POH Perfomance Based on?
 

Hey,

Just a quickie.

Trying to plan flight and looking at POH. It says for %BHP. This may be a stupid question but in flight how to I measure that? Is it by IAS or RPM?

Thanks in advance!

With fixed-pitch prop I would say RPM in level flight with regards to altitude - higher you go, more RPM you need in order to achieve same amount of power (% BHP).

Depending upon the format of the POH, I would expect that you are being provided with the %HP for the power setting stated on the chart, at the stated altitude, rather than having to flight plan to it. Other than referring to the cruise performance chart, you as the pilot have no way of measuring %HP in flight.

%HP is not something that you really need to know for flight planning, other than for fuel consumption, which is generally also provided in the cruise performance charts.

If this has not answered your question, perhaps a more specific question, with an example from your POH, will make an even more specific answer possible.

Fora fixed pitch prop, power should be set on RPM, variable pitch should be on a combination of RPM and MAP. The POH should state what the value(s) is/are for any given altitude.

G

G t E
 

Great reply, all summed up in two lines !

% BHP
 

The Rule of 48


This is an old rule, I have copied this from the Van's site.

For normally aspirated engines this gives a good approximation of % BHP


For those unfamiliar with the "rule" it goes like this... take your rpm in 100s and your manifold pressure in inches and add them together. A sum of 48 equals 75% power (such as 24" MAP and 2400 rpm), each reduction of 3 is a reduction of 10% power, so 45 would be 65% and 42 would be 55%.

A table listing all the combinations and settings in between that is used in the cockpit is a handy tool for cruise power management.

All that said, does anyone know how accurate this is? Any empirical data to compare with it?





Charlie

I'm newly qualified PPL and trying to figure out cruise performance and fuel consumption. So, I take it that I figure out my pressure altitude. Look across for corresponding temp. column and look at what RPM or speed I want to fly the aircraft in cruise and read the corresponding fuel burn? Is that the correct way of doing it for Cessna?

Thanks a mil!

I don't know, but it does look like it'll probably only work on one particular range of engines.

In any case, why on earth try and remember a fiddly "rule of thumb" like that, when all you need to do is copy a few numbers into a table and stick it on your kneeboard - something like power/height/rpm/map/fuel burn - shouldn't take up more than half an A5 sheet for just about anything in the SEP class given you are probably only going to look at maybe 4 power settings, and for most purposes 2,000 / 4,000 / 6,000fpm will cover pretty much any GA cross country.

I did use to own a share in a PA28-161 which had this on a placard on the pilots sun visor, which was a thoroughly sensible idea, but sadly have not seen that bit of common sense very often.

G

Genghis, I've seen it on a range of a/c from PA28s to PA31s to Bonanzas & a Kingair. If a power table isn't on the visor of an aircraft I operate, it doesn't take long before one appears! :E

Adding rpm/100 to the MP cannot mean anything, except as a coincidence of numbers. The two parameters are different units.

A more accurate HP estimator is to take the LOP fuel flow and multiply it by a certain factor (which I can't remember ;) ). This should work for any petrol engine with a given compression ratio.

I fly a TB20 and one aspect of POH performance which has intrigued me is fuel economy. Socata's figures for best economy cruise are pessimistic by some 15-20%. It appears that they just lifted Lyco's power settings and derived the performance (range) from those. But the practical difference between a zero-fuel range of 1100nm and 1300+nm is huge, when doing real flights around Europe's often sparse avgas+customs airfield situation.

Since Socata did not change their POH since the late 1970s (to greatly simplify certification of the GT version in 2000) and since accurate fuel totalisers were not around in the early years (for GA) I can understand they probably never knew the real fuel flow rate.

OTOH Socata's runway performance appears spot on, suggesting they derived it from test flights.

Private_Flyer

Percentage of power is used as a way to describe desired performance

The rule of thumb is use

- 75% power for max cruise speed

- 65% power for "normal" cruise

- 55% power for economy or to maximize range.

For simple fixed pitch prop trainer/touring aircraft Cessna POH's give cruise performance in tables so you simple select the cruise altitude and temperature, decide on what percentage of power you want to use and you will get the RPM you must set and the resulting cruise true airspeed and fuel consumption.

Piper unfortunately insists on using charts so you first have to find cruise speed and fuel flow for the specified conditions and percentage of power than go to another chart to figure what RPM setting this corresponds to.
(Note later models did have a handy table which gives you RPM values directly, on the windscreen sunshade but I would not count on this as it was rather flimsy so a lot of club aircraft no longer have an original (or even any sunshade))

A few other points.

