AGI Study Questions

[t0r0nad0]

So I was studying for my AGI last night, and I came across a little section on multi-engine operations in the part about climb rates. I had forgotten that I'll be certified to teach multi-ground school, so I'll have to learn this stuff. Anyway, I digress...

It said that on a multi-engine plane with props that rotate clockwise as looking from the back, that the left engine is the "critical engine" as it's center of thrust is closer to the centerline of the airplane than the right engine's, due to P-factor. After taking a couple of minutes to mull that one over, it made sense, and I can see what they mean. But, I do have a couple of questions that weren't answered in the Gleim outline:

1) So the left engine is the "critical engine"... so what? I mean, what does that mean to the pilot? Will the plane automatically crash because the left engine goes out instead of the right one?

2) In a multi-engine airplane with counter-rotating props (like a Piper Aerostar), is there not a critical engine since the center of thrust of both props will be equidistant from the centerline of the airplane?

Thanks in advance for the help. I'll probably post more questions here as studying commences (My goal is to take the exams on 6/30).

 

[poadeleted20]

1) So the left engine is the "critical engine"... so what? I mean, what does that mean to the pilot? Will the plane automatically crash because the left engine goes out instead of the right one?

No, but it does mean that Vmc is higher with the left engine out than with the right engine out. This is because the center of thrust on the right engine is farther from the cg of the plane, increasing the yaw moment when only the right engine is running than when only the left engine is running. Thus, with the left engine out, you need more rudder deflection at the same speed to hold the nose from yawing than with the right engine out. That means as you decelerate, you run out of rudder sooner with the left engine out/right engine running, and that means Vmc is higher in that situation.

2) In a multi-engine airplane with counter-rotating props (like a Piper Aerostar), is there not a critical engine since the center of thrust of both props will be equidistant from the centerline of the airplane?

Yes, with c/r props, there is not a critical engine -- Vmc will (at least theoretically) be the same with either engine out.

 

[Areeda]

PJ,

#1 No you can still fly with the critical engine inop, it is just more difficult to control if than if the non-critical engine is out. In other words Vmc will be higher.

#2 Correct, with counter rotating engines there is no critical engine, the plane is equally difficult to control with either one out.

Joe

edit: A bit slow and Ron's answer is more complete, but I didn't say anything stupid so I'll leave it.

 

[t0r0nad0]

Thanks Ron and Joe, that makes sense. So by that token, are there actually 3 VMC's in a twin (both/neither, right, left)?

 

[Areeda]

Thanks Ron and Joe, that makes sense. So by that token, are there actually 3 VMC's in a twin (both/neither, right, left)?

Well not by definition.

Vmc for a twin is defined under a specific set of conditions that is basically worst case of the three you list with the critical engine out, other engine at full throttle, aft CG limit, max gross weight, 5° bank... That is where the red line on the airspeed indicator is.

Change any variable and Vmc changes.

Joe

 

[t0r0nad0]

I assume the POH for a particular airplane will tell me approximately how much to raise the Vmc for a given change in any of those variables? Or is there just some rule of thumb I should use?

 

[poadeleted20]

So by that token, are there actually 3 VMC's in a twin (both/neither, right, left)?

There are actually an infinte number of Vmc's. Vmc is affected by many factors, but primarily:

• Bank angle - more bank reduces Vmc
• Critical engine - Vmc higher with critical engine failed than other engine
• Configuration - Vmc slightly reduced with gear/flaps down due to yaw stability
• CG location - Vmc increased with more aft cg due to shorter arm to rudder
• Density altitude - Vmc reduced with altitude since operating engine is putting out less power
• Feathered engine - feathering reduces drag on dead engine, requiring less rudder, reducing Vmc

The official POH Vmc is, by regulation, computed with 5 degrees bank, critical engine failed, gear/flaps up, aft-most cg, sea level density altitude, and the inoperative engine unfeathered. However, since several of these (bank, cg, and DA) are infinitely variable with the situation, and configuration/feathering/failed engine may also be different than book, the actual Vmc in any given real situation is also infinitely variable within the range from best case (high bank into the good engine, forward-most cg, right engine out, gear/flaps down, maximum denalt, and dead engine feathered) to worst case (banked away from the good engine, aft-most cg, left engine out, gear/flaps up, zero DA, and dead engine windmilling).


BTW, the mnemonic for those items is Baltimore City Cops Catch Dangerous Felons, invented by flight instructor and retired Baltimore City cop Harry Roop.

 

[poadeleted20]

I assume the POH for a particular airplane will tell me approximately how much to raise the Vmc for a given change in any of those variables? Or is there just some rule of thumb I should use?

No, there isn't, and in most cases, no the book doesn't tell you. The only way to find out is, as I was given as a homework assignment by the DPE for my MEI ride, to go out and try it in the various configurations (feathered/unfeathered, gear/flaps up/down, opposite engine, varying bank angles) and see how much difference it makes in that particular plane.

 

[t0r0nad0]

Okay, thanks for the help!

 

[Skip Miller]

I was given as a homework assignment by the DPE for my MEI ride, to go out and try it in the various configurations (feathered/unfeathered, gear/flaps up/down, opposite engine, varying bank angles) and see how much difference it makes in that particular plane.

How much difference did it make, Ron? Was it significant?

-Skip

 

[poadeleted20]

How much difference did it make, Ron? Was it significant?

Bank angle was huge -- the difference in Vmc between 5 into the good engine and zero bank was near 10 knots. The gear/flaps was very small, maybe a knot or two. Feathered engine was again upwards of 5 knots, critical engine was smaller. And this was in a PA23-150 -- no telling how it applies to other planes.

 

[TMetzinger]

I saw similar results as Ron in the Seminole. Bank angle was most important, feathering the dead engine helped the next amount, other things didn't matter that much.

