Cruise / Speed test

[Cluemeister]

Went up to 3500 feet plus or minus, put the autopilot on level, and set the RPM's to 5600-5650.

She leveled out at a constant speed, and here are a few screenshots of the speed and MAP.  113 KIAS, 122 True, RPM 5640 and MAP 32.4.

Prop is set to how it was delivered, I believe in the middle at 3.  I plan on pitching the prop to max cruise.  Not bad for how it is now though!

[geobuff]

If you have the throttle fire-walled and you are turning 5650 rpm, you are EXACTLY where you should be for max speed and power. Anything you change will make it worse. There is endless discussions on the Rotax-Owners Forum to validate this. NOW, if 5650 is not obtained at a fire-walled throttle setting, then you need to test it at the fire-walled setting. It the RPM goes higher at the fire-walled setting, then you need to pitch the prop so it is between 5600-5650 when the throttle is fire-walled. The term they use on the Rotax-Owners forum is "WOT" for fire-walled. WOT stands for Wide Open Throttle.

I see you are flying 122 TAS, which is exactly what I would expect, and what all the rest of us do at that RPM. Don't touch a thing. You got it. Now at that throttle setting you are burning a lot of fuel. Normally we should be flying at 5425 for cruise, as that is the best mileage/speed combo. You will get about 118 TAS at that RPM. These Tecnam's are fast aircraft compared to most all the others LSA's. You are traveling considerable faster that a C172 and near the speed of a C182, on 1/2 the fuel. We are fortunate. Some folks like to cruise at 5200 RPM, which will bring your TAS down to about 110. Yes, your fuel burn is less, but your speed is proportionally less also, but your "MPH" is about the same. These ROTAX engines are most efficient in the 5400 RPM range. Of course, all this is predicted on having the prop pitched correctly as described above. All these ideas come from the Rotax-Owners forum and from experimentation in my Astore.

I have posted my SPEED / ALTITUDE / RPM charts in other sections of this forum. In case some have missed it, I post them again below. There charts will be very close to the performance of your aircraft if your prop is pitched correctly. This chart was constructed by experimentation at the altitudes indicated, in smooth air. My engine is 912-IS, so if you have the carbureted engine, the fuel burns will be a little more, but the speeds will be accurate at the altitude indicated. Note I do not have Speed at WOT, because you cannot cruise at WOT for more that a few minutes (Rotax limitation and WOT is only intended for climbing), so there is no point. The chart only goes up to 5500 RPM, the max reasonable continuous performance.

[mackattack]

Just to confirm the 5650 is for a 914 engine or for the 912iS engine? Or are they the same numbers? I typically try to cruise around 5400 myself when max range is not the issue...

[geobuff]

5600-5650 is max efficiency of the prop for a Rotax engine. Doesn't matter if 912ULS, 912IS or 914 Turbo. In the attachment below, I show a clip from Roger Lee, the Rotax Prop Guru. If you search the forum, you will quickly realize he is the world expert in these issues.  

One thing he point out which offers a little wrinkle, is do this WOT adjustment at your most common flying altitude.  If you nearly always fly below 5000', do your test at 3500' altitude.  If you usually fly at 8000', do your test at 6000'.  He says it is a little different at each altitude, but since we do not have in-flight adjustable props, we must set WOT at the median altitude we fly at.  I think I set mine at 4000' because I fly both high and low, and I decided 4000 was my best average.  You may wish to choose a different altitude for your test.

[Cluemeister]

Oct 9, 2016 13:35:18 GMT geobuff said:

If you have the throttle fire-walled and you are turning 5650 rpm, you are EXACTLY where you should be for max speed and power. 

I do not have it firewalled at 5650.

[jetcat3]

Does firewalled mean 100% power in the case of the 914 equaling 100 continuous horsepower at 5600-5650?

[mackattack]

I believe it means all the way forward but not past the turbo detent.

[geobuff]

I have not flown the 914, so I don't know. Might be a good question for Roger Lee. I assume the 'turbo detent' is the mark on the throttle past which you engage the turbo. I guess before you engage you are at 85hp and 120 hp when past the mark and the turbo is engaged. Please educate me. Do you only use the turbo on climb or can you use it anytime for cruise continuous power?

If you can use the turbo for continuous cruise, I would believe WOT would be full forward on the throttle past turbo detent. I will post a question on the Rotax-Owners forum.

[geobuff]

Oct 10, 2016 3:37:35 GMT geobuff said:

I have not flown the 914, so I don't know. Might be a good question for Roger Lee. I assume the 'turbo detent' is the mark on the throttle past which you engage the turbo. I guess before you engage you are at 85hp and 120 hp when past the mark and the turbo is engaged. Please educate me. Do you only use the turbo on climb or can you use it anytime for cruise continuous power?

If you can use the turbo for continuous cruise, I would believe WOT would be full forward on the throttle past turbo detent. I will post a question on the Rotax-Owners forum.

OK, I have the answer from the Rotax-Owners forum.  It is BEFORE the turbo-Detent

[Cluemeister]

Full throttle is stopped at the detent. The 115% power (past the detent) is only available for up to 5 minutes.

[mackattack]

Roger was kind enough to post in response to exactly this question that I asked him a while back ... he is a super guy and a great resource ...

[jetcat3]

The 914 takes advantage of turbo boosting at any power setting which is nice. The computer constantly regulates that process. 115% power limited to 5 minutes. Maximum rated horsepower (115) is good all the way up to 8,000ft and maximum continuous power (100) is good up to 16,000ft.

[stroudj]

I have found that there are a lot of confusing concerning the operation of the 914. The following link will answer a lot of questions for the 914 operator.

www.rotax-owner.com/en/support-topmenu/technical-information/rotax-technical-articles/17-understanding-the-rotax-914/file


The following is an excerpt from the article:

 
As previously discussed, using the physical or visual means of verifying the

100% throttle position, which should equate to 35” airbox pressure and 5500

RPM, helps verify that the system is calibrated correctly. The normal

procedure to set maximum cruise is to bring the throttle back to 100% throttle

position mark, then adjust the RPM via the prop pitch (with adjustable props)

to 5500 RPM, and finally verify that the airbox pressure is 35inHg (±1inHg)

If you want to increase power, you would first adjust to the desired RPM,

using the prop pitch, then increase the throttle to the corresponding throttle

position, and finally verify that the airbox pressure is correct. The following is

the correct airbox pressure for a given RPM, for all full and cruise power

settings.

RPM MAP inHg Percent of power Throttle position

5800 40” 100% Power (Full Throttle) 115% Throttle Position

5500 35” 85% Power (Maximum Cruise) 100% Throttle Position

5000 31” 75% Power (Normal Cruise) Approx. 85% Throttle Position.

4800 29” 65% Power (Economy Cruise) Approx. 65% Throttle Position

It is important to note that 115% throttle position equates to 100% engine

power (full take-off power), don’t get the two number confused.

[mackattack]

Thanks much!!! Interesting that the article suggests WOT as 5500 but Roger Lee says 5650 in his posts...

[jetcat3]

Are those power readouts measured on the ground or in the air?