|
|
 Up |
|
|
  |
Author: terryterry Date: Mar 14, 2008 06:19
I posted this question a while back on rec.aviation.piloting and
rec.aviation.student and couldnt get an answer that made any sense. I
have also sent off the question to Cessna who "are looking at it" and
have been for the last 3 weeks. But maybe we have some smarter folks
on Aus.Aviation?
We get taught that take off performance is all dependant on density
altitude. Now if you look at the Cessna flight manual , and I have
done this for a Cessna 172N ( the one I fly) and a 172R ( VH-SCN
of Airborne Aviation, who have the data on the net) and also a Cessna
Cardinal from a canadian contact, They give take off and landing
distance in a table as a function of both pressure altitude and
temperature ( temps of 0,10,20,30,40 deg C) and pressure altitudes of
- to 8000 ft in steps of 1000 ft.
Now I like to work with density altitude so I thought I would take all
those combinations of pressure altitude and temperature and convert
them all to density altitude and then plot graphs of takeoff...
|
| Show full article (2.35Kb) |
|
| | 54 Comments |
|
|
Author: YaguYagu Date: Mar 17, 2008 03:01
Paul, May I please have a copy of it, when you are finished!
Yagu
"Paul Saccani" omen.net.au> wrote in message
news:slfmt3tvcpi9o2pmnqf1m5i2c45vqb52k1@4ax.com...> On Fri, 14 Mar 2008 06:19:40 -0700 (PDT), terry
> iprimus.com.au> wrote:
>>>Happy to email the excel spreadsheet with the raw data and the graphs
>>if anyone is interested in looking at it in more detail.
>
> Yes please.
> --
> Cheers
> Paul Saccani
> Perth, Western Australia.
|
| |
|
| | no comments |
|
|
Author: Sylvia ElseSylvia Else Date: Mar 17, 2008 21:03
terry wrote:
>
> So why would you require a greater takeoff distance for lower
> temperature at the same density altitude?
If the density altitude is the same, but the temperature is lower, then
so must the pressure be. What does reducing the pressure do to a piston
engine's ability to suck in air?
Sylvia.
|
| |
| no comments |
|
|
Author: KwyjiboKwyjibo Date: Mar 17, 2008 21:45
> terry wrote:
>>
>> So why would you require a greater takeoff distance for lower
>> temperature at the same density altitude?
>
> If the density altitude is the same, but the temperature is lower, then so
> must the pressure be.
Not neccesarily. You are thinking of dropping temperature within a specific
volume, which results in a pressure drop. The earth's atmosphere is not a
fixed volume.
--
Kwyj.
|
| |
| no comments |
|
|
Author: Sylvia ElseSylvia Else Date: Mar 17, 2008 22:04
Kwyjibo wrote:
>> terry wrote:
>>> So why would you require a greater takeoff distance for lower
>>> temperature at the same density altitude?
>> If the density altitude is the same, but the temperature is lower, then so
>> must the pressure be.
>
> Not neccesarily. You are thinking of dropping temperature within a specific
> volume, which results in a pressure drop. The earth's atmosphere is not a
> fixed volume.
>
Since we're talking about a constant density, for a given mass of air,
the volume is indeed constant.
Sylvia.
|
| |
| no comments |
|
|
Author: terryterry Date: Mar 17, 2008 23:31
On Mar 18, 3:03 pm, Sylvia Else wrote:
> terry wrote:
>
>> So why would you require a greater takeoff distance for lower
>> temperature at the same density altitude?
>
> If the density altitude is the same, but the temperature is lower, then
> so must the pressure be. What does reducing the pressure do to a piston
> engine's ability to suck in air?
I wouldnt have thought it would do anything to the ability to suck air
in. Then engine is also exhausting to a lower pressure.
|
| |
| no comments |
|
|
Author: terryterry Date: Mar 17, 2008 23:39
On Mar 18, 3:45 pm, "Kwyjibo" ozdebate.remove.com> wrote:>> terry wrote:
>
>>> So why would you require a greater takeoff distance for lower
>>> temperature at the same density altitude?
>
>> If the density altitude is the same, but the temperature is lower, then so
>> must the pressure be.
>
> Not neccesarily. You are thinking of dropping temperature within a specific
> volume, which results in a pressure drop. The earth's atmosphere is not a
> fixed volume.
>
The volume is irrlevant. density = PM / RT
M and R are constants. As Sylvia says if you lower T you must
necessarily lower P. ...
|
| Show full article (0.74Kb) |
| no comments |
|
|
Author: CoopCoop Date: Mar 18, 2008 00:21
On Mon, 17 Mar 2008 23:31:47 -0700 (PDT), terry
iprimus.com.au> wrote:
>On Mar 18, 3:03 pm, Sylvia Else wrote:
>> terry wrote:
>>
>>> So why would you require a greater takeoff distance for lower
>>> temperature at the same density altitude?
>>
>> If the density altitude is the same, but the temperature is lower, then
>> so must the pressure be. What does reducing the pressure do to a piston
>> engine's ability to suck in air?
>
>I wouldnt have thought it would do anything to the ability to suck air
>in. Then engine is also exhausting to a lower pressure.
|
| Show full article (1.75Kb) |
| no comments |
|
|
Author: Sylvia ElseSylvia Else Date: Mar 18, 2008 00:23
terry wrote:
> On Mar 18, 3:03 pm, Sylvia Else wrote:
>> terry wrote:
>>
>>> So why would you require a greater takeoff distance for lower
>>> temperature at the same density altitude?
>> If the density altitude is the same, but the temperature is lower, then
>> so must the pressure be. What does reducing the pressure do to a piston
>> engine's ability to suck in air?
>
> I wouldnt have thought it would do anything to the ability to suck air
> in. Then engine is also exhausting to a lower pressure.
|
| Show full article (1.12Kb) |
| no comments |
|
|
|
|
|
Author: Sylvia ElseSylvia Else Date: Mar 18, 2008 00:25
Coop wrote:
> The air is being pushed into the cylinder by external pressure, so
> anything which reduces the pressure pushing the air in will reduce the
> charge. I guess the reduced temperature might also reduce the amount
> of evaporation of the fuel (but I'm willing to be corrected on that,
> as I guess pressure also affects evaporation). So maybe the charge in
> the cylinders is significantly reduced. Perhaps this makes more
> difference than the effect on the exhaust gases being removed, as
> they are forced out by the piston sweeping up the cylinder.
Oh - seems you got there first.
Sylvia.
|
| |
| no comments |
|
|
|
|
