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Allegro Cruise Speeds

 
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Thom Riddle



Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

PostPosted: Tue Jan 31, 2006 6:55 am    Post subject: Allegro Cruise Speeds Reply with quote

Paul, at Aero-Siam asked, me about the cruise speeds I've been seeing on the Allegro so I'm posting the results of the tests I've run on our Allegro with 80 hp Rotax 912UL engine and Woodcomp 633R propellor set for best cruise speed at relatively low altitudes, i.e., 5,500 rpm at full throttle. I've attached a file that shows the actual performance numbers at various RPM settings. Please note the following:

Tests were done at sea level density altitude (cold day at about 1,800' msl). The Airspeed Indicator reads rather high so I did upwind/downwind GPS groundspeed averages to get True Air Speed (TAS). Note that any change in prop pitch setting will invalidate the actual speed results. Also, with this pitch setting, I could not attain 5,500 rpm at a Density Altitude above 6,500' (~2,000 m). With less pitch the Allegro will climb better and be able to achieve higher speeds at higher altitudes.

I hope this information is helpful. Let me know if you have any questions.


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Thom Riddle
Buffalo, NY (9G0)



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Aero Siam



Joined: 30 Jan 2006
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Location: Thailand

PostPosted: Thu Feb 02, 2006 11:52 am    Post subject: Re: Allegro Cruise Speeds Reply with quote

Thom

Thanks for the detailed info on the performance of the 80hp which I have incorporated into the website.

If someone has similar performance figures for the speed/RPM/fuel burn for the 100hp, please add them to the topic and we can expand the information available.

Paul


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Thom Riddle



Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

PostPosted: Fri Feb 03, 2006 5:13 am    Post subject: Allegro Cruise Speeds Reply with quote

Paul,

The airspeed varies approximately with the square root of the power.
The 912UL engine has a maximum continuous rating of 79 HP at 5,500 rpm.
The 912ULS is rated at 95 HP at 5,500 rpm, also max. continuous.

95 / 79 = 1.18987 18.99% greater max cont. power
1.18987 ^ .5 = 1.0908 9.1% approx. expected speed difference

Since the torque and horsepower curves over entire RPM range are
similar for the two engines, it is reasonable to expect about 9%
difference in airspeeds between the same airplane with different
engines. If an Allegro 2000 with the 912ULS engine and a three blade
Woodcomp prop is pitched for best cruise, then this airplane under the
same conditions could expect True Air Speeds (TAS) of about 9% higher
than the figures in the table I provided. Examples shown below.

Zero Density Altitude
Actual Aircraft Flying Weight 940 lb. or 427 kg.

RPM 912 UL 912 ULS
5,500 126 mph 137 mph
5,000 112 122
4,500 102 111
4,000 92 100
3,500 73 80

I hope this helps.

Thom in Buffalo


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Aero Siam



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PostPosted: Fri Feb 03, 2006 6:47 am    Post subject: Re: Allegro Cruise Speeds Reply with quote

Thom

Thanks for the info. I will update the Performance page to incorporate the speeds for the 100hp.

Is it possible to extrapolate the figures for fuel burn at various RPM to complete the picture? Rotax's website has a graph for fuel consumption. Can the figures be used directly or is there a difference between bench tests and actual in-flight results?

Paul


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Thom Riddle



Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

PostPosted: Sat Feb 04, 2006 6:26 am    Post subject: Allegro Cruise Speeds Reply with quote

Paul,

Based on what I've been told by a guy I know flying a Kolb Mark III
experimental the 912S is not as fuel efficient at the 912. He flew his
Mark III for a few years with the 912, putting over 1800 hours on it,
then installed a 912S and has been flying that for a little over 2000
hours. If anyone should know it is him. He flies that airplane all
across the USA and Canada, include three trips to Alaska north of the
arctic circle. He was happy with the 912 in every way except he needed
more power to fly over the Rocky Mountains which he does regularly,
even though he lives in Alabama, over 1,000 miles east of the Rockies.
With all his cross country flying he says that in his normal cruise
configuration he got 4 gph with the 912 and 5 gph with the 912S with
only about 5 mph difference in cruise speed. Of course this is with a
slower airplane so the speed difference is less than in the Allegro.

I know that the 912 burns 4.0 gph at 75% power (max cruise HP). Based
on this fact, I calculate the following:

75% x 79 hp = 59.25 hp at cruise
4.0 gph x 6 lb. per gal = 24.0 lb. per hr.
24.0 lb. per hr / 59.25 hp = .405 lb. per HP per hour at 75% cruise
power

Assuming the Kolb pilot is flying the 912S at 75% cruise (max cruise
HP), the following similar calculations yield:

75% x 95 hp = 71.25 hp at cruise
5.0 gph x 6 lb. per gal = 30.0 lb. per hr.
30.0 lb. per hr / 71.25 hp = .421 lb. per HP per hour at 75% cruise
power

The lb. per hour per HP figures are used to compare efficiencies at
specific power settings. Within normal cruise power ranges, say 60% to
80% or so, these numbers should hold close to constant. Typically for
aircraft engines, at higher or lower power settings the efficiencies
are not quite as good.

