tgrayson
New Member
I considered it, but then thought of the xkcd cartoon: "Someone is wrong on the internet." There are a limited number of battles I can fight.
Vmc...
- Thread starter meritflyer
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I considered it, but then thought of the xkcd cartoon: "Someone is wrong on the internet." There are a limited number of battles I can fight.
Can you give more detail. What he said sounds like BS, but it sounds like believable BS with my education on this topic. The arm (in green) he depicts, is where I smell BS. However, it sounds feasible that the moment arm of thrust (if that is even the right terminology) would be effected.
Do you believe this is accurate, "equaling the amount of drag that would be produced by a disc of plywood the same size as the propeller."? It is obviously what screws some of us up, like me, by embedding in our heads that a spinning propeller represents a flat disk when it doesn't. I curse this man for this page just for this comment!
The gear section sounds as if the people from alice and wonderland spoke to him in a dream. The way I see it, the flaps/gear on the operating engine side block, for lack of a better word, some of the thrust thereby reducing that thrust. Reduced thrust equals lower VMC.
It is interesting he doesn't even mention the keel aspect that many attribute gear to as a stabilizing platform.
Finally, "This decreases the overall drag that would otherwise result from the increase in “wetted area” caused by the oblique airflow across the airframe in uncoordinated flight." WHAT! Is it just me or does this make sense to you?
And as for the remaining paragraph in that section,
"The asymmetrical drag increases the rotational force vector and raises Vmca."
tgrayson
New Member
He doesn't know what a lever arm is. The lever arm is the perpendicular distance from the thrust line to the CG. That doesn't change with forward/rearward changes in the CG, but it would change with lateral movements of the CG, if, for instance, there were a wing locker full of bowling balls.
Understood, thanks.
tgrayson
New Member
No. The drag is likely far less than this.
Wetted area is the wrong term to use here; that's a fixed characteristic of an airplane and used to compute friction drag. The drag due to non-coordinated flight would all be of a pressure drag type, which is produced by flow separation.
dasleben
That's just, like, your opinion, man
Not sure where that question comes from, but yeah, I used to do a pretty decent Vmc demo when I was an instructor. Vmc in most light training twins is largely negligible when in zero sideslip. Honestly, gear/flaps/power, whatever, a normally-aspirated training twin tends to mush around and stall long before you actually see any real loss of control.
However, take it out of zero sideslip and you can have some fun. Cross-controlled, you'll lose directional control a full 20-30 knots above published Vmc.
Plane Trouble
Well-Known Member
Oh come now... The Pax can always go up front in sit in the pilots laps
nosehair
Well-Known Member
Yep. And the airplane is more stable with gear down. In most light twin trainers, the difference in Vmc gear down or up is not really readable on the airspeed indicator, but the controllability is noticeable.
I have discovered, in a lifetime of training vs. operating that a lot of the things we read about in aerodynamics, like slow flight, "behind the power curve", accelerated stalls, spin entry, even a simple thing like "coordinated turns", is much different in big airplanes.
This huge difference in the operation of big, or fast, airplanes is the root cause of the "pitch/power" debate.
In my experience, most light airplane pilots pitch to airspeed on final, while most big fast airplane pilots pitch to glideslope on final.
As to the B-17 "myth", Hacker, I use that example as "possible situation" where it might come in handy. I really like the B-17 manual you dug up, but producing a manual has nothing to do with the "myth".
The manual prescribes procedures that fall within "normal operations". My scenario describes "Battlefield conditions" where pilots (and all soldiers) do what is necessary to win the war. I'm sure many airplanes took off overloaded. ...aaand it may be a total 'myth', but it serves as a tool of instruction. See, you remembered it.
I do know, with absolute certainty, that all 'retract' airplanes I have trained in demonstrate a more stable platform with gear down. That is part of a complex check-out with me. Do slow flight with the gear up a bit, then put the gear down and watch how the ship stabilizes.
Hacker15e
Who am I? Where are my pants?
Those pages came from a thread on a warbird forum in which I re-posted your statement and posed the question if anyone knew anything about such a story. I've been around warbird pilots for decades, including several B-17 pilots, and I've never heard such a myth. But, since I'm not a B-17 pilot myself, I figured I'd ask some people who are.
The result? Several B-17 operators, both past and present, say that they've never heard of the "myth" in any way whatsoever.
http://warbirdinformationexchange.org/phpBB3/viewtopic.php?f=3&t=35063
I don't see the point in using the story as a teaching aid if there's no evidence that it's true.
tgrayson
New Member
If it's not readable on the airspeed indicator, then I suggest you don't have any real evidence that you noticed anything. Placebo effect, maybe?
