jawright
Well-Known Member
We use that same one...at least that's the one I was taught. "LC Will Buy Fat Cows."
acronym for Vmc factors
- Thread starter FOD
- Start date
We use that same one...at least that's the one I was taught. "LC Will Buy Fat Cows."
fish314
Well-Known Member
Ah, yes. I had an instructor from the class of '79!
tgrayson
New Member
The larger the HCL, the more of a sideslip to the good engine you have and the greater the directional control.
It's more of a paper reduction that a real one. In reality, you're going to be shooting for zero sideslip and won't be able to take advantage of the greater weight; it will only reduce your climb rate.
In all, better to be light, rather than heavy, no matter what it does to Vmc.
aloft
New Member
Class of '79, IIRC.
SmitteyB
Well-Known Member
Poster said:
Most unfavorable weight- I think by this they're assuming you understand with increased weight you will get a decrease in performance. Therefore unfavorable weight would obviously be MGW.
tgrayson
New Member
Performance doesn't play any role in Vmc. You'll fly into the ground with good directional control.
So, Lets get this straight. You dont bank toward the good engine You'll have degraded performance due to the side-slipping and less rudder authority leading to higher VMC.
You bank 2-3º or whatever necessary to obtain zero side-slip (ZSS), and you'll have the best performance and fair VMC. You bank more than whats recommended for ZSS, and you'll have lower VMC and degraded performance again due to slipping toward the good engine!
Now, Are the performance numbers in the PA44 or any A/C for that matter, published based on ZSS bank angle or the maximum 5º the FAA allows? Because that would make a difference in climbing performance!
Also, I understand how horizontal lift is a factor in maintaining directional control, but how can we compare it between light and heavier aircraft?
I mean off course the heavier one would have higher HCL but thats because the mass is bigger and the aircraft is having to generate more lift even in level flight. The heavier aircraft might be producing more lift but guess what!.. it's still in level flight!! cuz its HEAVY, it needs bigger force to accelerate it. so that horizontal lift is not just being used to gain directional control only. I think what I'm trying to say is.. whether it's light or heavy, whatever force it takes to get the airplane to turn at certain rate. again.. thats just what I think, I might be wrong I'll have to see what TG says.
What about the Higher AOA higher P-Factor theory?? is the yawing moment really that great to cause a big change in VMC?
Is that what makes Flaps UP (or T/O position) and out of Gound Efect factors give us higher VMC?
Thanks.
You bank 2-3º or whatever necessary to obtain zero side-slip (ZSS), and you'll have the best performance and fair VMC. You bank more than whats recommended for ZSS, and you'll have lower VMC and degraded performance again due to slipping toward the good engine!
Now, Are the performance numbers in the PA44 or any A/C for that matter, published based on ZSS bank angle or the maximum 5º the FAA allows? Because that would make a difference in climbing performance!
Also, I understand how horizontal lift is a factor in maintaining directional control, but how can we compare it between light and heavier aircraft?
I mean off course the heavier one would have higher HCL but thats because the mass is bigger and the aircraft is having to generate more lift even in level flight. The heavier aircraft might be producing more lift but guess what!.. it's still in level flight!! cuz its HEAVY, it needs bigger force to accelerate it. so that horizontal lift is not just being used to gain directional control only. I think what I'm trying to say is.. whether it's light or heavy, whatever force it takes to get the airplane to turn at certain rate. again.. thats just what I think, I might be wrong I'll have to see what TG says
.What about the Higher AOA higher P-Factor theory?? is the yawing moment really that great to cause a big change in VMC?
Is that what makes Flaps UP (or T/O position) and out of Gound Efect factors give us higher VMC?
Thanks.
nosehair
Well-Known Member
This term "paper reduction" might help explain it.
A heavier aircraft will provide more HCL when the banking technique is used. The more bank, the more directional control. Remember, we are talking about directional control, not climb performance.
The 3-5 degrees used by the manufacturer is all that is allowed for certification purposes, but you can use more. Imagine using 15 or 20 degrees, and no rudder. This wouldn't be good for climb performance, but it illustrates the effectiveness of using some bank for directional control.
If push comes to shove, and you are losing directional control with the recommended 3-5 degrees bank, and full rudder, I would keep cranking in bank until I have directional control. I would rather descend into terrain straight ahead relatively level, than roll upside down losing directional control.
tgrayson
New Member
The regs don't specify the condition, so I'd expect that the manufacturer is going to take every opportunity to maximize their performance figures, which would be at ZS. That said, a test pilot will only be able to maintain zero sideslip if he can determine it. The only real way to do that (that I know of) is with a yaw string; in lieu of that, they maybe be using 5 degrees as an approximation to ZS. One test pilot I discussed this with seemed to equate ZS with 5 degrees, but when I pointed out that you'd be slipping the other way, he agreed that this was probably the case. I highlight probably because it indicated that he didn't know, which suggested they weren't making a serious effort to determine the ZS bank angle. This is likely to vary among test pilots and manufacturer, so I'm inclined to say "No way to know."
If I understand what you're saying, you're arguing that the higher HCL is needed in order to accelerate a heavier airplane sideways. Although a higher force is needed to accelerate a heavier airplane sideways at the same rate, the rate at which we accelerate isn't important. All that affects is how long it takes to establish a steady sideslip, not whether one will occur or not, nor its magnitude. The Vmc maneuver is from an equlibrium condition, where the sideslip has already been established, regardless of how long it took. With a particular bank angle, the aircraft will accelerate sideways until the drag is equal to the lift force.
Seems unlikely to be significant, particuarly since the direction in which it would change Vmc is opposite what we know to occur.
We don't know that those things make Vmc higher in any particular airplane. Some of the criteria for the Vmc measurement are just for consistency of testing among all airplane. However, flaps have been shown to increase directional stability in some airplanes (which would reduce Vmc). This is why many manufacturers suggest using partial flaps when landing in a crosswind.
I've never seen any tests or theory that shows that GE has any particular effect on Vmc. I could offer arguments either way, but until you have test data, you have nothing.
Fly_Unity
Well-Known Member
Ive seen studies that showed the difference in performance at different bank angles. The Seminole in an engine out has the best performance at 2.7 degrees of bank to be exact, I believe at 5 degrees of bank, there is a loss of 50 FPM from the 2.7 degrees of bank. Sure 5 degrees of bank has a lower VMC, but if you lose an engine, who cares about that as long as your above VMC speed, Many people I talk to always flys the full 5 degrees of bank in an engine out, which is only (maybe) slightly better performance as 0 degrees of bank in an engine out
tgrayson
New Member
Melville Byington, a professor at Embry Riddle, did flight tests on a number of aircraft, including the Seminole. His figures:
Code:
[FONT=Courier New] Cessna[/FONT][FONT=Courier New] Piper Beech
[/FONT][FONT=Courier New]Crusader Seminole Baron 58
[/FONT][FONT=Courier New]ZS Bank: 1.5 2.1 2.7
[/FONT][FONT=Courier New]Ball Defl: .3 .4 .7
[/FONT][FONT=Courier New]ROC Change[/FONT][FONT=Courier New] (0 to ZS) +42 +62 +105
[/FONT][FONT=Courier New]ROC Change[/FONT][FONT=Courier New] (ZS to 5) -91 -92 -76
[/FONT]PilotBeckfizzle
Well-Known Member
S - Standard day @ sea level
M - Max power on operating engine
A - Aft CG
C - Critical engine prop windmilling
M - Most unfavorable weight (light)
U - Up to 5 degrees of bank into operating engine
F - Flaps/Gear Up
*Probably not a appropriate to say out loud if your DPE is a female.
M - Max power on operating engine
A - Aft CG
C - Critical engine prop windmilling
M - Most unfavorable weight (light)
U - Up to 5 degrees of bank into operating engine
F - Flaps/Gear Up
*Probably not a appropriate to say out loud if your DPE is a female.
Fly_Unity
Well-Known Member
Interesting
Are you saying that VMC wouldn't change between the light and heavy aircraft if both are banking for ZSS only?
tgrayson
New Member
Correct. The heavier aircraft will need less of a bank to achieve ZS. (The bank is correlated with the T/W ratio.)
skycowboy
New Member
How about:
Critical Engine Wind milling - increase in Vmc, drag, asym-thrust
Aft Center of Gravity - Increase in Vmc - less rudder moment
Standard day - aircraft is Vmc certified on standard day - nonstandard temps and pressures affect the Vmc
Take-off Configuration - Gear down acts as three vertical stabilizers thus lowering the Vmc
Bank 5 degrees into good engine (unless your in an MU-2), thus helping the zero-sideslip situation thus reducing drag, thus decreasing Vmc
Operating engine max power - asym thrust causes Vmc to increase
Weight most unfavorable - max gross + aft CG = bad = increase in Vmc
C
A
S
T
B
O
W
Four factors that determine critical engine:
P-factor
Accelerated slipstream
Spiraling slipstream
Torque
Hope this helps,
Critical Engine Wind milling - increase in Vmc, drag, asym-thrust
Aft Center of Gravity - Increase in Vmc - less rudder moment
Standard day - aircraft is Vmc certified on standard day - nonstandard temps and pressures affect the Vmc
Take-off Configuration - Gear down acts as three vertical stabilizers thus lowering the Vmc
Bank 5 degrees into good engine (unless your in an MU-2), thus helping the zero-sideslip situation thus reducing drag, thus decreasing Vmc
Operating engine max power - asym thrust causes Vmc to increase
Weight most unfavorable - max gross + aft CG = bad = increase in Vmc
C
A
S
T
B
O
W
Four factors that determine critical engine:
P-factor
Accelerated slipstream
Spiraling slipstream
Torque
Hope this helps,
tgrayson
New Member
If that's from our discussion of months ago, I had made the point that there was no evidence that slipstreams curve the way that some people show in their ME prep material; from everything I've ever seen slipstreams travel pretty straight. The only diagram that I've ever seen showing how a slipstream affects directional control showed the airplane in a slip.
That said, I would not discount the direction of prop rotation from producing slipstream effects on directional control. There are other ways this could happen beside having the slipstream curve. The slipstream itself widens at an angle of about 15 degrees; depending on the geometry, the edges of the slipsteam could have some influence on the vertical stabilizer. Even disregarding that, the propeller sidewash on the sides of the fuselage would mix with the relative wind, creating pressure differences on one side of the aircraft relative to the other, affecting directional control in some way. Unless I see some supporting data, I'm skeptical whether these phenomena produce consistent effects across all airplanes.
I probably woudn't recommend someone walking into a checkride with this degree of agnosticism, but I'd also rather not see them drawing a curving slipstream!
Fly_Unity
Well-Known Member
The Turbine Pilot's Manual shows a diagram of a curving slipstream, I doubt that anyone would bust a checkride because of that, though I agree that there is no evidence for it, I never even thought about it accuring in a slip though.
