Why is total energy difficult using delta pitot pressure?

The subject says it all

Bill Snead
6W

On 7/22/2012 7:11 PM, wrote:
The subject says it all

It wasn't for me, using a 302 on my ASH 26 E, as it has a very accurate
pitot/static system. If it's not accurate, it's going to be harder.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz

A pitot connection alone cannot give you total energy. In fact, it gives you backwards total energy. Kinetic energy increase results increased pitot pressure yet potential energy increase is associated with reduced pitot pressure. This problem can get resolved when you also connect to a static port and do the math.

Now, pitot ports are fairly foolproof and always give good signal. But, not so for static ports. Static ports are easily fooled by changes in AOA and yaw. On fuselage static ports are usually hopeless for use in delta P varios. The best bet for a static port are those tied to a modern triple probe. And that begs the question: why bother if you are going to install a modern triple probe anyway?

On 7/23/2012 1:18 PM, Steve Koerner wrote:
A pitot connection alone cannot give you total energy. In fact, it
gives you backwards total energy. Kinetic energy increase results
increased pitot pressure yet potential energy increase is associated
with reduced pitot pressure. This problem can get resolved when you
also connect to a static port and do the math.

Now, pitot ports are fairly foolproof and always give good signal.
But, not so for static ports. Static ports are easily fooled by
changes in AOA and yaw. On fuselage static ports are usually
hopeless for use in delta P varios. The best bet for a static port
are those tied to a modern triple probe. And that begs the question:
why bother if you are going to install a modern triple probe anyway?

The triple probe is usually in the prop wash of a self-launcher,
rendering it almost worthless under power. Using the fuselage
pitot/static avoids that problem. That's why I do it, but I'm not sure
the original poster had in mind.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz

On Sunday, July 22, 2012 8:11:02 PM UTC-6, wrote:
The subject says it all

Bill Snead
6W

I would say 90 percent plus of competition pilots use a vacuum total energy probe. From the posts, I would say it is simpler. To use the pitot you must have a good pitot and static system plus a way to reverse the pressure delta (electronic or diaphragms). Any thing else? It also sounds like the pitot input can be made to work. Do you think that the drag from the vacuum probes is very great?

Bill Snead
6W

On 7/25/2012 9:16 PM, wrote:
On Sunday, July 22, 2012 8:11:02 PM UTC-6, wrote:
The subject says it all

Bill Snead 6W

I would say 90 percent plus of competition pilots use a vacuum total
energy probe. From the posts, I would say it is simpler. To use the
pitot you must have a good pitot and static system plus a way to
reverse the pressure delta (electronic or diaphragms). Any thing
else? It also sounds like the pitot input can be made to work. Do
you think that the drag from the vacuum probes is very great?

There is no need to "reverse the pressure delta" in instruments like the
302. The vario uses the pitot/static ports to compute the airspeed, and
from the airspeed, it can calculate the TE compensation. Older analog
varios used various schemes involving diaphragms, calibrated leaks,
etc., but I think those were for netto and other functions, not TE
compensation.

I don't think the drag of the probe is significant.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

At 23:17 23 July 2012, Eric Greenwell wrote:
On 7/23/2012 1:18 PM, Steve Koerner wrote:

The triple probe is usually in the prop wash of a self-launcher,
rendering it almost worthless under power. Using the fuselage
pitot/static avoids that problem. That's why I do it, but I'm not sure
the original poster had in mind.

--
Why do you need total energy when running your engine?

On Friday, July 27, 2012 1:35:01 AM UTC-7, Jim White wrote:
At 23:17 23 July 2012, Eric Greenwell wrote:
>On 7/23/2012 1:18 PM, Steve Koerner wrote:
>
>The triple probe is usually in the prop wash of a self-launcher,
>rendering it almost worthless under power. Using the fuselage
>pitot/static avoids that problem. That's why I do it, but I'm not sure
>the original poster had in mind.
>
>--
Why do you need total energy when running your engine?

Because you want to find lift, put the noise maker away and go soaring. An electronic compensated C302 works great for doing that while under power as described. The -MG version of the 302 with loud external speaker mounted near the pilot's head can be heard over engine noise/and through most headsets.

Darryl

On 7/27/2012 1:35 AM, Jim White wrote:
At 23:17 23 July 2012, Eric Greenwell wrote:
On 7/23/2012 1:18 PM, Steve Koerner wrote:

The triple probe is usually in the prop wash of a self-launcher,
rendering it almost worthless under power. Using the fuselage
pitot/static avoids that problem. That's why I do it, but I'm not sure
the original poster had in mind.

--
Why do you need total energy when running your engine?

The TE response isn't really needed because I'm flying at a fairly
steady airspeed; instead, the problem is the prop wash makes the
pressure at the probe very unsteady, and that makes the vario needle and
audio very unsteady, making the vario almost useless for finding and
centering a thermal.

Some pilots work around the problem with a panel-mounted pneumatic
switch to connect the vario TE input to cockpit static, and I'm told
that works well enough.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz