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Intepretation of VMCG

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lilo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Topic: Intepretation of VMCG
    Posted: August-01-2005 at 6:36pm

Hi,

I searched this forum but found no satisfying explanation of "V1 limited by VMCG".

I read the ATR72 FCOM which comes with the F1 ATR 72-500 package. While trying to calculate the various speeds I step over the chart "V1 LIMITED BY VMCG (FLAPS 15)" on page 208. My intepretation of this chart is as follows: Given a fixed airport pressure altitude, V1 limited by VMCG decreases when the outside air temperature increases. The other way round: Given a fixed temperature, V1 limited by VMCG decreases when the airport altitude pressure increases.

Hm, by intuition I would say that V1 and VMCG should increase when OAT and/or airport pressure altitude increase. I think that the air becomes thinner then and it would take higher speeds to safely control the aircraft.

What's wrong with my thoughts?

 

Rgds

lilo

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musicpete View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote musicpete Quote  Post ReplyReply Direct Link To This Post Posted: August-02-2005 at 3:29am
I think, you are right. This has been puzzling me for a long time: It really seems to be the wrong way around... Maybe one of the real ATR-pilots in this forum can clear it up. But I would not count on it: Because of the summer holidays this place is pretty dad.

The good news is: You do NOT need that table to calculate the takeoff speed V1! To be honest, I am not even sure why ATR put that table in the manual...
Greetings!

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: August-02-2005 at 4:28am

musicpete,

that's what I was hoping for. A simple diagram like this would help a lot. I already started created an excel sheet where all the calculation is done automatically. I got so far to implement the calculation of VSR, VmLB, VmHB, the runway correction and VMCG. While playing with my VMCG function it crossed my mind that the diagram looks a bit odd.

So I hope there will be more answers. In the mean time I will implement the determination of limiting/non-limiting runway.

Kind regards

lilo

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Post Options Post Options   Thanks (0) Thanks(0)   Quote musicpete Quote  Post ReplyReply Direct Link To This Post Posted: August-02-2005 at 4:41am
Hey lilo, did you already take a look at my takeoff data calculation? I do not think that an excel sheet will be able to compuate all the necessary data. The process is very complicated.

I also am working together with a programmer, to create a complete calculation suite for the ATR. Without the need for Excel, which I personally hate. So maybe your work will become redundant soon.
Greetings!

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: August-02-2005 at 6:58am

musicpete,

I studied your takeoff data calculation document. I appreciate it because you try to throw light on the parameters, formulas, tables and diagrams. Still it is too time consuming. It might be interesting to do that once or twice. But at the end of the day I want to fly. So what is a computer for? Right, doing calculations.

Your software project sounds interesting. I had only a quick and easy solution in mind. After all, I want to safe my time. At first glance I also thought it more efficient to implement it in C#, Java or something similar with a database behind it to store all the tables and diagram grid points. But then I started to play with some of the formulas in Excel and got the first results at once. It becomes more complicated when you want to implement the determination of values in one of the iso-line diagrams. I miss the database here. But it is possible without too much programming. Excel is open to everyone. You can send the document to whoever want's it. Others can have a quick look at the formulas, tables, even diagrams and the VB code of course. They could easily make changes and all that without the need to install a development environment. This might be of great help when it comes to evaluation and bug fixing. But if the cost/profit ratio is not your concern the software project is certainly more challenging. Wish you success!

Rgds

lilo

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Post Options Post Options   Thanks (0) Thanks(0)   Quote musicpete Quote  Post ReplyReply Direct Link To This Post Posted: August-04-2005 at 2:54am
Hey lilo!

I agree 100% that doing the computation is boring, hard work. Flying on the PC should be fun, though.

I decided for the .exe-Version of a calculation program for 2 main reasons:

1) I can't code to save my life. Not in C++, not in Excel, ... When I was 16, I wrote my last programs in MS-DOS BASIC and Turbo Pascal 6. Since then I forgot everything, and am too busy with my real life to bother re-learning it.

2) Excel-Files must be opened by Excel, or the horrible Excel viewer. The latter one gives me tons of problems and headaches. On the other hand: A .exe works for everyone!

I must say: I am looking forward very much to what you will come up with! My own little prjoect will take some more months, because my programmer is enjoying his holidays (and rightly so!). We already got as far, as determining L and "Icing conditions". The main work is still ahead, but we both find the challenge very interesting.

If you are interested, I could send you a copy of my design document. While the version for manual calculation is all pretty and shortened, the "real" version has everything taken apart.
Greetings!

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: August-04-2005 at 4:01am

musicpete,

I sent you a PM.

By the way: On page 4 in your take-off and landing calculation data booklet you provide a formula for calculating the pressure altitude. When I use this formula with the min and max pressure in the table the result highly differs from the corresponding ZP value in the table. For example, the pressure 31.01 in HG results in:

(29.92 - 31.01) * 1000 = -1090

The table value is -989. May it be that this formula is a bit too linear? I searched the internet for propper calculations and found:

- a scientific approach on http://mtp.jpl.nasa.gov/notes/altitude/StdAtmos1976.html

- a practical approach on http://www.4wx.com/wxcalc/formulas/pressureAltitude.php

By using the formula provided on the second page you get -993 which is still not the same as the table value. But it is definitely closer to that.

Rgds

lilo

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musicpete View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote musicpete Quote  Post ReplyReply Direct Link To This Post Posted: August-04-2005 at 6:33am
Wow! Thank you for these links!

I never was satisfied with that formula, but wanted to offer it for people who dislike the value-searching in the table. I searched for some hours, but was unable to find the webpages you dug up... I only found compiled calculators, which were useless for me.

Thank you!

That will surely make it into the next version of the calculation! Together with the other minimal errors I found, it will be only a small (and hopefully the last) update.
Greetings!

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2005 at 11:25am

What a stupid question that was! Forgot the basics! Shame on me!

IAS means indicated airspeed. It is measured by the amount of air pressing a valve and it is different from ground speed. Given a fixed ground speed, the IAS decreases as the air becomes thinner (lower pressure, higher altitude). So the diagrams are correct. Of course they are correct, they ever were!

rgds

lilo

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Post Options Post Options   Thanks (0) Thanks(0)   Quote musicpete Quote  Post ReplyReply Direct Link To This Post Posted: August-23-2005 at 1:55am
So I don't need to change my booklet? Sound like less effort. Less effort is good.

I will sometime implement the better pressure altitude formula, though.
Greetings!

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: August-23-2005 at 8:24am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote musicpete Quote  Post ReplyReply Direct Link To This Post Posted: August-23-2005 at 10:34am
Oops...   

I have to think about that. But not today. Maybe tomorrow I will have to work less, then I will have time for thoughts.
Greetings!

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: August-24-2005 at 5:01am

Take your time.

By the way, I guess you also missed my question in

http://atr.flight1.net/forums/forum_posts.asp?TID=2200&P N=1,

did you?

rgds

lilo

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adverse_Yawn Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2005 at 3:17am
Quote V1 limited by VMCG decreases when the outside air temperature increases. The other way round: Given a fixed temperature, V1 limited by VMCG decreases when the airport altitude pressure increases.

To understand that it is important to understand what Vmcg means. Essentially it is the minimum speed at which the rudder is able to counter and engine failure (asymetric thrust). As density altitude increases, the thrust produced by the engines decreases. If the engine thrust is less, then the asymetric effect will be less and hence lower demands on the rudder when an engine fails.

Quote Hm, by intuition I would say that V1 and VMCG should increase when OAT and/or airport pressure altitude increase. I think that the air becomes thinner then and it would take higher speeds to safely control the aircraft.

Not at all. The V speeds refer to your IAS not TAS. TAS increases relative to IAS with reduced density. The aircraft at a given IAS will always behave the same at that IAS regardless of density (except for dampening effects, but not relevant to this explanation) are less. Think of IAS as a Molecule Meter. It will only show 100kts when 100kts worth of molecules have passed within a timeframe.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2005 at 6:43am
Why the sharp edges in the middle of the iso-lines in both diagrams for VMCG and VMCA? Is this only a linear aproximation of the real but not easy to be read sufrace in the three dimensional space or is there a physical explanation for that?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adverse_Yawn Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2005 at 5:37pm
The figures won't be primarily mathematically sourced. The data is gathered during the test flight phase. The engineers will then interpolate the empirical figures to a comprehensive set of charts and tables.

I'm not sure, but I imagine that the "sharp edges" are related to the engines being fully rated below a certain temperature.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2005 at 12:24pm
That doesn't count for VMCA. It is the minimum control speed for approach. And there is no full rating of engines in that situation.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adverse_Yawn Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2005 at 1:08pm
Quote That doesn't count for VMCA. It is the minimum control speed for approach. And there is no full rating of engines in that situation.

Vmca is the min control speed in the air (as opposed to the ground) at full thrust. Again it is related to the rudders ability to countrol the asymetric thrust. The likely scenario is just after lift off and when initiating a go-around.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lilo Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2005 at 1:55pm
Hm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adverse_Yawn Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2005 at 3:22pm
Quote Hm

I'll take back the just after lift off bit (brain fart). V1 is limited by Vmcg/Vmca so you will always be above the min control speeds by Vr anyway. However, it stands that if you go-around will be select GA power which will be max if not near max power. When you do it with a dead engine and you do this below Vmca you will yaw uncontrollably. Vmca also takes into account the critical engine and an allowance of small bank (usually 5deg) to the dead engine to help counter yaw too.
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