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A Couple Of V-speed Questions  
User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Posted (8 years 2 months 17 hours ago) and read 3301 times:

What is the definition of Vmca (minimum control speed, air) and Vmcg (minimum control speed, ground)? Somewhere I heard that the Vmcg or Vmca have something to do with the critical engine being inoperative, is that true?

I was also wondering about the definition of V2, is that speed supposed to be achieved by the pilot, or is it a speed that the manufacturer guarantees the aircraft will always get to if you work out all the previous speeds well?

Here's one final question, not about a v-speed. Is the TAS equivalent to an IAS corrected for pressure, using the 2% (approximate) speed indication loss per 1000 feet? Are there any corrections made for temperature?

Thanks in advance everyone.

Alfredo

18 replies: All unread, jump to last
 
User currently offlineSkookum From Canada, joined Jul 2006, 115 posts, RR: 0
Reply 1, posted (8 years 2 months 17 hours ago) and read 3292 times:

TAS is the speed of the aeroplane through the air mass. Therefore, if no wind exists, TAS = GS (ground speed).

Cheers
Skookum



Good flying
User currently offlineN8076U From United States of America, joined Jun 2006, 425 posts, RR: 9
Reply 2, posted (8 years 2 months 17 hours ago) and read 3285 times:

Quoting Bio15 (Thread starter):
Is the TAS equivalent to an IAS corrected for pressure, using the 2% (approximate) speed indication loss per 1000 feet? Are there any corrections made for temperature?

The relationship between TAS and altitude is not linear. TAS is calculated using pressure, temperature and indicated speed.

Chris



Don't blame me, I don't work here...
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 3, posted (8 years 2 months 17 hours ago) and read 3285 times:

Quoting Bio15 (Thread starter):
Is the TAS equivalent to an IAS corrected for pressure, using the 2% (approximate) speed indication loss per 1000 feet?

I'll come back to the V-speeds as I get a chance but I wanted to address this one first.

I've never heard of such a rule of thumb but doubt that it would be very accurate for very much altitude change. When I had to do it myself I always used a whizwheel:


The scales on these are pretty much logarithmic rather than linear which is why I doubt the constant rate of change.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSkookum From Canada, joined Jul 2006, 115 posts, RR: 0
Reply 4, posted (8 years 2 months 17 hours ago) and read 3285 times:

I'll add another:

V2 is the takeoff safety speed, and is the minimum speed in the second phase of a climb following an engine failure. Just for reference, usually jet airliners will initially climb out at V2 + 10-20 knots.

Cheers
Skookum



Good flying
User currently offlineSkookum From Canada, joined Jul 2006, 115 posts, RR: 0
Reply 5, posted (8 years 2 months 17 hours ago) and read 3277 times:

Feeling brave, so I'll go for another:

Vmca: Minimum Control Speed (air). The minimum speed at which the aeroplane is controllable with a max of 5deg bank when the critical engine become inop and with the remaining engine(s) at takeoff thrust. Increases with aft of CG. Marked with red line on some ASI, usually in general aviation twins.

Vmcg: Minimum Control Speed (ground). The minimum CAS on the ground with which a takeoff can be safely completed when the critical engine becomes inop, and with the remaining engine(s) at takeoff thrust.

Corrections welcomed.

Cheers
Skookum

EDIT: Forgot to mention, Vmcg decreases with increase of density altitude due to less less adverse yaw from the critical engine. Also decreased by reduced takeoff thrust, larger rudder areas, forward CG, and engines closer to fuselage.

[Edited 2006-07-23 21:14:02]


Good flying
User currently offlineAlias1024 From United States of America, joined Oct 2004, 2760 posts, RR: 2
Reply 6, posted (8 years 2 months 9 hours ago) and read 3201 times:

Here is the FAA definition of Vmc:

Vmc is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.

When an aircraft is certified for Vmc it must be under the following conditions:

Takeoff power on operating engine

Windmilling propeller on inoperative engine, unless the aircraft has an autofeather feature

No more than 5 degrees of bank toward the operative engine

Most unfavorable CG location

Trim in takeoff position

Gear in takeoff position

Flaps in takeoff position

Cowl Flaps in takeoff position

Maximum takeoff weight

Aircraft airborne and out of ground effect


An important thing to remember about Vmc is that it does NOT ensure that the aircraft will climb. It only ensures you will be able to keep the nose pointed straight by applying no more than 180 pounds of force on the rudder pedal.