That said, I would feel comfortable with the following numbers for the
912S:

Power Fuel Consumption (US gph)
60% 4.0 gph
65% 4.3
70% 4.7
75% 5.0
80% 5.3

Outside this range the fuel burn will probably be greater(per HP) than
this but I don't know by how much. You may be able to make educated
guesses by comparing real world fuel burn rates with what is in the
Rotax manuals. BTW, the fuel burn numbers in the Rotax manuals are
notoriously conservative, especially for the 2-stroke engines, so I
would not use them.... too pessimistic.

Worth noting is the fact that the lower compression 912 requires only
87 Octane fuel vs. the higher compression 912S which requires 91 Octane
fuel. I burn 87 Octane auto fuel in both my 912 engines and they are
quite happy with that fuel. Some people run higher octane than is
required but it is a waste of money.
Thom


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Thom Riddle
Buffalo, NY (9G0)



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Aero Siam



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PostPosted: Sat Feb 04, 2006 8:35 pm    Post subject: Re: Cruise Speeds - 75% power Reply with quote

Thom
Thanks again for your detailed info about fuel consumption and the experiences of your friend with the Kolb. I am still working on an update to the website.

A couple more points - how is 75% power defined? I have seen it quoted by people at anywhere between 4800 and 5400 RPM. If 75% power equates to about 71hp for the 912S then looking at Rotax's website, 71hp hits the line at about 4200 RPM which seems way low. What's the solution?

Pitching the prop. Does anyone have any info about the performance differences with the Woodcomp prop set fine and coarse. It would be interesting to do an article sometime for both engines showing the performance range at both ends of the scale.

Paul
Aero Siam


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Thom Riddle



Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

PostPosted: Sun Feb 05, 2006 6:02 am    Post subject: Re: Allegro Cruise Speeds Reply with quote

Paul,

Rotax uses Max. Continuous Cruise RPM and power as the base or 100% since anything higher than 5,500 is limited to 5 minutes. There are two curves on one chart shown in the Rotax manual. One curve is for power availalbe and the other is for power required, if my memory is correct. There is also a table for RPM and Manifold Pressure settings which equate % power. With an in-flight adjustable prop, like on my Titan Tornado, or constant speed prop. this table is the key to setting power. But this table is of little use for fixed-pitch props. That is why I stated in the data I gave you for the power/speed of 912 engine, that a change in prop. pitch will invalidate all the data.

5,400 rpm on a 912 series engine (AT SEA LEVEL) is about 98% power if the prop is pitched in the normal range, i.e. it is loaded. At least one article I've read about the Allegro states this rpm to be 75%. Just not true and a greviously mis-leading statement in my opinion.

That said, the way a ground adjustable prop is pitched is one factor that will determine at what rpm a given amount of power is actually being delivered and thus fuel consumed. The load is also a big factor. For example, go to straight and level flight and set your rpm to 4,800 RPM. Notice that in level flight the throttle is no where near wide open. Set your trim for Vy and advance throttle to wide open. With the prop set like mine is, the Allegro will start climbing and at Vy the RPM is still about 4,800 rpm. Which one of these conditions is consuming more fuel? They are at the same rpm, so why not the same fuel? The load is far greater during climb at this rpm and the throttle is wide open too.

As I've stated before, my prop is set for best cruise speeds at relatively low density altitudes. That is, wide open throttle at DA of less than 7,000 feet will produce 5,500 rpm. At this pitch, our engine turns only 4,800 rpm at Vy and wide open throttle. At wide open throttle, 4,800 rpm at sea level, the engine is producing about 84% power. If I wanted/needed better climb performance I could reduce the pitch a little bit which would result in higher RPM under the same conditions, say 5,000 rpm, which would yield about 89% power and higher climb rate. BUT, this means that in straight and level I could pass the 5,500 rpm limit for max. continuous cruise, because with less ptich there is less load on the engine at a given throttle setting. However, this would also allow for higher rpm at higher altitudes this producing a little higher cruise speeds at higher density altitudes at wide open throttle.

As you can see the relationship between power, RPM, and airspeed is complex and the main reason that airplane performance can benefit greatly from an in-flight controllable ptich propellor. I can pitch the prop. on my Titan Tornado for 5,500 or even 5,8000 rpm at wide open throttle at Vy climb speed and get 1,400 ft./min. climb rate (5,500 rpm), even with its very short 20' wingspan. When I reach cruise altitude, I adjust the pitch so that I get 5,500 rpm at wide open throttle, then reduce the throttle to about 4,200 rpm for low fuel consumption, and low noise level cruising at nice comfortable cruise speed. Even though both airplanes have very close to the same max. cruise speed, my normal cruise speed in the Titan with the adjustable pitch prop is only 4,200 rpm but 4,700 rpm in the Allegro, all because of the in-flight adjustable prop.

In the USA, aircraft with Light Sport Aircraft air-worthiness certificates are not allowed to have in-flight adjustable propellors. So this is not an option for Allegro owners in the USA. I don't know the rules in other countries. That said, if one wanted a good compromise between climb and cruise, I would set the prop. pitch so that the eingine would turn about 5,000-5,100 rpm with wide open throttle at Vy.

Sorry for the long-winded response but the relationship bewteen prop pitch, engine RPM, power and thrust produced is complex.

Thom in Buffalo


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