% power from RPM?
 

I fly a DR400, 180hp with a fixed pitch prop.
Is there an easy way to calculate % power from RPM, or do I have to worry about altitude etc?
Cheers

There is quite possibly a chart in your poh giving you the information that you need.

Yes power at a given rpm will change depending on altitude. Imagine taking it to an extreme. If you managed to get so high that there was almost no air left it would take almost no power to turn the prop at 2500 rpm as there would be no resistance.

Yet on the ground it takes quite a lot of power to achieve the same rpm.

Strictly, you need to know RPM, manifold pressure and OAT to calculate power output. Mixture setting is also a variable unless you are consistent with your setting procedure.

For a fixed pitch set-up you will either need to use the POH if it has figures for your propeller (a fine prop will give less power than a cruise prop at the same RPM). Alternatively if you have an MP gauge and an OAT gauge, you can use the standard Lycoming power curves from the operators manual.

A rough rule of thumb for FP is to increase the RPM by 1000 for every 2-3000' increase in altitude to maintain the same cruise power and set mixture accordingly.

DR400 power setting
 

The first thing is that the RPM required for % power setting changes with density altitude and so each attitude will have a different RPM for a fixed power setting.

For all practical application you can use pressure altitude unless the temperature is extremely away from ISA.

I plotted a graph with RPM on one axis and pressure altitude 1013 mb on the other with the 60 & 75 % lines as these settings are tabulated in the DR400 manual and are a straight line between zero and the attitude at which the engine reaches max RPM for the given power setting.

The graph gives a quick and accurate RPM setting for % power that is close enough given the relatively narrow tempriture band the DR400 is likely to fly in and the accuracy of the RPM gauge. Ofcouse you must referance your altitude to 1013 as all the numbers are based on ISA and the engine leaned as you increase altitude.

I had the same question about my engine, Its a Lycoming o-360 c4f which is rated for 180hp at 2700 rpm, being a fixed prop that on take off can not produce more than 2550 rpm I wondered what the actual HP being being delivered at that rpm was , Lycoming,s website ( and am sure your engine manufacturer will also) have graphs and a matrix that lists the power produced at sea level and altitude at differing temperatures and RPM,s. Mine was 172HP at 2550 RPM at sea level at standard temps.

I asked this question in the context of wanting to know when it was okay to lean the mixture, given that, as I understand it, you're not meant to do so with power greater than 75% with the O-360.

I've just checked the POH which states that with a 76.58 prop at sea level, 2700rpm gives a power output of 70%. At higher altitudes, this value decreases as expected. I normally cruise at 2400-2500rpm so I guess I can lean the mixture at will?

Genrege
 

I think that you need to go back and read the book because I think you have misunderstood it, from memory ( my DR400 flight manual is at the other end of Europe) at an altitude of 8000 ft the engine makes 75% power at 2700 rpm. The 75% rpm at sea level is about 2550 rpm. So anything above 2550 at sea level and you should not lean the mixture.

This is the reason that I have drawn the graph and keep it in the aircraft because it is the only accurate way of making the speed and fuel consumption numbers.

I have a house in France just north of the Spanish border and can only make it non stop in the DR400-180 from Oxfordshire if I get all the numbers set correctly.

I've included the page from the POH that I was referring to. This is a 180R with probably a different propellor to yours?


Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting
http://tinypic.com/r/34eevy1/6

genrege, the table you have shown only shows the cruise performance. It doesn't say anything about the engine power output during a climb.

Here is a possible scenario: During the climb you fly with a low-ish airspeed, say 90 knots. You have full throttle, the engine is developing about 90-100% power but due to the low-ish airspeed the prop is limited to 2700 RPM.

You now convert your climb to a cruise. The speed rises, you close the throttle gradually to keep the RPM below 2700. Once you reached your cruise, the throttle now limits the prop to 2700 RPM and engine power is 70%. Airspeed will be 124, according to the table. (Sea level power according to the POH picture you supplied.)

So you can lean this engine when it is cruising at 2700 RPM (at sea level) since engine power is 70%, but you cannot lean this engine when you are climbing at 2700 RPM, since the engine is producing more than 75% power.

Remember that prop RPM is dependent on a lot of factors, and one of them is airspeed itself. In one situation (cruise climb) 2700 RPM might equate to 90-100% engine power, while in another situation (Vne dive, for instance) 2700 RPM might equate to maybe 20% engine power.

With a fixed pitch prop, one could get HP from the RPM but one would need to take in altitude and temperature.

With a CS prop, it can't be done that simply. The governor is holding the RPM constant (within certain limits) so RPM alone is useless. You can get a rough indication from the engine fuel flow and some other factors, and it gets more complicated because the only time the HP is proportional to fuel flow is when at peak EGT or LOP.

Genrege
 

Yes the prop is different, the 180R is a glider tug and it has a prop that is optimized for climbing at about 70 KTS as this is the best lift/ drag speed for the airframe and so the best speed for towing gliders into the air, Unfortunatly the propeller severely limits the engines ability to produce power at altitude or higher TAS due to the RPM limit on the engine. ( but it is a very good glider tug !)

With the cruise prop fitted things are totally different with the engine making 75% power at 8000 ft at 2700 RPM on a ISA day.

If you intend to use the aircraft for touring then it is another prop that you require, if you are thinking of doing this then take a look at all the options as Sensenich, MT & Hoffman have props approved for the DR400-180.

There are tables on the Lycoming website

OK lets make this simple.

On the Rich side of peak EGT, the % power is determined by MASS AIR FLOW. In other words you have more fuel than you need and hence the volume or mass of air being consumed detremines the power.

Simply, less swept volume (RPM) and less MP (O2 per said volume) determines HP.

Sure the cam design etc affects this, but lets say 20% less MP, and 10% less RPM, is 0.8x0.9 = 0.72 this 72% power. Now this si not always accurate but it may be close enough.

On the lean side of peak EGT, in a normally aspirated 8.5:1 engine take fuel flow in US GPH x 14.9 = HP.

How is that?

Another thing you could learn if you did an APS course ;) :ok:





PS, Out of the three levers available, Throttle/RPM/Mixture, which is the most effective power lever? ;)

Yep. And another thing you could have learned from reading the thread, is that we were talking about a fixed pitch prop, with no MAP information available.

(In fact, we are even talking about a very fine pitch, fixed pitch prop, optimized for glider towing and capable of overspeeding the engine by a fair margin, if used to deliver 100% power at cruise speeds.)

What did your APS course say about that?

Backpacker
 

A superbe post that well and truly hits the nail on the head !

FlyinDutch.


The Lycoming charts are of very little practical use with this aircraft as the propellor is optimized to give full power (2700 RPM) at 70 Kts, this way all 180 hp is avalable at the best lift drag speed for the airframe and at a speed that most modern gliders will climb well.

The assumption from the Lycoming charts is that the 2700 Rpm limit will be reached at the full throttle altitude for 75% power, ths is how most aircraft light aircraft are set up to get the best cruising performance, the DR400-180R has only one mission, that is to drag a glider as efficiently up to about 3000ft.

To do this the prop is very fine and the engine will overspeed as soon as you ask it to fly any faster than about 70kts so to keep within RPM limits the power must be reduced as TAS increases.

The aircraft performs its designed task very well but as with all very specialized equipment it is not very good at anything else.

Not a great deal of info to go on there, but the real question was the last bit.

My answer explained about the effect of RPM, and generally speaking consider the RPM directly proportional for the purpose at hand. The other bit being mass air flow I though was well explained, so in answer to his original question, I could have simply said NO.

However original poster then went on to say, So the answer to this different question is YES!!!!

Sorry I bothered some of you but there are no doubt others that will have found the extra info useful.

Backpacker, I trust you are familiar with the saying, Give a man a fish you feed him for a day, teach him to fish and he will feed his family forever. Or did I waste my time again? :ugh:

I'd be careful with that.

When I had too much time on my hands a while back I delved into the performance graphs and produced a table for the sake of having a reference by which I can quickly determine whether or not it is safe to lean the mixture. The essence of the table is that by checking RPM against pressure altitude I am presented with the OAT value that corresponds to 75% power; if the actual OAT is warmer than the presented value then it is safe to lean the mixture.

What struck me about the data is that although it seems that you are generally producing less than 75% power at typical cruising RPMs and at typical temperatures from 1000' PA and above a 50RPM increase necessitates an increase in OAT of 15 degress celsius if it is to remain safe to lean the mixture. We all know how easy it is to inadvertantly gain 50rpm with a fixed-pitch propeller, especially when it's turbulent.

Sure. But you're not handing out fish, nor teaching fishing. You're organizing pop quizzes about fishing, where you hint at being able to catch fish that are so big that even the experts and the manuals fail, without handing out the correct answers afterwards.

Incidentally, you have still not told us how to run the O-320 as fitted to, say, the R2160, LoP without rough running.

Some of the folk in that last thread worked out some stuff, I made it clear there was no cookbook secret to it, but the fantastic little O-320 does it quite nicely in other airframes, and while I have not flown the Robbin I suspect it would also.

Even if I spent all night typing away, I figure I would end up flogging a dead horse. It is not unlike hundreds of years ago convincing folk the earth ws not flat but indeed round (spherical), but that does not seem to matter.

You would catch more flies with honey......

Bring one to me, of your choice and we'll go play. :ok: Better than average chance we would have a win, maybe not, but is the Robbin representitive of all GA Aircraft, no, so maybe it would be fine, maybe it would not play nice. But most will.;)


I am in part, but when I point folk in the direction of where the fishing classes are, the know it alls all pop up with smart a$$ comments and scoff. Why would I waste my time on people who do not want to learn. :ugh:

Jabawoky
 

Are we getting a little miffed ?

What you say is no doubt true in most cases however in this case your answers have little practical application as the overriding limitation to power out put on the DR400-180R is the ease at which the 2700 RPM limit is reached.

It matters not how much theory you apply you can't get past the fact that at anything above 3000 ft and 75 KTS TAS it is just not posable to get 100% power out of the engine and 75% power will be limited by the RPM limit IRO 85kt TAS, all because of the super fine prop. Other DR400-180 variants with cruise props are more than happy to make 75% power up to 8000ft ISA with TAS IRO of 135 Kts at that point they too hit the 2700RPM limit.