It is a mistake to think you can solve any major problems with just potatoes.
User currently offlineRaggi From Norway, joined Oct 2000, 1001 posts, RR: 1
Reply 7, posted (8 years 2 months 2 hours ago) and read 3171 times:

Quoting N8076U (Reply 2):
The relationship between TAS and altitude is not linear. TAS is calculated using pressure, temperature and indicated speed.

TAS is CAS, not IAS, corrected for temp & pressure.


raggi



Stick & Rudder
User currently offlineAirfly From United Kingdom, joined Mar 2005, 322 posts, RR: 1
Reply 8, posted (8 years 2 months 2 hours ago) and read 3164 times:

Quoting Bio15 (Thread starter):
using the 2% (approximate) speed indication loss per 1000 feet?

Here is a rule of thumb, which may be helpful:

TAS: [(IASx2%)x(ALT/1,000ft)] +IAS

So: if IAS= 300 and ALT= 20,000:
300 x 0.02 x 20 + 300= 420TAS

The TAS is the speed of an aircraft into the air mass it is in, whatever the density of this air mass is.

Cheers
Lucca M.



L - I - B - E - R - A - L
User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 9, posted (8 years 1 month 4 weeks 1 day 20 hours ago) and read 3124 times:

I know what TAS is, True Air Speed, but what is IAS? Is that the same as ground speed?


norwegianpilot.blogspot.com
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (8 years 1 month 4 weeks 1 day 20 hours ago) and read 3121 times:

Quoting BoeingOnFinal (Reply 9):
but what is IAS? Is that the same as ground speed?

IAS is "Indicated" AirSpeed. Therefore it is the number read directly off the Airspeed Indicator.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Indicated Airspeed
Corrected for installation error

gives
Calibrated airspeed
corrected for compressibility

gives
Equivalent airspeed
corrected for temperature and pressure

gives
True Airspeed

+++++++++++++++++++++++++++++++++++++++++++++++++++++

True Airspeed when properly applied over a plotted course, with known or forecast winds and correctly applied magnetic deviation AND variation will yield
GroundSpeed

But Groundspeed is a NAVIGATION consideration and not one that relates to aerodynamics, performance etc. It has nada to do with V-speeds.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 11, posted (8 years 1 month 4 weeks 1 day 18 hours ago) and read 3093 times:

Ok, I see. I have flown a Cessna, and I think the stall speed was 43 knots with 0 flaps (not really sure), that is 43 knots in TAS or IAS? Cause you read of the indicated air speed, but if the conditions are right you might stall at IAS 50 or 55 knots?
I know you should have a margin though, but how does this work?



norwegianpilot.blogspot.com
User currently offlineFlyMatt2Bermud From United States of America, joined Jan 2006, 563 posts, RR: 7
Reply 12, posted (8 years 1 month 4 weeks 1 day 18 hours ago) and read 3092 times:

Quoting Bio15 (Thread starter):
I was also wondering about the definition of V2, is that speed supposed to be achieved by the pilot, or is it a speed that the manufacturer guarantees the aircraft will always get to if you work out all the previous speeds well?



Quoting Skookum (Reply 4):
V2 is the takeoff safety speed, and is the minimum speed in the second phase of a climb following an engine failure. Just for reference, usually jet airliners will initially climb out at V2 + 10-20 knots.

Alfredo, V2 is a calculated speed affected by ambient temperature, airport pressure altitude and the takeoff weight of the aircraft. Holding V2 after the loss of one (critical) engine will assure that you meet the 'second segment' climb requirements as required under FAR Part 25 and/or JAR's. The second-segment climb gradient with one engine inoperative must not be less than 2.4% for twin-engined aircraft, or 2.7% for three-engined, or 3% for four-engined aircraft. Second segment begins at gear retraction (normally 35' above runway) and goes to between 400' and 1,500' above the airport.

Think of it as a target speed if you loose an engine during takeoff and prior to flap retraction.

Most operational procedures call for the crew to bug this speed prior to takeoff as a reference in the event of an engine failure during or immediately following takeoff for quick reference. If you loose an engine after accelerating beyond V2, the pilot may elect to hold present speed as long as it can determined their rate of climb is sufficient, if there is any doubt you just pull the nose back gradually until you reach V2. If you loose an engine, flaps and slats must remain in the 'takeoff position' with takeoff thrust on the remaining engine(s) until the aircraft reaches the top of the second segment climb at which point the aircraft speed is accelerated to Final Segment (or in some cases 3rd segment) profiles.

Second Segment Climb performance may be a limiting factor in your Takeoff Performance calculations. Also, keep in mind, under the worst conditions, 2.4% gradient is barely climbing, we're talking 24 feet altitude for 1,000 feet across the ground.

