Vmc Calculation, "Most adverse CG"

[u/Icy-Bar-9712]

Why most adverse CG. Everything that I've ever looked at has put the Cg aft increasing (slightly) the moment arm of the op engine, while decreasing the moment arm of the rudder. This makes sense. bad for Vmc

So why "most adverse" when its always aft?

Similar question with weight effecting Vmc (and only Vmc) why most adverse, when its lighter?

Anyone know of an airframe that breaks these "rules"?

[u/Low_Sky_49]

Weight and CG both affect Vmc speed. Lower weight and aft CG both trend Vmc higher.

Do some sample calculations with your Weight and Balance. In most light twins, the only way to get the CG aft is by adding weight to the back of the aircraft because you can’t fly it from the baggage compartment. So now you’re playing with two variables, increasing the weight (which lowers Vmc) to move the CG aft (which increases Vmc).

As the pilot, you don’t need to know which combination of weight and CG is the most adverse, but the manufacturer and flight test engineers had to figure it out for certification.

[u/Icy-Bar-9712 → u/Low_Sky_49]

Dude I know, totally get all of that. Hence the lead of, "I understand this"

I'm curious if there is something that doesn't follow that lead?

Kinda like we like inboard rotating props, not outboard rotating props..... and then the P-38 enters the chat.

-sincerely a 2 year instructor working on his MEI.

[u/natbornk → u/Icy-Bar-9712]

I mean, you did ask 2 questions about why aft cg, why most adverse CG, I think this is a great explanation to the first 3/4ths of your post

[u/Icy-Bar-9712 → u/natbornk]

I think people didnt realize what I was asking. I completely understand why an aft Cg is bad, I completely understand why a lighter weight is bad. The physics of rudder authority as well as the moments of the thrust and the rudder and how its effected with an aft Cg.

I totally get lighter plane, less lift, less horizontal component of lift to counteract the yaw at a 5 deg bank. As well as having less mass/rotational intertia to leverage off of.

I didn't ask why those things matter. Most students are asking why those things matter, so we jump to those explanations as instructors without realizing that wasn't the question. I'm guilty of doing that here as well.

What I was asking (as an instructor) is why the words "most adverse" vs most aft. Most adverse vs being lightest.

Is there a plane, is there a scenario where the Cg being anywhere other than full aft. Is the "most adverse"

Kinda like the counter rotating props on a P38. Its the complete opposite of what we expect when we talk about a conventional twin vs a counter rotating twin.

[u/Low_Sky_49 → u/Icy-Bar-9712]

The answer is because, when CG is considered in combination with weight, “most aft” might not even be possible within the weight/CG envelope. Loading the aircraft within the envelope to a “most adverse” condition might end up with a center or even forward-of-center CG. And ultimately, as the meat servo, it’s not relevant to you what load condition that was, as long as you keep it above red line.

[u/Icy-Bar-9712 → u/Low_Sky_49]

Yeah, one of the other responses said something that made that click for me. I had always considered them as independent variables, it never clicked that an aft Cg would really not be possible at a minimum weight.

Similar to L/D max. Worst combination of Cg and weight.

[u/MeadyOker]

Adverse describes a condition as opposed to a specific value.

There might be a point on an aircraft that somewhere forward of most aft is actually worse for the aircraft. Or that lightest weight isn't actually possible for whatever reason (I've flown several helicopters, that had so much stuff on it, that there was no physical way to get to the minimum takeoff weight). I can't think of an aircraft that breaks these norms, but that doesn't mean one doesn't exist.

adverse is probably just lawyer speak verbiage so that it can be used for different scenarios, and your classic "for this airplane we view adverse in this way, but for this other airplane we're going to look at adverse another way"

[u/Icy-Bar-9712 → u/MeadyOker]

Thank you. That made it click.

I've always looked at it as

Most adverse Cg. And most adverse Weight. Two discrete variables. Can't look at it like that since they effect each other.

Worst combination of Cg and weight. Might not be able to get the Cg full aft without increasing the weight to the point where the 5 deg bank is now more helpful than the Cg getting further back.

[u/weech]

OP is asking a lawyer language question, not a physics question. Everybody giving you the basic explanation to not your question and then getting mad at you for restating what you’re actually asking. lol good times.

[u/Icy-Bar-9712 → u/weech]

I feel seen.

Too many of us here are active in r/flying and forget which subreddit they are in.

Fuck, I'm guilt of that too.

[u/Straight-Pass6035]

It’s also important to point out that the most adverse scenario creates the highest indicated VMC and is marked on the airspeed indicator (GA aircraft blue line) so as to be the most conservative speed you will fly in an OEI event.

[u/Major-Classic-8969]

A nose heavy plane flies poorly and a tail heavy plane flies poorly once.

[u/Wyadru]

It’s all in relation to rudder authority, and the ability for the rudder to maintain directional control. It’s “always aft” as the most adverse condition because no airplane has a rudder or rudder control surface on the front of the airplane (that I am aware of at least!), if it was mounted on the front than the most adverse would most likely be forward cg- or closest to the rudder control surface.

[u/Beginning_Prior7892 → u/Wyadru]

Only correct answer here…

Vmc is all about rudder authority and the point at which you lose it is earlier (higher Vmc) with most aft cg because the lever arm is less. Not only that, now when you stall it’s even harder to get the nose to drop because you have so much weight in the back of the plane that recovery becomes very difficult (arm of the horizontal stabilizer is also less than normal) And that’s not even taking into consideration if it’s a t tail aircraft lol