS. Help me out though, if you think those questions would fall under a subject area that I outlined from the PTS above, tell me where and why? I'm open to discussion and suggestions.

Grumble

To the OP: an MEI needs to be able to clearly teach/explain the 4 factors that make up a critical engine, and certification of Vmc (23.149) and why each factor is important.

You lose an engine right after take off, you're going to pitch for blue line (wing down, ball deflected) and trim right?

Cubdriver

I am with Grumble and others that for an MEI applicant to overdo this topic is not a good idea. You are not applying for an engineering degree, you are applying for an instructor certficate. Just give your examiner what you learned from an approved pilot textbook on the subject. There are lots of them, and they have all you need to know as a pilot. I have an engineering degree in the field of aircraft design and such a course of study takes 3+ years of full-time effort, none of which is required to fly safely.

A nice book for walking a somewhat closer line between engineering depth and pilot-level knowledge of the subject is-

Aerodynamics for Naval Aviators.