[Edited 2006-07-24 20:13:32]


"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
User currently offlineSkookum From Canada, joined Jul 2006, 115 posts, RR: 0
Reply 13, posted (8 years 1 month 4 weeks 1 day 17 hours ago) and read 3082 times:

Quoting BoeingOnFinal (Reply 11):
Ok, I see. I have flown a Cessna, and I think the stall speed was 43 knots with 0 flaps (not really sure), that is 43 knots in TAS or IAS? Cause you read of the indicated air speed, but if the conditions are right you might stall at IAS 50 or 55 knots?
I know you should have a margin though, but how does this work?

Stall speed is always considered in IAS for practical purposes, and the aircraft will always stall at the same IAS as long as conditions (flaps, acceleration (G), etc.) remain constant.

For example, a given Cessna might stall at 40 KIAS (knots IAS) at 3000' with full flaps, so therefore it will stall also at 40 KIAS at 7500' with full flaps, but the TAS will be higher (the aircraft will have to move faster through the less-dense air at 7500' to acheive the 40 KIAS.)

Stall speed ranges occur because of factors such as wing flaps, which reduce stalling speed, or acceleration or G-loading, which increases stalling speed.

Cheers
Skookum



Good flying
User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 14, posted (8 years 1 month 4 weeks 1 day 16 hours ago) and read 3062 times:

Thanks for the responses.

Quoting SlamClick (Reply 3):
've never heard of such a rule of thumb but doubt that it would be very accurate for very much altitude change. When I had to do it myself I always used a whizwheel:

I have a flight computer myself, but still hadn't learned how to use it until I saw your response. I read the manual and it is pretty simple to use. I took the time to plot the Flight computer results versus the 2% rule of thumb mentioned above to compare and here's the result:

I used an IAS of 250 for all the calculations, and temperature vs. altitude numbers corresponding to a high/low tropical region, for the flight computer calculations.



It is clear that the rule of thumb works fine up to around FL220 for practical purposes. Above that level I'd prefer the whizwheel.

Quoting SlamClick (Reply 10):

Calibrated airspeed
corrected for compressibility
gives
Equivalent airspeed

I assume that large airliners, which work on the high subsonic ranges, undergo internal EAS computations to show the correct mach number on the airspeed indicator, is this correct? Using the whizwheel above 0.3 Mach might probably indicate wrong values since it doesn't account for compressibility.


----


Quoting FlyMatt2Bermud (Reply 12):
Also, keep in mind, under the worst conditions, 2.4% gradient is barely climbing, we're talking 24 feet altitude for 1,000 feet across the ground.

Not until you mentioned this I realized how poor that climb gradient really is. A typical landing would have a higher slope, around 3°, and landings are slow descents.



Alfredo


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 15, posted (8 years 1 month 4 weeks 1 day 15 hours ago) and read 3047 times:

Quoting FlyMatt2Bermud (Reply 12):
Also, keep in mind, under the worst conditions, 2.4% gradient is barely climbing, we're talking 24 feet altitude for 1,000 feet across the ground.

Actually 2.4% by definition is 2.4 per hundred. So it would be 24 feet of climb in a thousand feet of transit.

Formula for gradient is "rise over run" so 2.4 / 100 = 0.024 or 2.4%

Still not much of a climb, especially if you are looking at trees or powerlines.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFlyMatt2Bermud From United States of America, joined Jan 2006, 563 posts, RR: 7
Reply 16, posted (8 years 1 month 4 weeks 1 day 14 hours ago) and read 3042 times:

Quoting FlyMatt2Bermud (Reply 12):
2.4% gradient is barely climbing, we're talking 24 feet altitude for 1,000 feet across the ground.



Quoting SlamClick (Reply 15):
So it would be 24 feet of climb in a thousand feet of transit

I thought that's what I wrote? What am I missing here? If you're climbing 24' in 1,000 feet is that not the same as transit?



"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 17, posted (8 years 1 month 4 weeks 1 day 14 hours ago) and read 3035 times:

Quoting FlyMatt2Bermud (Reply 16):
I thought that's what I wrote? What am I missing here?

Okay, I dusted my screen and the decimal point went away.
My apologies sir.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFlyMatt2Bermud From United States of America, joined Jan 2006, 563 posts, RR: 7
Reply 18, posted (8 years 1 month 4 weeks 1 day 5 hours ago) and read 2987 times:

Quoting SlamClick (Reply 17):
My apologies sir

Slamclick, never a need to apologize. I just thought it might be time for me to go get another eye exam. When do the nearsighted prescriptions quit changing? <--- Aw, thanks I'm off to start a new thread!



"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
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