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Rotax performance charts and what it means to us


Roger Lee

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I know a couple here that like charts.

 

First we need to know something about the Rotax HP/torque chart. It is a calculated chart for the person in the field since it was done under very controlled circumstances and not an actual run time chart that may apply directly to you under field conditions and rules out many in field variables. It was charted for a sea level run at 59F and with an in flight adjustable prop and they may have used a resistance device instead of a real prop. Plus most props would yield a slightly different result. It is also charted that you can achieve 5800 rpm. The chart shows both torque and HP climbing from throttle advancement and at 5000 rpm the torque peaks and as throttle is advanced further and rpm climbs the torque starts back down and HP increases until you get to 5800 WOT. This is where Rotax considers peak HP at optimal conditions. Anything less than this and you're losing performance and add a few other variables and you lose even more. This is why they want you to take off at 5800 WOT and then change the prop pitch with your in flight adjustable prop. Some will leave the prop pitch alone and reduce power to 5500 or less. Some will set it to get 5500 WOT and reduce power and some will reduce pitch and fly WOT at 5500 rpm. This is the smaller number of most fliers and the engine can handle this.

 

So what does this chart tell me since we only have a ground adjustable props. Well you could set the pitch to get 5800 WOT on take off and that would give us a killer climb prop. Good for people with special needs like consistent short fields, float flying and of heavily weighted planes. Maybe even some flying out of fields over 7K on a regular basis. Special needs folks, but isn't good for cruise performance or fuel economy, but still might be necessary..

 

What if we set the prop to get only 5500 WOT in level cruise and now I'm 300 rpm below what Rotax says is max power. Well that means you are somewhat behind the 8 ball. Most are not flying out of sea level with temps 59F or below so the engine has lost some of its HP and torque before you even leave the ground. That means the engine has to work harder which also increases oil temps and fuel consumption. The higher your field elevation the more lost HP you will have to turn a prop that can only get 5500 WOT in level cruise and may only be getting 4800+/- rpm at take off.

 

You will lose even worse if you always set the prop pitch for low altitudes at a field and then always fly high like over 8K. Because of HP and torque loss at the higher altitudes causes the engine work harder to try and turn the prop you set for lower altitudes and it's trying get you to your max rpm of 5500 WOT. Depending on altitudes you may or may not make 5500 WOT. So a guy at 12K MSL will lose approximately 36% of his HP just at altitude alone and then add density altitude on to the heap and his engine is working harder to turn the prop pitch set at 5500 WOT. The guys at only 1K may only have a 3% loss and of course this is based on atmospheric conditions at the time too. These are just general numbers. If the guy at high altitudes is a tad overpitched then his performance really suffers and he can't turn the prop efficiently since he has lost engine power.

 

So if you under pitch to get max power on take off we have a good climb prop, but it sucks for cruise and has to run at higher rpms to achieve a decent cruise speed and fuel burn.

 

If you over pitch (anything less than 5500 rpm WOT at your average altitude) then you lose out on climb, cruise, fuel economy and increased engine temps because this engine hasn't the HP or torque to turn the prop efficiently. So over pitch has no redeeming qualities and cost you everything and possible engine damage.

 

So where to set a prop for a decent compromise?

Bottom line is engine and prop should be balanced for your type of personal flying. So balance for climb, balance for cruise, balance for fuel and balance for engine temps.

 

Well we don't want 5800 on take off or even WOT at cruise altitude. Too flat for cruise and fuel economy.

 

If we pick an rpm in the middle of 5500 continuous allowed rpm and max power rpm of 5800 that puts us in a balanced state around 5650.

 

A balanced state between climb, cruise, fuel economy and engine temps. This will allow us to still keep some good climb, keep some good cruise, best fuel economy and best engine temps. Now we can get around 4950-5000 rpm at take off and WOT in flat and level cruise we can get 5650 knowing that we won't fly there since most of us throttle back to 5100-5300 on average. These are averages for most fliers. Yes there are some that fly lower rpms and some higher.

With two exact aircraft with the same props and one set for 5500 rpm WOT at whatever altitude you pick, will it be faster or slower than the one that gets 5650 at the same altitude?

If everything is equal the one that gets 5650 will be faster at its WOT of 5650 over the 5500 WOT. Its getting to use another 150 rpm and a tad more HP/torque to help turn the prop.What about these pitch settings at lower rpm cruising.

If both planes reduce throttle to 5200 rpm the one that is set for 5650 will slowly be out pacing the one set for 5500. It will work less hard to turn the prop more efficiently.

If either plane goes up above their average altitude the one that gets 5650 WOT will turn more rpm easier than the one that was only set for 5500 WOT. Remember as we climb higher the carbs richen, less oxygen and decreased HP so the engine will work harder to turn the prop as we climb higher and higher. The engine will work easier and turn the prop more efficiently with less pitch.

 

Another scenario is if one plane is set to get around 5600 at WOT and the other to get 5750 WOT in level flight the one that is set to get 5600 will be quite a bit faster than the 5750 one and get better fuel economy. Plus the one set for 5750 WOT will have to fly at 5300-5350 rpm when the other one is only at 5200 rpm in cruise.

