OK. These numbers are based on an extract from a power setting table for a
Lycoming 180HP :-
For 55% rated power with a FF of 7.4G/h at SL with an RPM of 2100, the MP
is 20.9.
For 55% rated power with a FF of 7.4G/h at 12,000 ft PA, again with an RPM
of 2100, the MP is 18.2.
I understand (and please correct me if I’m wrong) that fuel flow has a
linear relationship to power, i.e. a certain fuel flow will produce a
certain amount of power. Therefore, the FF figure of 7.4G/h producing 55% at
both alts is consistent with that understanding. However, what I don’t
understand immediately is why at altitude the MP required is only 18.2
rather than 20.9.
Anyone have a simple explanation?
Thanks in advance. I’m working on the "no stupid questions" maxim here….
although some of this stuff makes me feel like a moron from time to time.
In article <b_Z%e.5408$U51.5…@news-server.bigpond.net.au>,
- Hide quoted text — Show quoted text -
"xerj" <n…@nowhere.com> wrote:
> OK. These numbers are based on an extract from a power setting table for a
> Lycoming 180HP :-
> For 55% rated power with a FF of 7.4G/h at SL with an RPM of 2100, the MP
> is 20.9.
> For 55% rated power with a FF of 7.4G/h at 12,000 ft PA, again with an RPM
> of 2100, the MP is 18.2.
> I understand (and please correct me if I’m wrong) that fuel flow has a
> linear relationship to power, i.e. a certain fuel flow will produce a
> certain amount of power. Therefore, the FF figure of 7.4G/h producing 55% at
> both alts is consistent with that understanding. However, what I don’t
> understand immediately is why at altitude the MP required is only 18.2
> rather than 20.9.
> Anyone have a simple explanation?
> Thanks in advance. I’m working on the "no stupid questions" maxim here….
> although some of this stuff makes me feel like a moron from time to time.
It sounds like an error. Have you checked the horsepower curves? You
cannot get 20.9 in. MP at 12Kft on a normally aspirated engine. IMHO,
the RPM for 12Kft should be higher to accommodate the lower available MP.
> It sounds like an error.
Pretty sure it’s not.
It’s not the same engine, but here’s a scan of another power setting table:-
http://www.experimentalairplane.com/power-setting.gif
This one doesn’t show fuel flows, but you’ll notice that the MP goes down as
alt goes up for the same power setting.
"xerj" <n…@nowhere.com> wrote in message
news:b_Z%e.5408$U51.5131@news-server.bigpond.net.au…
> [...] what I don’t understand immediately is why at altitude the MP
> required is only 18.2 rather than 20.9.
> Anyone have a simple explanation?
Sure: at the higher altitude, lower air density allows the necessary air to
go through the throttle at a lower MP.
Or: at the higher altitude, lower air temperature provides greater air
density at a given MP, allowing for more power at a lower MP.
Or… :)
You didn’t say you wanted a *correct* explanation. I have no idea if either
of the above theories are correct. Personally, I’m leaning toward the
second (the first seems a little fishy to me). Does the power setting table
you’re looking at assume standard temperature at both altitudes?
Pete
> You didn’t say you wanted a *correct* explanation. I have no idea if
> either of the above theories are correct. Personally, I’m leaning toward
> the second (the first seems a little fishy to me). Does the power setting
> table you’re looking at assume standard temperature at both altitudes?
Yes, it does.
The second explanation seems pretty logical to me.
Xerj,
not sure about the answer. Have you looked into John Deakin’s excellent
engine management columns over at avweb.com for an explanation? The
basic ones are several years old, but they are truly excellent. Look
for one titled "Manifold pressure sucks" (get it?), the others were
published around that time.
–
Thomas Borchert (EDDH)
Peter Duniho wrote:
> "xerj" <n…@nowhere.com> wrote in message
> news:b_Z%e.5408$U51.5131@news-server.bigpond.net.au…
>>[...] what I don’t understand immediately is why at altitude the MP
>>required is only 18.2 rather than 20.9.
>>Anyone have a simple explanation?
> Sure: at the higher altitude, lower air density allows the necessary air to
> go through the throttle at a lower MP.
simple but wrong explanation
The important question is: what MP is neccessary to drive the required
amount of air through the engine. (assuming same air/fuel ratio for both
conditions) At lower ambient pressure slightly less MP is required to
get a given airflow into the cylinder, as there is less residual gas
left there. However this effect is rather slim.
> Or: at the higher altitude, lower air temperature provides greater air
> density at a given MP, allowing for more power at a lower MP.
Correct. This seems to me to be the more important explanation, if
standard altitude temperatures are is assumed.
regards,
Friedrich
–
for personal email please remove ‘entfernen’ from my adress
> not sure about the answer. Have you looked into John Deakin’s excellent
> engine management columns over at avweb.com for an explanation?
Yeah, those articles are great. I can’t find the exact answer in them, but I
think it has something to do with density as posited by other posters in
this thread.
xerj opined
>OK. These numbers are based on an extract from a power setting table for a
>Lycoming 180HP :-
>For 55% rated power with a FF of 7.4G/h at SL with an RPM of 2100, the MP
>is 20.9.
>For 55% rated power with a FF of 7.4G/h at 12,000 ft PA, again with an RPM
>of 2100, the MP is 18.2.
>I understand (and please correct me if I’m wrong) that fuel flow has a
>linear relationship to power, i.e. a certain fuel flow will produce a
>certain amount of power. Therefore, the FF figure of 7.4G/h producing 55% at
>both alts is consistent with that understanding. However, what I don’t
>understand immediately is why at altitude the MP required is only 18.2
>rather than 20.9.
>Anyone have a simple explanation?
>Thanks in advance. I’m working on the "no stupid questions" maxim here….
>although some of this stuff makes me feel like a moron from time to time.
Temperature.
-ash
Cthulhu in 2005!
Why wait for nature?