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Spaetzuender
27th May 2007, 06:38
Hi,

can somebdy tell me why the Vmcg in higher altitude is less and not more than on msl?

Thanks

Rick

411A
27th May 2007, 06:45
Gas turbine engines are not 'supercharged' at higher altitudes...IE: they produce less thrust up high(er), therefore the Vmcg is slightly less.

Thylakoid
27th May 2007, 14:07
I has to do with the ambient temp as well.
Eg.: for the B777-200, MSL, 30 degrees C, VMCG 114 kts;
4000 ft elev, 30 degrees C, VMCG 110 kts.
These values are for max T.O. thrust.

Cheers

barit1
27th May 2007, 15:21
While thrust AT THE SAME SL N1 OR EPR is certainly less at altitude, many (most?) engines have some "bump" built in to partially or fully compensate for altitude - up to a certain critical limit.

Therefore, don't take it for granted that Vmgc will be less. Go by the book!

Schnowzer
27th May 2007, 15:21
I always understood that control power was proportional to IAS. At higher alt and temp you will need a higher TAS/Gnd Speed to achieve that IAS but once there you are looking good. As to the loss of thrust, if you have less available, you'll need less control authority.

Might be talking pants though!

Spaetzuender
27th May 2007, 18:20
Hi all,

thanks for the answers. They helped.
Thats`s for my ATPL preparation.

Cheers
Rick

Airgus
27th May 2007, 19:06
I think the point you want to see here is DENSITY.
The higher the density (Lower airports) the engine provide more thrust/power.
If you loose an engine, the operational one will provide more thrust/power and you will need a higher yaw force to counteract this effect, therefore you will need a higher VMCG/VMC speed to help this yawing force at higher density airport or lower altitude airports.

In this case (high altitude airport), your operational engine will provide you with less power/thrust that in the similar situation at a lower altitude airport, therefore your VMCG will be lower.

Viele Gruesse and viel Glueck!

Gus.-

Jaguar Pilot
28th May 2007, 13:41
Airgus is quite correct.

In assymetric conditions, minimum control speeds reflect aerodynamic forces available (roll and yaw) to counter assymetric roll/yaw moments.

The assymetric roll/yaw moments are a function of the thrust/power being produced by the propulsion units(s).

As altitude increases, the thrust/power being produced is less, so the yaw/roll moments are less. Therefore less aerodynamic forces are required and the the aeroplane may be flown at reduced speed to counteract these moments.

There are at the moment four questions in the CPL/ATPL question database on this subject.

JP

Old Smokey
28th May 2007, 16:27
All of the above sounds fine in general terms, yes, turbine engines do decline in thrust as altitude increases, but................

Point 1. A great many jet engines are 'flat rated', i.e. Pressure Limited, and this pressure limit prevails to an altitude where there is insufficient density to produce the pressure limit, and are, thereafter, 'Full Rated' or Temperature limited. Thus, up to the level where the Flat Rating prevails, thrust will be constant, and, to take it one step further, thrust may INCREASE due to lower back pressure from lower ambient atmospheric pressure. I know of at least one engine that is flat rated up to 8,000 feet.

Point 2. If the Net thrust remains constant, as alluded to in Point 1, the EAS for Vmcg will remain the same. For a constant EAS, IAS or CAS will be HIGHER.

Having said all that, the Vmcg for certification purposes is calculated for the WORST CASE, and not the prevailing case, thus the official Vmcg published in the AFM will be the same irrespective of the Altitude.

Did that throw a spanner in the works?:8

Regards,

Old Smokey

Mad (Flt) Scientist
28th May 2007, 16:40
Having said all that, the Vmcg for certification purposes is calculated for the WORST CASE, and not the prevailing case, thus the official Vmcg published in the AFM will be the same irrespective of the Altitude.


But there is provision to be allowed to schedule minimum control speeds with variation in temperature and altitude, precisely so that you are not unduly penalised for unfeasible amounts of thrust at high altitudes.

If, at the option of the applicant, the AFM value of VMCA is to vary with pressure altitude and temperature,....
(Yes, I know that quote says VMCA...)

Of course, if the OEM didn't do so, then you have to abide by the "single value" rule...

Jaguar Pilot
28th May 2007, 17:17
411A

Gas turbines are not "supercharged" at any altitude,
they are turbocharged at all altitudes.
That's what the compressor, driven by the turbine(s) does.
As in "turbojet" or "turbofan".

Thrust decreases with altitude because of reducing density.

Old Smokey
28th May 2007, 19:27
Jaguar Pilot, would you consider amending your statement to "Thrust decreases with altitude because of reducing density, after that Pressure Height is reached where the Flat Rating no longer applies?"

Point well taken Mad (Flt) Scientist, I was trying to make the point that the worst Vmcg may well be at an increased altitude (the WORST case) for a Flat Rated engine, after which point, Vmcg (actual) does reduce, and alleviation during certification is indeed available above that Altitude, or, to be precise, Pressure Height.

Regards,

Old Smokey

Mad (Flt) Scientist
29th May 2007, 04:12
Agree with the possibility that the worst case may not be SL; I think one of our types has a marginal max thrust increase with altitude for some temp conditions, due to the details of the flat rating applied.

Jaguar Pilot
29th May 2007, 12:50
Old Smokey,

Very happy to do so - I was trying to keep it simple in lieu of the original question.

JP

Old Smokey
29th May 2007, 13:53
Jaguar Pilot,

I feel a bit guilty for 'nit picking' on your good response, if one adopts the principal of keeping the replys simple (as you did, and I often do), along come some pedant (me in this case) to say "but have you considered such and such":= Sorry if I seemed to come from that direction.

The problem is this, it appears that the original poster may be studying for Performance 'A' or such like, that is the assumption, but then again he/she may be studying Aeronautical / Performance Engineering where a more complete understanding is required. It's impossible to tell unless the original poster is specific, and then again, other people who may be seeking a more complete understanding also read the threads.

As a person who is GUILTY of preparing examination papers for both ATPL and Aeronautical Engineering, I do agonise over all of the possible considerations that must go into a set of multiple choice answers. It seems that this is not always the case in the examining fraternity, the trend seems to be to ask, and expect a response for a generic general principals aircraft. A pox on them!:*

Sorry, rant over, that assuages my conscience for nit picking a well put together response from you.:ok:

Regards,

Old Smokey

barit1
29th May 2007, 13:56
MFS sez:
Agree with the possibility that the worst case may not be SL; I think one of our types has a marginal max thrust increase with altitude for some temp conditions, due to the details of the flat rating applied.

A bit out of the ordinary, a custom rating for a particular operator at a particular airfield; probably the result of a negotiation by an OEM to forestall the operator buying a competitors' bird!

There are some economic (parts life) penalties associated with such a rating, but as long as it's with the certified limits ... :ok:

john_tullamarine
30th May 2007, 06:40
.. along comes some pedant ...

Ah .. pedantry .. that delightful skilset which allows one to drill down to the pedagogically insignificant minutiae which generally doesn't count for much at day's end ...

.... pedants of the world .... UNITE !!

... which reminds me OS .. we still haven't caught up for a cleansing ale or two.

Jaguar Pilot
30th May 2007, 14:29
Old Smokey,

No offence taken whatsoever.
I have also acted as a CAA consultant on the question database.

The reduction in VMCG/A/L with increasing altitude forms the basis of one of the questions, and a simplified view must be taken.

And for you and everyone else:

We all know that, overall, the power output of a normally-aspirated piston engine at constant RPM decreases as altitude is gained due to reducing air density and thus a reduction in the weight of charge on induction.

Therefore how come the power output can actually increase with increasing altitude with a normally-aspirated engine?.

It is not a trick question in any way, but has to be thought about carefully.

JP

Old Smokey
31st May 2007, 04:23
Good grief Jaguar Pilot, the goal posts keep moving! I thought that we were discussing jet engines.

My personal quick summary -

(1) Vmcg/Vmca for a normally aspirated piston engine aircraft will decrease with increasing Pressure Height,

(2) Vmcg/Vmca for a supercharged piston engine aircraft will increase with increasing Pressure Height up to full throttle height, and then decrease,

(3) Vmcg/Vmca for a Fully Rated (EGT Limited) Jet engine aircraft will decrease with increasing Pressure Height, and

(4) Vmcg/Vmca for a Flat Rated Jet engine aircraft will increase with increasing Pressure Height up to the height where temperature (EGT) becomes the prevailing limit, and then decrease.

