Fuel flow and leaning this time

[xerj]

Yet another question.

Say you're at altitude and it's time to lean the constant speed engine(s).

You pull back on the mixture control, the EGT starts to go up. The engine
"sounds" more powerful as you do this. The prop might speed up momentarily
until the governor does its stuff and keeps it at the selected speed. You
reach peak EGT, and if for argument's sake you're going for a certain
degrees ROP, you ease the mixture forward until you hit the target temp.

While you're doing this, you're also watching the fuel flow meter that you
have installed. As you pull the mixture back, the fuel flow starts to go up
as the power increases, and peaks with peak EGT. After peak the fuel flow
would go down as you keep on pulling mixture back.

The thing that has me a bit confused is this:-

I understand that roughly speaking increased power = increased fuel flow.
Therefore, it makes theoretical sense to me that as you pull the mixture
back and get closer to stoichiometric that you will see an increase in fuel
flow because the power output of the engine is increasing. What I can't do
is explain why mechanically.

If you've got a constant speed engine that is running full rich and is at a
certain RPM and MP setting, when you pull the mixture back, it'll still be
running at the same MP and RPM, right (after the governor has done its
thing)? Therefore, the pistons will be hopping up and down at the same rate,
won't they? If so, why is more fuel getting used when leaned as opposed to
full rich? For some reason I seem to think that it should be *less* fuel
flow but burned more efficiently, but that obviously isn't the case,
demonstrated empirically by the fact that fuel flow increases, and also due
to the maxim that more power requires more fuel.

I realise the answer must be pretty ~duh~ obvious, but it's escaping me at
the moment.

Once again, thanks in advance!

[[email protected]]

It helps if you clarify if you are piloting a constant pitch or
constant speed propeler.

[Peter Duniho]

"xerj" > wrote in message
...
> [...]
> I understand that roughly speaking increased power = increased fuel flow.
> Therefore, it makes theoretical sense to me that as you pull the mixture
> back and get closer to stoichiometric that you will see an increase in
> fuel flow because the power output of the engine is increasing. What I
> can't do is explain why mechanically.

Perhaps it's because that doesn't happen? :)

Most of the planes I've flown don't even have a fuel flow gauge. But the
ones that do have one, they show exactly what one would expect: as the
mixture is leaned, fuel flow decreases.

For example, with the TIO-540 in my airplane, flow flow readings are
roughly:

30 gph at full power, full rich
20 gph at cruise power, full rich
15 gph at cruise power, leaned to 50 degrees ROP

I do not see the fuel flow increase while leaning from full rich to 50
degrees ROP at cruise power. The fuel flow correlates directly to the
mixture control, and I do not see the fuel flow increase as mixture is
leaned. Ever.

Now, not that I've seen this personally, but I can imagine that a fuel flow
"meter" that is actually a fuel *pressure* meter might show you the behavior
you're seeing. Given Cessna's preference to label a low-voltage light as an
"overvoltage" light, I wouldn't be surprised to see some manufacturer
install a fuel pressure gauge but then call it a fuel flow meter. But I
have a hard time seeing how such a gauge would ever be able to show anything
approximating the actual fuel flow. That's not to say it couldn't...just to
say I'm not familiar with such a design, and so don't know how it would
work.

Pete

[Peter Duniho]

> wrote in message
oups.com...
> It helps if you clarify if you are piloting a constant pitch or
> constant speed propeler.

It seems like a pretty safe assumption to assume that by "constant speed
engine", he really means "constant speed propeller".

[xerj]

Sorry, I mean constant speed prop.

> wrote in message
oups.com...
> It helps if you clarify if you are piloting a constant pitch or
> constant speed propeler.
>

[xerj]

> Perhaps it's because that doesn't happen? :)

Thanks, Peter.

I think I've been mistaken in what I thought I saw happening, and/or how I
remembered it.

When (if!) I have the spare coin again to rent a plane with an FF gauge,
I'll go have a look myself.

[Newps]

xerj wrote:

> Yet another question.
>
> Say you're at altitude and it's time to lean the constant speed engine(s).
>
> You pull back on the mixture control, the EGT starts to go up. The engine
> "sounds" more powerful as you do this. The prop might speed up momentarily
> until the governor does its stuff and keeps it at the selected speed. You
> reach peak EGT, and if for argument's sake you're going for a certain
> degrees ROP, you ease the mixture forward until you hit the target temp.
>
> While you're doing this, you're also watching the fuel flow meter that you
> have installed. As you pull the mixture back, the fuel flow starts to go up
> as the power increases, and peaks with peak EGT.

That's wrong. The fuel flow goes down as you pull the mixture out. The
fuel flow always goes down when you pull the mixture out.

>
> The thing that has me a bit confused is this:-
>
> I understand that roughly speaking increased power = increased fuel flow.
> Therefore, it makes theoretical sense to me that as you pull the mixture
> back and get closer to stoichiometric that you will see an increase in fuel
> flow because the power output of the engine is increasing. What I can't do
> is explain why mechanically.

You are wrong. The flow goes down because you were over rich for
takeoff and climb.

[Newps]

Peter Duniho wrote:


>
> Now, not that I've seen this personally, but I can imagine that a fuel flow
> "meter" that is actually a fuel *pressure* meter might show you the behavior
> you're seeing.

It doesn't, it works just like a flow meter.

[Steve Rubin]

In article >,
Newps > wrote:
>That's wrong. The fuel flow goes down as you pull the mixture out. The
>fuel flow always goes down when you pull the mixture out.

Not always.

On my T210 it goes up about 1-2gph before starting to go back down.


--
Steve Rubin / AE6CH / http://www.altdb.net/
Email: / N6441C / http://www.tch.org/~ser/
"Why don't you mind your own business?" -- John Navas 01/04/05
"If you don't like it, keep it to yourself" -- John Navas 01/04/05

[John Clonts]

>>That's wrong. The fuel flow goes down as you pull the mixture out. The
>>fuel flow always goes down when you pull the mixture out.
>
>Not always.
>
>On my T210 it goes up about 1-2gph before starting to go back down.

Interesting. Is that per the Cessna factory "fuel flow" (pressure)
gauge, or is it per an actual (e.g. Shadin, etc) fuel flow meter?
--
Cheers,
John Clonts
Temple, Texas

[Steve Rubin]

In article om>,
John Clonts > wrote:
>Interesting. Is that per the Cessna factory "fuel flow" (pressure)
>gauge, or is it per an actual (e.g. Shadin, etc) fuel flow meter?

Shadin. I don't pay too much attention to the factory gauge when I lean,
but I will look at it next time.



--
Steve Rubin / AE6CH / http://www.altdb.net/
Email: / N6441C / http://www.tch.org/~ser/
"Why don't you mind your own business?" -- John Navas 01/04/05
"If you don't like it, keep it to yourself" -- John Navas 01/04/05