Factors affecting Vmc
01-24-2011, 07:13 AM
#21
On Reserve
Joined APC: Jan 2011
Position: Jet SIC
Posts: 20
Originally Posted by aviationbill
Oh wow, I can't believe I'm going to type this but here goes, hope it helps
CRM LOFT ACE
C - Critical engine inop and windmilling (windmilling drag is significant compared to feathered)
R - Most rearward CG (rudder effective arm is reduced)
M - Most unfavorable weight (light weight. Remember your horizontal component of lift and how it opposes your operating engine)
L - Landing gear (as required by manufacturer certification)
O - Operating engine at max takeoff power (and remember this can only occur at sea level in standard conditions
F - Flaps (again as required by manufacturer certification
T- Trim for takeoff (an aircraft not trim for takeoff will require control loads outside of those necessary. Especially in the case of a rudder trim which is an anti-servo tab style.)
A - Airborne and out of ground effect (remember the effect that better performance can have when close to the ground)
C - Cowl Flaps (as required again by manufacturer setting. Especially for those flying a seminole this is required to be open)
E - Eliminate sideslip (max 5 degrees towards good engine)
If you really need more theory explanation behind some of these I would be more than happy to help. I've got a few thousand hours of dual given behind me and I am more than happy to point you at some resources as well.
CRM LOFT ACE
C - Critical engine inop and windmilling (windmilling drag is significant compared to feathered)
R - Most rearward CG (rudder effective arm is reduced)
M - Most unfavorable weight (light weight. Remember your horizontal component of lift and how it opposes your operating engine)
L - Landing gear (as required by manufacturer certification)
O - Operating engine at max takeoff power (and remember this can only occur at sea level in standard conditions
F - Flaps (again as required by manufacturer certification
T- Trim for takeoff (an aircraft not trim for takeoff will require control loads outside of those necessary. Especially in the case of a rudder trim which is an anti-servo tab style.)
A - Airborne and out of ground effect (remember the effect that better performance can have when close to the ground)
C - Cowl Flaps (as required again by manufacturer setting. Especially for those flying a seminole this is required to be open)
E - Eliminate sideslip (max 5 degrees towards good engine)
If you really need more theory explanation behind some of these I would be more than happy to help. I've got a few thousand hours of dual given behind me and I am more than happy to point you at some resources as well.
01-24-2011, 07:14 AM
#22
On Reserve
Joined APC: Jan 2011
Position: Jet SIC
Posts: 20
Originally Posted by Cubdriver
Thanks Ace & welcome to APC. This thread is bit out of date now although the topic never is.
Item 10 looks new. Who is including that, AllATPs? I will have to think about how drag reduction at the wing would affect one side more than the other. One thing to remember is the low-speed (ie. vmc) regime is one where the drag curve goes up exponentially with decrease in speed. So there is a lot of induced drag to begin with, and any change due to ground effect will receive an amplified effect. Ground effect would greatly reduce drag and it would affect the wings unequally since the wings are loaded unequally. That would be my theory anyway.
Item 10 looks new. Who is including that, AllATPs? I will have to think about how drag reduction at the wing would affect one side more than the other. One thing to remember is the low-speed (ie. vmc) regime is one where the drag curve goes up exponentially with decrease in speed. So there is a lot of induced drag to begin with, and any change due to ground effect will receive an amplified effect. Ground effect would greatly reduce drag and it would affect the wings unequally since the wings are loaded unequally. That would be my theory anyway.
01-29-2011, 07:57 PM
#23
On Reserve
Joined APC: Jan 2011
Position: Jet SIC
Posts: 20
This is the way I understand it: Out of Ground Effect requirement is because the airflow is more streamlined over the wings, and therefore more airflow over the rudder, when the airplane is in ground effect. When it leaves ground effect, the wash over the wing has more of a downward deflection, reducing flow over the rudder, decreasing it's effectiveness. So if manufacturers could certify their aircraft with 30degrees bank, and in ground effect, the speed would probably be 30KIAS! haha
01-07-2014, 05:46 PM
#24
Line Holder
Joined APC: Feb 2013
Posts: 65
Ace,
If you're still around, do you have a reference for that info?
Does anyone else have any info on how ground effect contributes to Vmc?
Aside from being part of the certification process, I have not found anything in any FAA publications that addresses it.
If you're still around, do you have a reference for that info?
Does anyone else have any info on how ground effect contributes to Vmc?
Aside from being part of the certification process, I have not found anything in any FAA publications that addresses it.
01-07-2014, 09:29 PM
#25
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Joined APC: Nov 2011
Position: Volleyball Player
Posts: 3,982
Originally Posted by TheStranger
Ace,
If you're still around, do you have a reference for that info?
Does anyone else have any info on how ground effect contributes to Vmc?
Aside from being part of the certification process, I have not found anything in any FAA publications that addresses it.
If you're still around, do you have a reference for that info?
Does anyone else have any info on how ground effect contributes to Vmc?
Aside from being part of the certification process, I have not found anything in any FAA publications that addresses it.
Unfortunately this subject, more than most, gets way out of hand way fast. No where in the PTS does it say "knows the factors of Vmc", in fact, it specifically states "which factors" an applicant ought to know. Things like the relationship between stall speed and Vmc, how it may vary with conditions and aircraft, the effects of CG and weight, the effects of having the proper control inputs. The PTS lists these specifically for a reason IMO. The mythical "9" (or 8 if you actually read the FAR) "factors of Vmc" is not extremely valuable, it just helps you understand the testing criteria. It's far more important to know which factors will affect you and which factors you may vary or that you may have control over.
While the pamphlet above does a great job of describing Vmc and the factors, another pamphlet "Always Leave Yourself an Out" does a great job of explaining the performance aspect, although they both overlap.
Last edited by JamesNoBrakes; 01-07-2014 at 09:42 PM.
01-08-2014, 01:35 PM
#26
Gets Weekends Off
Joined APC: Dec 2011
Posts: 177
Originally Posted by exerauflyboy5
Since I'm currently stuck in Las Vegas at ATP's CFI’s school and I’ve been over this more times than I can count. All I can add is this,
SMACFUM- The conditions that affect VMC,
Sea level pressure-increases VMC
Max power on the operating engine-Increases VMC
Aft legal C.G-Increases VMC
Critical engine Wind milling-Increases VMC
Flaps/gear down-decreases VMC
Up to 5 degrees of bank into operating engine-Decreases VMC
Most unfavorable weight-A lighter airplane has a higher VMC than heavier airplane
YUM-YUM
SMACFUM- The conditions that affect VMC,
Sea level pressure-increases VMC
Max power on the operating engine-Increases VMC
Aft legal C.G-Increases VMC
Critical engine Wind milling-Increases VMC
Flaps/gear down-decreases VMC
Up to 5 degrees of bank into operating engine-Decreases VMC
Most unfavorable weight-A lighter airplane has a higher VMC than heavier airplane
YUM-YUM
Also, for teaching SMACFUM I would say flaps up / gear up to increase VMC (as long as were talking 23.149).
01-08-2014, 06:04 PM
#28
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Joined APC: Nov 2011
Position: Volleyball Player
Posts: 3,982
Originally Posted by csucbrown
I use SMACFUM too but add PAT at the end. Props forward, Airborne & out of ground effect, and trimmed for takeoff.
Also, for teaching SMACFUM I would say flaps up / gear up to increase VMC (as long as were talking 23.149).
Also, for teaching SMACFUM I would say flaps up / gear up to increase VMC (as long as were talking 23.149).
Objective: To determine that the applicant exhibits satisfactory
knowledge of the elements related to engine
inoperative principles of flight by explaining the:
1. meaning of the term “critical engine.”
2. effects of density altitude on the VMC demonstration.
3. effects of airplane weight and center of gravity on control.
4. effects of angle of bank on VMC.
5. relationship of VMC to stall speed.
6. reasons for loss of directional control.
7. indications of loss of directional control.
8. importance of maintaining the proper pitch and bank
attitude, and the proper coordination of controls.
9. loss of directional control recovery procedure.
10. engine failure during takeoff including planning, decisions,
and single-engine operations.
knowledge of the elements related to engine
inoperative principles of flight by explaining the:
1. meaning of the term “critical engine.”
2. effects of density altitude on the VMC demonstration.
3. effects of airplane weight and center of gravity on control.
4. effects of angle of bank on VMC.
5. relationship of VMC to stall speed.
6. reasons for loss of directional control.
7. indications of loss of directional control.
8. importance of maintaining the proper pitch and bank
attitude, and the proper coordination of controls.
9. loss of directional control recovery procedure.
10. engine failure during takeoff including planning, decisions,
and single-engine operations.
01-09-2014, 01:59 PM
#29
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Joined APC: Feb 2013
Posts: 65
JNB,
Thanks for your response and links. I had not seen the "Leave Yourself An Out," but it's a great document.
I agree that being in ground effect would decrease induced drag, requiring less power to be airborne, and as a result decrease Vmc.
To clarify, your position is that the items mentioned in 23.149 are to simulate the "most critical" engine failure situation (after takeoff w/ gear up), rather than each of those items specifically affecting Vmc. Is that correct?
In the "Leave Yourself an Out" document, it states (as you have stated) that the Part 23 test describes the "worst conditions." Are there any other FAA sources that confirm that this is the purpose of 23.149? If not, how did you come to this conclusion?
BTW, in the "Leave Yourself an Out," it does mentioned that having the landing gear extended seems to reduce Vmc for most light twins. And the Airplane Flying Handbook states that it has a stabilizing effect which tends to decrease Vmc.
Thanks.
Thanks for your response and links. I had not seen the "Leave Yourself An Out," but it's a great document.
I agree that being in ground effect would decrease induced drag, requiring less power to be airborne, and as a result decrease Vmc.
To clarify, your position is that the items mentioned in 23.149 are to simulate the "most critical" engine failure situation (after takeoff w/ gear up), rather than each of those items specifically affecting Vmc. Is that correct?