 

We have demonstrated this hundreds of times at our field with those same planes. These aren't speculation numbers, but real time flights with the same planes side by side. Numbers are kept and recorded for comparison.

 

If you have a need for a great climb prop pitch then set it so. Never over pitch as there are no redeeming qualities from over pitching. You lose out on all items.

 

So where should the average fliers set our prop pitch if we look at real time flight testing with multiple LSA props and aircraft to achieve a BALANCE? Well the statistics show around 5650 WOT at your average altitude. This will give you a tad better climb over lower rpms at take off. A tad better fuel economy at cruise and a tad better performance with engine workload trying to turn the prop at a reduced rpm below that 5650 and lower engine temps. Less work equals less temp and fuel burn.

I set my prop this year going to Page, AZ Fly-In to get 5750 and the other guys were at 5600. I had to fly at 5300+ to keep up with them at 5200, but I had a good climber at our higher elevation airfields and the fact that we were heavier. I dropped it back to 5650 when I got home. Fuel and cruise speed suffered more than I would like.

 

Applying the Rotax chart as it is written in the manual doesn't apply if you have anything different from the way they tested. If you only get 5500 rpm WOT or 5650 WOT the chart would need to be re-done because HP and torque curves will be different. It's a good guideline, but not exactly specific to many of us.

 

Bottom line is engine and prop should be balanced for your type of personal flying taking into consideration performance for climb, cruise, fuel and engine temps.

 

The but here is you get to set it wherever your heart desires, but do it for the right reasons and considerations.

 

I have about 80 steady LSA clients and would say almost all are right around the 5600-5650 mark at WOT in flat and level flight at their average altitude. We will all fly a little higher and lower at times. It's just the nature of the beast.

 

Engines, props and fuselages are a marriage that engineers have to bring together and like any marriage balance seems to be better for a long happy relationship.

 

This post isn't about max anything. It is about balance and understanding the whys and why nots of setting prop pitch.

Set it where you want, but do it with some understanding.

 

 

I can't believe I stayed up to midnight for this. I have an early flight and annual tomorrow. Off to bed.

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That is a mouthful.

 

We explain it quite differently but we end up in the same place.

 

Optimize your speed for 7.5K or higher, at lower altitudes simply throttle back.

 

You can consider all that is written above and pitch for 5650 at your average ...  and will have the same result.

 

Is it a good result because of the word 'balance' instead of 'best speed'?  Its the same result, balance at 4K = best speed at 10k.

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I know a couple here that like charts.

 

First we need to know something about the Rotax HP/torque chart. It is a calculated chart and not an actual run time chart. It was predicated and calculated out for a sea level run at 59F and with an in flight adjustable prop. Also predicated on that you can achieve 5800 rpm. The chart shows both torque and HP climbing from throttle advancement and at 5000 rpm the torque peaks and as throttle is advanced further and rpm climbs the torque starts back down and HP increases until you get to 5800 WOT.

 

Roger,

 

Are you sure?  Is this documented somewhere?

 

Predicted / fictitious power / torque charts serve what purpose?  Add in a variable throttle position and what can be learned?

 

Don't you think the charts are from dyno runs?  A dyno run where throttle is fixed at WOT and the load is variable so achieving 5800 is a given?

 

If it is a dyno run at WOT as I assume then I can pick a point, like 5500 and know how much power I can achieve setting for that RPM @ WOT @ standard conditions.  From that I can infer where best speed is (5500) and where best climb is (5800).  If all this is a result of variable throttle settings than It doesn't help me.

 

The earliest views I had at the Rotax charts included that the graph was done at WOT.

 

What makes you think this is predicted/caculated at various throttle settings with a prop installed? If it was an in flight adjustable props wouldn't we see bracketed results, one bracket for each prop setting? 

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Hi Ed,

 

I told you it was late last night and I'm certainly not a night owl. I turn into a pumpkin by 2200 hrs. I actually fell asleep in the chair then tried to get up and write this.

I should go back when I get home an amend the wording. It was POORLY done. Sorry.

The chart comes out of the Operators manual and is published for standard conditions. The but the test are setup to dramatically optimize their numbers and hy may not have used a prop at all, but a resistance device. They don't use our props, we aren't at standard conditions, air intakes are different, ect.... The "dyno" test are done under very controlled situations which we will never see. So the chart only gives us generalities to interpolate for our field use.

So because of my terrible wording we do the calculating. The charts are quite a bit off for us. We do nothing like it was done at the factory so our results and calculated interpretations are usually off. For instance you would be a perfect example. You have a different air intake, a different prop, different atmospheric conditions, max throttle of 5500 rpm, a Warp prop and of course the biggest variable is the pilot and the list goes on. We as users have so many variables that the chart really only speaks in generalities and sometimes doesn't even apply when we get too far off track with setups.

 

There are so many variables that the chart becomes more of a feel good thing to refer to.

