Why does Vmc decrease while in ground effect

[Bernoulli]

Here is a multiengine aerodynamics question for you that I don't quite understand. Why would Vmc decrease while in ground effect. I understand why performance increases while in ground effect... but I don't quite understand why ground effect would lessen the effect of an operating engine yawing the plane toward a failed engine. The reason ground effect improves performance is because the ground changes the upwash and downwash of the plane and ultimately reduces INDUCED DRAG... So why would a decrease in induced drag reduce Vmc? Thanks in advance for any replies.

 

[CSY Mon]

VMC in ground effect?


Hmm, that would only be important on take-off and if ya are in a non-transport category craft, like a Seneca, just cut the power and land.

If in ground effect on landing, ya only consider VMC in case of a full power G/A.

I had a similar question years ago when I flew the C-402 with a Robertson STOL kit.

The VMC was several knots below "normal" VMC, but I never found out why....

A mod giving more rudder perhaps?

Anybody else..?

 

[Snoopy58]

VMC may not decrease, but the airspeed indicator may be reading differently! On the C-130, we had VMC in & out of ground effect charts, with separate charts for airplanes with a particular pitot-static system. IIRC, with one pitot-static system, the VMC IGE was lower, while with the other system the OGE was lower.

Knowing your IGE VMC was very important in a Herk (or any airplane with high VMC's), as you don't want to rotate slower than that speed... because if you do & then lose one, you're instantly in a world of hurt. Putting it back down at that point may not be a good option, since there is no performance data for stopping beyond when you start to rotate, and if you're not on an extremely long runway, you could find yourself running off the end at a high speed, when the airplane would have flown just fine on 3 engines. For about everything else, you'd use the OGE speed, but (if at all possible) you computed performance based on delaying rotation until the IGE VMC speed. (There was a way around it if you absolutely positively could not increase the rotation speed, but it amounted to a waiver to accept the increased risk should you happen to lose an outboard engine at exactly the wrong moment.)

Hope that helps!

 

[Bluto]

Here's a really shady explanation for why Vmc decreases in ground effect:

Ground effect is more pronounced the closer you get to the surface. If an aircraft began to lose directional control due to Vmc, it would begin to roll and yaw toward the inoperative engine, as this occurs, the wing with the inoperative engine gets closer to the ground reducing its induced drag further while the wing with the operating engine rises farther out of ground effect increasing its induced drag. The resulting differential drag yields an opposing force to the Vmc yaw, thus (minutely, if at all) reducing Vmc.

While this explanation is lame, it's pretty much the only thing I can come up with. Now, more importantly, I think people generally misuse the "factors affecting Vmc". The purpose of learning these factors is simply to have a frame of reference when it comes to Vmc. The reason the FAA cares about these factors is for aircraft certification so that Vmc means the same thing to all certified multi airplanes.

This whole stupid discussion really bothers me: "How does this factor affect Vmc? How does that factor affect Vmc?" because many of the factors vary by aircraft and there is no authoritative source for the answers. Many MEI's teach what they were taught by rote having no idea how the forces and aerodynamics really work.

 

[91100 100 set]

It was a long time ago that I thought about this, but I think it has something to do with the fact that the aircraft angle of attack is slightly (very slightly) less in ground effect. With a lower AoA on the entire plane, the difference between the descending and ascending propeller blades on the critical engine is slightly less, creating a more symetrical "area" of thrust coming off that propeller. In other words (now that my thoughts are more organized) the descending blade takes a bigger bite than the ascending blade (same thing that contributes to a turning tendency in a single engine plane). Increase the angle of attack, the thrust is more asymetric, where more of the total thrust for that propeller is coming from the descending blade. This only applies to the critical engine (the effect would be opposite on the non-critical engine), but all certification criteria (which is where the 9 factors of VMC come from) are based on the critical engine becoming inoperative.

 

[bigD]

Yeah, the lower AOA and thus lower P-Factor while in ground effect was always the reason I've been given. Seems to make sense to me.

CSY Mon - I'm assuming the STOL kit included a VG kit for the vertical stabilizer. My understanding is that (like a VG kit for a wing) it delays airflow separation slightly, which in turn gives more rudder effectiveness at slower airspeeds. Therefore, Vmc decreases.

 

[Oakum_Boy]

You're in trouble if you VMC in ground effect.

 

[VNugget]

This whole stupid discussion really bothers me: "How does this factor affect Vmc? How does that factor affect Vmc?" because many of the factors vary by aircraft and there is no authoritative source for the answers. Many MEI's teach what they were taught by rote having no idea how the forces and aerodynamics really work.

That attack may be valid, but woldn't it be an argument FOR having this type of thread? I can only benefits in people learning more through discussion. Especially those "many MEI's."

 

[Bernoulli]

Ground effect is more pronounced the closer you get to the surface. If an aircraft began to lose directional control due to Vmc, it would begin to roll and yaw toward the inoperative engine, as this occurs, the wing with the inoperative engine gets closer to the ground reducing its induced drag further while the wing with the operating engine rises farther out of ground effect increasing its induced drag. The resulting differential drag yields an opposing force to the Vmc yaw, thus (minutely, if at all) reducing Vmc.

It was a long time ago that I thought about this, but I think it has something to do with the fact that the aircraft angle of attack is slightly (very slightly) less in ground effect. With a lower AoA on the entire plane, the difference between the descending and ascending propeller blades on the critical engine is slightly less, creating a more symetrical "area" of thrust coming off that propeller. In other words (now that my thoughts are more organized) the descending blade takes a bigger bite than the ascending blade (same thing that contributes to a turning tendency in a single engine plane). Increase the angle of attack, the thrust is more asymetric, where more of the total thrust for that propeller is coming from the descending blade. This only applies to the critical engine (the effect would be opposite on the non-critical engine), but all certification criteria (which is where the 9 factors of VMC come from) are based on the critical engine becoming inoperative.

Hey Bluto and 91,100,100 set...thanks guys. Great answers. I think both are accurate. None of the book I have on Multiengine have answers to this so thanks a bunch.

 

[Bluto]

That attack may be valid, but woldn't it be an argument FOR having this type of thread? I can only benefits in people learning more through discussion. Especially those "many MEI's."

I agree entirely. My frustration is directed solely at a system that allows (fosters?) rote knowledge in such a safety-critical area, not at the original poster. I apologize if I didn't make that clear initially.

I feel searching for the 'whys' can be a waste of time for students when the impact on Vmc is either insignificant, or indeterminate. A thorough discussion on the factors you can control is much more valuable in my estimation.

Unfortunately, many students never even hear from their instructors where the "nine factors" come from. Thus they assume that they each have some deep significance when it comes to aircraft control. In reality, the nine factors are simply the conditions required to determine Vmc for aircraft certification so that a meaningful comparison can be made between different types. For anyone who is interested, the specific source is: 14CFR 23.149

Oh and 91,100, 100 set,
I like your explanation, it makes sense.

 

[User546]

And one thing that absolutely blows my mind is how many students never learn, or full understand that when things head south, and your facing a possible or imminent Vmc rollover - simply pull the power back to idle, thus eliminating the airplane's ability to roll.

I was flying with a guy in a 421 a month or so ago and we were talking about the single engine performance quality of the 421, and he was talking about how easy it was to Vmc roll it, yadda yadda. When I made the comment "if your going to Vmc, pull the power back to idle - no power, no Vmc." He looked at me with a look of amazement I've never seen before, as if I just gave him this secret revelation of flying multi-engine airplanes. A guy that doesn't understand that doesn't have any business flying his wife and four children around in a 421.

So much of the multi-engine rating is geared towards Vmc, but like the previous poster said, it's all rote knowledge, and I bet the majority of MEI instructors couldn't break down much further past the rote knowledge part of each Vmc factor.

Its important to understand those things, but its even MORE critical to know how to FLY and configure the airplane as quickly and safely as possible after an engine failure. Those 9 Vmc factors aren't going to save your life when your inverted and 3 seconds from hitting the ground.

 

[91100 100 set]

I also agree with the sentiment this thread is taking. Most of the 9 factors are so insignificant as to how much they actually change the speed where the plane will lose control. One or two knots, maybe. And if you can fly to one or two knots in the real world, I'm impressed. I think it's important to have a decent understanding of those factors yes, but only so far as how they are certification criteria only.

When I would teach this stuff, I would focus on two areas regarding single engine operations. Control (heading and roll) of the airplane and performance (climb/sink rate) of the airplane. They often contrast each other. Having a problem controlling the plane, reduce the power on the good engine. But your performance goes out the window. Another area that was not sufficiently stressed to me as a student is the role of bank angle and slip angle and how it relates to control versus performance of the plane. I had quite a few commercial ME students that seemed to think that 5 degrees into the dead engine was where it was at, based on their previous training. Again, a certification limit. The manufacter is only permitted up to 5 degrees of bank to establish Vmc during certification. You want a lower Vmc, just bank more, but again, the performance suffers. If you are seeking maximum performance (best climb or minimum sink), I would teach 2-3 degrees of bank and the ball "about half way out". But I would stress how this relates to the actual slip angle. Because it's the slip angle that really matters most. Whatever bank angle and skid/slip ball indication gives you zero side slip is where you will find the best performance. Some of my school's twins had those little strings somewhere on the nose, and that was handy to demonstrate this. The instrumentation in your typical twin (only bank angle and skid-slip ball) are actually pretty lousy when it comes to determining where exactly zero side-slip will occur.

 

[91100 100 set]

Those 9 Vmc factors aren't going to save your life when your inverted and 3 seconds from hitting the ground.

I'm not sure knowing much of ANYTHING will save your life in that situation!