In the "Leave Yourself an Out" document, it states (as you have stated) that the Part 23 test describes the "worst conditions." Are there any other FAA sources that confirm that this is the purpose of 23.149? If not, how did you come to this conclusion?
BTW, in the "Leave Yourself an Out," it does mentioned that having the landing gear extended seems to reduce Vmc for most light twins. And the Airplane Flying Handbook states that it has a stabilizing effect which tends to decrease Vmc.
Thanks.
01-09-2014, 07:32 PM
#30
Gets Weekends Off
Joined APC: Nov 2011
Position: Volleyball Player
Posts: 3,982
Originally Posted by TheStranger
JNB,
Thanks for your response and links. I had not seen the "Leave Yourself An Out," but it's a great document.
I agree that being in ground effect would decrease induced drag, requiring less power to be airborne, and as a result decrease Vmc.
To clarify, your position is that the items mentioned in 23.149 are to simulate the "most critical" engine failure situation (after takeoff w/ gear up), rather than each of those items specifically affecting Vmc. Is that correct?
Thanks for your response and links. I had not seen the "Leave Yourself An Out," but it's a great document.
I agree that being in ground effect would decrease induced drag, requiring less power to be airborne, and as a result decrease Vmc.
To clarify, your position is that the items mentioned in 23.149 are to simulate the "most critical" engine failure situation (after takeoff w/ gear up), rather than each of those items specifically affecting Vmc. Is that correct?
In the "Leave Yourself an Out" document, it states (as you have stated) that the Part 23 test describes the "worst conditions." Are there any other FAA sources that confirm that this is the purpose of 23.149? If not, how did you come to this conclusion?
Take stall speed for example:
§23.49 Stalling speed.
(a) VSO (maximum landing flap configuration) and VS1 are the stalling speeds or the minimum steady flight speeds, in knots (CAS), at which the airplane is controllable with—
(1) For reciprocating engine-powered airplanes, the engine(s) idling, the throttle(s) closed or at not more than the power necessary for zero thrust at a speed not more than 110 percent of the stalling speed;
(2) For turbine engine-powered airplanes, the propulsive thrust not greater than zero at the stalling speed, or, if the resultant thrust has no appreciable effect on the stalling speed, with engine(s) idling and throttle(s) closed;
(3) The propeller(s) in the takeoff position;
(4) The airplane in the condition existing in the test, in which VSO and VS1 are being used;
(5) The center of gravity in the position that results in the highest value of VSO and VS1; and
(6) The weight used when VSO and VS1 are being used as a factor to determine compliance with a required performance standard.
(b) VSO and VS1 must be determined by flight tests, using the procedure and meeting the flight characteristics specified in §23.201.
(c) Except as provided in paragraph (d) of this section, VSO at maximum weight may not exceed 61 knots for—
(1) Single-engine airplanes; and
(2) Multiengine airplanes of 6,000 pounds or less maximum weight that cannot meet the minimum rate of climb specified in §23.67(a) (1) with the critical engine inoperative.
(d) All single-engine airplanes, and those multiengine airplanes of 6,000 pounds or less maximum weight with a VSO of more than 61 knots that do not meet the requirements of §23.67(a)(1), must comply with §23.562(d).
(a) VSO (maximum landing flap configuration) and VS1 are the stalling speeds or the minimum steady flight speeds, in knots (CAS), at which the airplane is controllable with—
(1) For reciprocating engine-powered airplanes, the engine(s) idling, the throttle(s) closed or at not more than the power necessary for zero thrust at a speed not more than 110 percent of the stalling speed;
(2) For turbine engine-powered airplanes, the propulsive thrust not greater than zero at the stalling speed, or, if the resultant thrust has no appreciable effect on the stalling speed, with engine(s) idling and throttle(s) closed;
(3) The propeller(s) in the takeoff position;
(4) The airplane in the condition existing in the test, in which VSO and VS1 are being used;
(5) The center of gravity in the position that results in the highest value of VSO and VS1; and
(6) The weight used when VSO and VS1 are being used as a factor to determine compliance with a required performance standard.
(b) VSO and VS1 must be determined by flight tests, using the procedure and meeting the flight characteristics specified in §23.201.
(c) Except as provided in paragraph (d) of this section, VSO at maximum weight may not exceed 61 knots for—
(1) Single-engine airplanes; and
(2) Multiengine airplanes of 6,000 pounds or less maximum weight that cannot meet the minimum rate of climb specified in §23.67(a) (1) with the critical engine inoperative.
(d) All single-engine airplanes, and those multiengine airplanes of 6,000 pounds or less maximum weight with a VSO of more than 61 knots that do not meet the requirements of §23.67(a)(1), must comply with §23.562(d).
BTW, in the "Leave Yourself an Out," it does mentioned that having the landing gear extended seems to reduce Vmc for most light twins. And the Airplane Flying Handbook states that it has a stabilizing effect which tends to decrease Vmc.
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