Remember a real emergency is no time to play test pilot, and sometimes the smartest thing to do in a light twin is to pull BOTH engines back and maintain positive control.

 

[Noah Werka]

Quote: Originally Posted by t0r0nad0 View Post 1) So the left engine is the "critical engine"... so what? I mean, what does that mean to the pilot? Will the plane automatically crash because the left engine goes out instead of the right one?

Quote: 2) In a multi-engine airplane with counter-rotating props (like a Piper Aerostar), is there not a critical engine since the center of thrust of both props will be equidistant from the centerline of the airplane?

The 600 series Aerostar had both engines rotating clockwise when viewed from the cockpit. In the 700 series Aerostar, both engines rotated outward, making both of them critical. The only other airplane that had outward rotating props was the Lockheed P-38 of WWII Fame.

Buddy of mine had lots of Aerostar time and said the 700 was a handful with one engine out.

Check out the position of the de-ice boots on the props on this 700P

.

 

[t0r0nad0]

Quote: Originally Posted by t0r0nad0 View Post 1) So the left engine is the "critical engine"... so what? I mean, what does that mean to the pilot? Will the plane automatically crash because the left engine goes out instead of the right one?

Quote: 2) In a multi-engine airplane with counter-rotating props (like a Piper Aerostar), is there not a critical engine since the center of thrust of both props will be equidistant from the centerline of the airplane?

The 600 series Aerostar had both engines rotating clockwise when viewed from the cockpit. In the 700 series Aerostar, both engines rotated outward, making both of them critical. The only other airplane that had outward rotating props was the Lockheed P-38 of WWII Fame.

Buddy of mine had lots of Aerostar time and said the 700 was a handful with one engine out.

Check out the position of the de-ice boots on the props on this 700P

.

Wow, I knew that the props counter-rotated, but I didn't realize that they both rotated outwards. That's kind of a strange design, I wonder why they didn't just have them both rotate inwards for this very reason? That said, I have only seen one Aerostar (pictured below)... must be a 700 because the props counter-rotated.

Looking at the boots on the left prop here, it appears that this rotates outward as well.

 

[Skip Miller]

The P-38 was first flown with "normal" counter-rotating props and the plane's handling suffered. Evidently when the props' slip stream impacted the tail surfaces, bad handling resulted. I had always thought the long horizontal stabilizer between the two vertical stabilizers might have been a contributing factor - but that is nothing but a guess. The wartime solution was to fit counter-rotating engines that turned the "wrong" way and the problem was solved although single engine ops suffered. This was not an elegant solution, but in wartime, they didn't have a lot of time to get this bird out of the hangar and fighting the war.

I don't believe the Aerostar is going to go to war anytime soon so I have no idea why they have the engines turning the "wrong" way.

Edit: From Wikipedia:

the propeller rotation was reversed, with the blades rotating outwards (away) from the cockpit at the top of their arc rather than inwards as before. This improved the aircraft's stability as a gunnery platform.

-Skip

 

[TMetzinger]

If they both rotate outwards, why are they both "critical"? My understanding is that a "critical" engine is the one whose failure most reduces performance. Thus, in a twin, there can be only one critical engine if there is one.

In the -700, it sucks when an engine fails as you're going to have a big "arm" on the running engine, but it sucks equally no matter which engine fails. Just need to have enough rudder to manage it.

 

[Noah Werka]

Tim, with an inward rotating prop, the effect of an engine out, (either side) is not as great as with an outward rotating prop. On a "normal twin, which both engines rotate clockwise, think of the moment arm of the downward moving of the propeller blade which is farthest from the center line of the aircraft. It creates the most moment(think P factor). That is why the right engine is the critical engine. On an aircraft that both engines rotate inward, the loss of either engine will not give you the problems of the outward rotating(right side) prop on a "normal' twin thus no critical engine. On an aircraft with both engines rotating outward, the moment arm(think P factor) is the same as a normal twin(left engine out), big problems.

 

[poadeleted20]

The critical engine is the one which affects controllability more. In the outward turning Aerostars, neither engine affects controllability more than the other. Thus, neither is "critical."

 

[TMetzinger]

Tim, with an inward rotating prop, the effect of an engine out, (either side) is not as great as with an outward rotating prop. On a "normal twin, which both engines rotate clockwise, think of the moment arm of the downward moving of the propeller blade which is farthest from the center line of the aircraft. It creates the most moment(think P factor). That is why the right engine is the critical engine. On an aircraft that both engines rotate inward, the loss of either engine will not give you the problems of the outward rotating(right side) prop on a "normal' twin thus no critical engine. On an aircraft with both engines rotating outward, the moment arm(think P factor) is the same as a normal twin(left engine out), big problems.

As I said, it doesn't matter WHICH engine fails, it sucks. But that means neither engine is critical - you don't care which fails.

 

[t0r0nad0]

Quick update... I took and passed both the FOI (90%) and AGI (86%) exams yesterday. I have an appointment with the FSDO tomorrow morning at 8:30a to make everything official.

Now, I already filled out my application via IACRA and printed out the "Unofficial Copy" 8710 form from that... does anyone else know if I need to bring anything else with me to the FSDO besides picture ID and my test scores?

 

[gprellwitz]

Quick update... I took and passed both the FOI (90%) and AGI (86%) exams yesterday. I have an appointment with the FSDO tomorrow morning at 8:30a to make everything official.

Now, I already filled out my application via IACRA and printed out the "Unofficial Copy" 8710 form from that... does anyone else know if I need to bring anything else with me to the FSDO besides picture ID and my test scores?

As I recall, that was about all I needed to bring. I may have had my log book with, but it isn't required.