1) Percentage of power does have application to some engine limits. For example both Continental and Lycoming do not recommend leaning below 75%of maximum rated power.


2) The POH cruise speed and fuel consumption figures were mesured on new perfectly rigged aircraft. There is no way a 10,000 club beater is going to make book figures. I tell my students to reduce the book cruise figures by 10% and add 10% to the fuel flow figures when nav planning, and

3) fuel flow figures are for engines running properly leaned. If the engine is left full rich it will burn up to 25% more fuel than the POH figures

Frankly I am rather surprised this was not covered in your PPL training.....

Should be "above" 75% power.

Remember too that it may not be possible to achieve 75% rated power at altitude in a normally aspirated aeroplane so therefore you may be better off leaving the engine leaned while climbing at altitude. Some high altitude airports (not in the UK ;) ) and you will need to lean on the ground before take off.

There is also the "constant EGT climb" technique which is perfectly OK for the engine, even though technically you are moving the red lever while the power is above 75%.

The doctrine of not touching the red lever anytime above 75% is not based on any engineering principles.

Not in the UK ;)

You are of course correct as I neglected the above 75% qualifier. Leaning at 75% power is permitted. You point is also well taken with respect to your comment about the engine my not being able to make 75% at altitude even at full throttle and thus could be leaned. This points to the requirement to know what power the engine is actually making at any time which will require consulting the POH and understanding what it is telling you.

There is one caveat to the above though. Overly rich mixtures will help reduce cylinder head temperatures therefor it is usually advisable to leave the mixture full rich on long climbs on hot day especially if the aircraft is not equiped with a cylinder head temperature guage.

Bottom line though is the mixture should always be leaned in cruise flight.

This method requires the accurate real time EGT and CHT information you can only get from a modern digital engine analyser and the even cylinder to cylinder fuel flow only possible with fuel injection. Since the original poster said he was a new PPL, I took this to mean he would be flying simple rental aircraft, fitted with carbs, and which in general only have the most basic engine monitoring instruments. The prohibition in leaning above 75% power IMO reflects less the posssibilty that it will always be dangerous to the engine and more to the fact that without full real time engine monitoring it is safer to just make a blanket prohibiton not to lean in a regime where damage could occur under some circumstances and would not be detectable in time due to a lack of information available to the pilot. (Ie the first indication of overheating would be a high oil temperature by which time cylinder head damage may allready have occured)

That's also true, but one could argue that the best advice for somebody flying a plane with no engine instruments is to leave the red lever fully forward during all climbs, and also to never climb more than say 6000ft which avoids the need to lean during high altitude climbs.

All in all, this makes a spamcan pretty useless :)

There is IMO nothing wrong with leaving the mixture full rich in the climb, other than you may waste a bit of fuel. I do not understand your comment about never flying above 6000 ft. I though I was clear in my earlier post. If the power being developed is less than 75%, a value which you can determine from the information in the POH, than obviously you can lean. As a rule of thumb the full throttle engine output in most light touring/trainers falls below 75% between 7000 and 8000 ft.

I have simplified the situation as there are cases when a more sophisticated
approach is required, like for example leaning for maximum power prior to takeoff at a high altitude airport, but given the fact that the original poster is obviously stuggling with very basic concepts I thought it more appropriate not to go into advanced concepts.

In any case my experience is the biggest problem new PPL's have is most were never taught how to properly lean in cruise flight and therefore all the book planning they do with respect to calculating fuel flows is utterly useless.

That's OK for "UK GA" levels (SFC to 2500 ft ;) ) but I don't think many (any) normally aspirated plane will reach its published operating ceiling if the mixture is left full-rich all the way up. The engine will be running mega rich well before then. I have to say though I have never tried going full-rich at FL200...

Very true. Most get away with it but only by wasting a large chunk of their plane's range.

IO540

I say again IMO there is no problem leaving the mixture full rich in the average club trainer/tourer for climbs to 6000/7000 ft. In fact it is the safest way to fly these minimally instrumented aircraft by low time new pilots like the original poster in this thread

Leaning so as to achieve the serrvice ceiling, management of the mixture control at 20,000 ft , constant EGT climbs etc are important concepts but they are advanced flying techniques so therefore I would suggest that you start a new thread to pursue this topic as I would suggest that they are concepts currently beyond the scope of experience of the original poster.

I would have thought it a sensible test for glider pilots ;). At a minimum it should leave a nice smokey black trail from the way way too rich mixture.