 

What would rally be helpful for us in the field is to be able to do real dyno test with our current setups and then we would all find out how wrong we probably are. LOL

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Here's the big one:

 

The chart shows both torque and HP climbing from throttle advancement and at 5000 rpm the torque peaks and as throttle is advanced further and rpm climbs the torque starts back down and HP increases until you get to 5800 WOT.

 

The testing is done at WOT, instead of advancing a throttle to get a higher RPM the load is reduced.  Charting the hp/torque at different rpm but always WOT provides numbers we can use.  I can refer to the chart and see 92hp or 95hp at 5500RPM. Lets go with 95hp that I see on the CPS chart.  Assuming its 5500 @ WOT what power setting do I have if I am realizing 5500 @ WOT @ 10,000' DA.  95hp x 70% = 66.5hp  That's warp drive or not.

 

If the chart advanced the throttle to get to 5800 then the 5500 number would be at partial throttle and it wouldn't give me my best continuous power setting. I have to assume the charted data are all from a WOT condition.

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"not sure how much 150 rpm would do for me at speed or climb."

 

It would be a small amount. People with more pitch a bigger difference. You would see some overall fuel economy, slightly better climb, better oil temps (more pronounced on a hot day), a little more topend speed. Some of this will also hinge on your flying style.

 

"I can refer to the chart and see 92hp or 95hp at 5500RPM"

 

But it still was testing completely different from what you personally have as a use and your individual variables. Even your air intake makes a difference or your carb jetting at your altitude. You just don't know the variables and how they figure into your equation and  idea of the chart. It just gives a good generalised numbers, but doesn't take into account for everyone's variables. All variables will impact your final HP answer.

Remember that many items in the manuals are lines in the sand that had to be put somewhere and each specific person's use may move those lines and so some things are ranges versus specifics because of that.

 

It puts you in the ballpark, but isn't a homerun.

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"The testing is done at WOT, instead of advancing a throttle to get a higher RPM the load is reduced.  Charting the hp/torque at different rpm but always WOT provides numbers we can use.  I can refer to the chart and see 92hp or 95hp at 5500RPM. Lets go with 95hp that I see on the CPS chart.  Assuming its 5500 @ WOT what power setting do I have if I am realizing 5500 @ WOT @ 10,000' DA.  95hp x 70% = 66.5hp  That's warp drive or not."

 

Hi Ed,

I agree it will help, but only generalising and interpolating for all your differences from the test itself.

Other than just saying you lose 3% of your HP for every 1K in altitude increase the chart will only help you in generalities. You have nothing physically and do nothing like the chart.

 

The HP output may be 66 HP, but the Warp doesn't get the performance that others do so your performance you were hoping for and looking to get is handicapped. It's just one of those variables, but actually one that can be changed.

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Unintended consequences of prop pitch:

 

The last item that over pitching affects is slow spin up of the rpm in an emergency situation.

As an example:

You're coming in for a landing at idle and you've set your idle rpm down around 1600 and the big thing you've done is have a prop pitch that will only get 5200 rpm WOT in flat and level flight.

All of a sudden you are in trouble and balloon at slow speed and you're going to dump the nose into the ground.

The two things that will hurt you. You go WOT trying to get rpm back, but you're prop doesn't gain rpm very quickly because you are over pitched so a slow spin up happens. Being at a really low rpm may make the engine slow to respond as instant WOT is applied. So you may or may not dump the nose.

 

Two things that will hedge your bets and has been tested with some white knuckles is:

Having a prop pitch a little flatter like 5650 rpm at WOT will allow the engine to spin up faster. Provides a little better climb to help level off before the nose contacts the ground. The second thing is a personal preference on landing technique. If you come in with some throttle and not at stall the rpm has already started and it recovers instantly with a WOT application and the little extra speed allows a quicker speed recovery away from stall and a better leveling of attitude. Bottom line it levels the aircraft in an instant and you don't contact the ground.

 

One reason jets come in with some throttle is it takes a while to spin up in case they need a go around.

 

Just a different landing technique to put in your expanding toolbox unless you only have a little toolbox.. :lol:

 

Playing with this to test isn't advised and causes white knuckles and bulging eyes. :laughter-3293:

 

The best prevention though is to stay away from any condition that may put you in one of these situations. 

An ounce of prevention is worth thousands to your pocketbook. <_<

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I just put it out there for sale and you wanted a Warp. That's what came on your plane and I had it added the day I picked mine up. It was an FD option back then.

It isn't a bad prop and has some good qualities for dirt or gravely fields, but there are better performers, but won't do as well in the dirt and debris fields.

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I think I asked this before a few years ago:  How did they measure the horsepower that shows up on the performance charts?  

 

I ask this because I see confusion in the explanations.  So, I ask the question to get in my two cents.

 

The answer is you can't measure horsepower.  You can only measure torque.  

So you calculate horsepower: (rpm x torque)/5252 for english units.

 

One last comment on engine life that Charles Lindbergh came up with a long time ago.  He ran his variable pitch props with high MP and low RPM for better fuel economy and his engines did not need to be overhauled as often, so that might be a life lesson, so to speak.   B)

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