That's why without objective, measurable data, I don't think that any claims about what affects Vmc in what direction are very reliable. Environmental conditions are almost never the same and it's hard to do the experiment in exactly the same way ever time. Test pilots generally have more training in this regard and the airplanes often carry test and recording equipment that make the gathering of reliable data much more likely.
For instance, I can change Vmc by adjusting my degree of bank a degree either side of the allowed 5. Making sure I have *exactly* 5 is impossible with the instruments we have in the airplane, since usually the attitude indicator is marked in units of 10 degrees. And, the rule of thumb is that each degree is worth 3 knots of Vmc.
trafficinsight
Well-Known Member
Random things I know about Vmc 
1.) CAR3 Airplanes were calculated at most aft C.G. with nothing said about weight, Part 23 Airplanes are calculated at "Most unfavorable C.G. and weight." There's a difference.
2.) Gear and flaps may or may not lower or raise Vmc. I always figured they chose gear and flaps up because it's a known quantity.
3.) Vmc is calculated at 5 degrees of bank into the operating engine... this may or may not be the most favorable condition depending on the airplane.
Obviously the key word here is "calculated" They didn't go Vmc twins at low altitude to make sure. In reality Vmc is almost never what's marked on the airspeed indicator, and in the old days it wasn't even marked.
1.) CAR3 Airplanes were calculated at most aft C.G. with nothing said about weight, Part 23 Airplanes are calculated at "Most unfavorable C.G. and weight." There's a difference.
2.) Gear and flaps may or may not lower or raise Vmc. I always figured they chose gear and flaps up because it's a known quantity.
3.) Vmc is calculated at 5 degrees of bank into the operating engine... this may or may not be the most favorable condition depending on the airplane.
Obviously the key word here is "calculated" They didn't go Vmc twins at low altitude to make sure. In reality Vmc is almost never what's marked on the airspeed indicator, and in the old days it wasn't even marked.
Papoo
Polar 1.
Up to 5 degrees of bank is what 23.149 states, and the pertinent point that you've raised is 'may or may not' be a favorable condition. A small bank in to the operating engine will (probably) lower Vmc; you're using your wings to create additional 'aid' to directional control.
However, what seems to be unknown to most, is that the 'split ball' condition MASSIVELY decreases performance. It is agreed that the split ball is the minimum parasite drag attitude, but test pilot reports do show that the increased induced drag thoroughly swamps any decrease in parasitic drag.
Not that this is too relevant to the discussion, but I just wanted to point it out in the event of a real engine loss, because I feel a great number of guys have been told the wrong thing, and they'll split the ball. Center the ball. The performance difference is VAST, and it may be the difference between life and death.
Definately worth proving this with your students, and ensuring they know what to do in the event of an engine out.
Incidentally, among the test pilot community, there is a lot of questioning as to the point in training pilots to do Vmc demos. It's a certification requirement for aircraft, and largely irrelevant. What the test pilots are saying, is that it is teaching pilots to do the wrong habits for when crunch time comes.
You can bet your bile duct that when you have an engine-out on climb out, start losing directional control etc, on a high density altitude day, the guy that banks 5, and reduces throttle (another idiotic habit) when loss of directional control comes, will be much worse off than the guy that centers the ball, and in the even of losing directional control, lowers the nose, maintains full throttle to regain airspeed and continues the climb..
Food for thought.
tgrayson
New Member
Evidence?
The flight tests I've seen show that optimal bank into the operating engine is 2-3 degrees. And this is what theory calculates. This equates to about 1/2 ball out of center. You won't find any knowledgable or authoritive source that advocates a centered ball.
Mike H
Well-Known Member
Actually, it's gear up and flaps set to Take Off position. The point is to establish a knows airspeed that will allow for aircraft control in the event an engine fails at the most inopportune time- which would be on take off, after gear retraction (with no usable runway remaining), and before you've gained enough airspeed to retract the flaps.
No man/women would, our balls must be off center for aesthetic purposes. Imagine a man or women with a center stack...or yourself.
That's a pretty bold definitive statement, what with the HUGE number of variables that come into play in such a scenario. To me, it really depends.....on any number of combinations of factors; and especially pilot technique and where the particular event is exactly happening (airspeed/alt/terrain below, etc, etc, etc), since even the same general techinques won't be done by every pilot the same exact way.
trafficinsight
Well-Known Member
trafficinsight
Well-Known Member
I'll freely admit that aerodynamics is not my strong point... but isn't the idea behind the split ball to give you zero sideslip? and wouldn't that give you the lowest drag?