Turbo-Props deliberately left out of the summary as you can play tricks with them (like injecting Water Methanol) to restore power loss with increasing Altitude and/or temperature.

It would have helped a lot if the original poster had been specific!:ugh: What's he/she studying, PPL, CPL, ATPL, Performance Engineering?

Regards,

Old Smokey

Jaguar Pilot
31st May 2007, 09:08
Sorry Old Smokey.

Didn't mean to move the goal posts at all.
Just thought I would change tack a bit to something different.
If you would like the question as a new thread I shall be happy to oblige.

JP

Spaetzuender,

Just what are you studying for?

5150
31st May 2007, 14:36
Turbo-Props deliberately left out of the summary as you can play tricks with them (like injecting Water Methanol) to restore power loss with increasing Altitude and/or temperature

You can inject water in piston engines too! :8

Old Smokey
31st May 2007, 14:55
Arrrgh! 5150, forgot that one too. Maybe we should also consider Nitrous Oxide boosting!

Welcome to the Unholy Alliance of Pedants! :E:ok:

Regards,

Old Smokey

Jaguar Pilot
31st May 2007, 17:24
OK Old Smokey.

Any takes on my piston engine question?

JP

Old Smokey
1st Jun 2007, 02:11
Hi Jaguar Pilot,

Boy oh boy, we did go off at a few tangents here, all good clean fun!:D

For the normally aspirated piston engine, provided that full throttle is used for Takeoff, power, and therefore thrust output, decreases as Pressure Height increases. Thus, ACTUAL Vmcg and Vmca will decrease as Altitude increases.

As mentioned earlier the AFM value of Vmcg and Vmca is fixed at the 'worst case' value, unless, as Mad (Flt) Scientist has indicated, the manufacturer has sought some alleviation for decrease at higher altitudes.

Thus, if we're talking of examination questions, one must be very careful to ascertain whether the examiner is asking after the ACTUAL, or the Certified AFM value of Vmca/Vmcg.

The "provided that full throttle is used for Takeoff caveat was added earlier just in case some normally aspirated engine, somewhere, is limited to less than full throttle, e.g. 27" Manifold pressure instead of the near 30" available at Sea Level. That instance will be similar to a 'Flat Rating' up until the altitude where (for the example) 27" is no longer available.

Regards,

Old Smokey

barit1
1st Jun 2007, 12:31
(I too have a pedant's license, right here in my wallet...)

Inasmuch as Vmc (a or g) is a function of assymetric thrust, and not necessarily engine shp, then propeller efficiency must be part of the equation. While I have no specific data at hand, I can readily conceive of a propeller optimized for higher altitude takeoffs, and thus sub-optimum at SL.

And gearbox limitations are common on turboprops, making them flat-rated up to an altitude where the gas generator no longer can overtorque the gears. Not sure if any recips have this limitation.

john_tullamarine
2nd Jun 2007, 13:29
.. the T&E boys and girls would drool at this thread ...

411A
2nd Jun 2007, 15:31
Not sure if any recips have this limitation.

Some do.
Examples...R4360, R2800, both Pratt&Whitney.
CurtisWright, R3350 including the turbocompound series.

Speaking of the R4360, it is, without a doubt, the smoothest piston engine I have ever flown, almost turbine-like.
These were on the Stratocruiser, a typical example of a Flight Engineers airplane.

mcdhu
2nd Jun 2007, 15:57
If I could throw in a spot more pedantry to this debate and in particular OS's very wise move in his summary to ignore turboprops because:

......you can play tricks with them (like injecting Water Methanol) to restore power loss with increasing Altitude and/or temperature.

Not only can you restore lost power with water meth, some turboprop engines allow you to boost power (as opposed to restore) using water meth and thus it is possible to have 2 sets of performance figures and speeds etc for the same configuration and conditions ie with and without w/m.

Switches back to pragmatism,
Cheers,
mcdhu

barit1
2nd Jun 2007, 16:59
411A, I once had a boss who was ex-BOAC, served as F/E on several types including the B377. He wasn't very impressed with its performance, but would agree about the 4360.