Stupid (or perhaps complex) question...

dans2992

dans2992

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Dans2992
Ok, so if you are at 1000msl, and it's a very hot day such that the DA is 6000ft, will your plane perform identically to being at 6000ft at standard temp?

Would the RPM of a constant speed prop be the same (assuming prop control is full forward?)

I don't think this question is as simple as it seems....
 
For example - I believe that jets have worse cruise performance when ISA is above standard.

If performance was simply based on DA, wouldn't it just be a matter of climbing/descending to the appropriate DA to match "standard ISA" performance?
 
That's 6000 ft true altitude, not pressure altitude or pressure altitude corrected to an airport, but yes, that's what DA means. Note that the FAA definition is technically correct, but it's misleading, as it's really about calculation technique and not physical response.

Keep in mind that such a configuration requires temperatures around 120 F, so I sure as heck wouldn't be flying in that.

Jets have worse performance above ISA because the density altitude thing breaks down in the stratosphere. Otherwise, you could just assign them a higher DA and be done. The ISA is isothermal above the tropopause.
 
OK, here's probably the most ignorant answer you will get.
I assume that if DA is 6,000 at ground level, then you plane will operate as if it is at 6,000 msl.

And if that is the case, then you will have great difficulty descending to meet ISA, which is at sea level.

I am waiting to be scolded and corrected.
 
Although I simplified the numbers a bit, I did have similar conditions the other day. OAT was 115. It was hot in the plane. :)
 
JOhnH said:
OK, here's probably the most ignorant answer you will get.
I assume that if DA is 6,000 at ground level, then you plane will operate as if it is at 6,000 msl.

And if that is the case, then you will have great difficulty descending to meet ISA, which is at sea level.

I am waiting to be scolded and corrected.
Click to expand...

ISA is defined up to ridiculous altitudes. It's 15 C at sea level, and has other values elsewhere.
 
What started me down this line of thinking is that my RPM seems to be slower than normal on takeoff. Redline is 2700

Departing from 1400msl with DA of 5600, it was only making 2500 (as I quickly glanced during TO roll)

The next morning, on departure from a 6500msl field, DA of 8500, RPM was 2550.

Cruise TAS still seems to be "in the ballpark" so I don't think I have an engine issue.

Either the tach is off, the governor needs adjustment, I glanced at the tach during different times in the takeoff roll, or this is just normal. I don't remember the last time I actually saw 2700... Then again I can't recall the last time I flew below a 4000' DA or had max RPM selected other than takeoff or climb. :)

I'm thinking that at a higher DA the engine makes less power, but also the prop should meet with less resistance because the air is "thinner". Does this balance out in such a way that takeoff RPM _should_ be the same for a constant speed prop under a wide range of DA?
 
dans2992 said:
Ok, so if you are at 1000msl, and it's a very hot day such that the DA is 6000ft, will your plane perform identically to being at 6000ft at standard temp?

Would the RPM of a constant speed prop be the same (assuming prop control is full forward?)

I don't think this question is as simple as it seems....
Click to expand...

Simplified, yes, you'll perform as if you were at 6000'; yes, you will make the same RPM on a constant speed prop since you'll still be making enough power to stay in the governed range to still make full RPM. It really is pretty much that simple.
 
asicer said:
Why not?
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'Cause it SUCKS.

There are safety related issues for heat. 100 deg is enough that I get concerned. 120 would be a no-go unless it was somehow critical, and even then would require countermeasures such as wet towels and lots of drinking water.

Even in Death Valley, there are times of day where it isn't THAT hot. It will hit 120 on Wednesday, but it's only 89 at 8AM, and into the 70s after midnight (though there is quite a bit of terrain around there, so night flying might be a bit risky). Plan better.
 
Toward the end of your takeoff roll you should be turning 2700 rpm, no matter the DA. If at cruise, push the prop in and if you get 2700, your governor is adjusted correctly. If the governor is adjusted correctly and you can't get more than 2500 rpm on takeoff/climbout, you might want to check timing, compression, adequate fuel flow, worn cam, etc., and anything else that might be depriving you of producing rated power.


Edit: (just saw this)

dans2992 said:
Cruise TAS still seems to be "in the ballpark" so I don't think I have an engine issue.
Click to expand...

If your MP, Tach, and FF indicators are accurate and if the plane does book speeds, then yes, you are making the correct amount of power. So if it fails to make enough power (when taking off at 8500 DA or otherwise heavily loaded) to allow the prop to turn at 2700, you might want to check your max FF.
 
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I was in Death Valley at 115 degrees at around 11 in the morning, flew out of there without issue, and landed at Lake Havasu when it was 122 degrees that afternoon. I had to go to 9500 that day to get to 75 degree air according to the OAT. But, other than that, the CHTs were still well under 400 even throughout the climb.

Would do it again without hesitation.
 
EdFred said:
I was in Death Valley at 115 degrees at around 11 in the morning, flew out of there without issue, and landed at Lake Havasu when it was 122 degrees that afternoon. I had to go to 9500 that day to get to 75 degree air according to the OAT. But, other than that, the CHTs were still well under 400 even throughout the climb.

Would do it again without hesitation.
Click to expand...

On one HOT summer day while leaving Phoenix, departure once asked me what altitude I wanted to climb to. I replied, 65 degrees please.

Believe me, they were not amused....:lol:
 
dans2992 said:
What started me down this line of thinking is that my RPM seems to be slower than normal on takeoff. Redline is 2700

Departing from 1400msl with DA of 5600, it was only making 2500 (as I quickly glanced during TO roll)

The next morning, on departure from a 6500msl field, DA of 8500, RPM was 2550.
Click to expand...
Then something ain't right, possibly the adjustment of your prop's low pitch stops. Have it checked.
 
Zeldman said:
On one HOT summer day while leaving Phoenix, departure once asked me what altitude I wanted to climb to. I replied, 65 degrees please.

Believe me, they were not amused....:lol:
Click to expand...

:rofl:

That's awesome. People need to laugh more.
 
JohnAJohnson said:
Toward the end of your takeoff roll you should be turning 2700 rpm, no matter the DA. If at cruise, push the prop in and if you get 2700, your governor is adjusted correctly. If the governor is adjusted correctly and you can't get more than 2500 rpm on takeoff/climbout, you might want to check timing, compression, adequate fuel flow, worn cam, etc., and anything else that might be depriving you of producing rated power.


Edit: (just saw this)



If your MP, Tach, and FF indicators are accurate and if the plane does book speeds, then yes, you are making the correct amount of power. So if it fails to make enough power (when taking off at 8500 DA or otherwise heavily loaded) to allow the prop to turn at 2700, you might want to check your max FF.
Click to expand...

Is it possible that with the reduced air density, there just isn't enough engine performance to get up to 2700 rpm, even WOT, and even with proper governor adjustment?
 
PPC1052 said:
Is it possible that with the reduced air density, there just isn't enough engine performance to get up to 2700 rpm, even WOT, and even with proper governor adjustment?
Click to expand...

That would have to be a really underpowered airplane.

6000 DA is doable even by a 152.
 
Ron Levy said:
Then something ain't right, possibly the adjustment of your prop's low pitch stops. Have it checked.
Click to expand...


We did some checking today with an optical tach, and our mechanical tach is off (and appears to be further off the hotter it gets).

Bottom line, full power static runup at 3500 ft DA is 2600 rpm as measured with optical tach. (Mechanical tach read 2500)

Since 2700 isn't expected until we have forward motion, I believe the problem is our mechanical tach and not much more...
 
dans2992 said:
We did some checking today with an optical tach, and our mechanical tach is off (and appears to be further off the hotter it gets).

Bottom line, full power static runup at 3500 ft DA is 2600 rpm as measured with optical tach. (Mechanical tach read 2500)

Since 2700 isn't expected until we have forward motion, I believe the problem is our mechanical tach and not much more...
Click to expand...

Very typical which is why guys go to the digital tachs when they have trouble, although most any speedometer shop can recalibrate it for you.
 
I'm thinking instead of fixing the tach, we just go digital and be done with it...
 
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