Cruise Speeds

[Direct C51]

I have been trying to clean up my Sonex, aerodynamically, in order to gain a few knots. I was feeling like something was off with my numbers compared to the published data from Sonex, until I started doing some calculations.

Sonex Published Cruise Speeds:
Aerovee 80hp - 150 mph.
Jabiru 3300 - 170 mph.

AEROVEE - 44" pitch prop. This prop moves forward 44 inches for every rotation. In order to cruise at 150 mph with 100% efficiency (not possible) the RPM would be 3600!
44 in x 3600 rpm = 158,400 in/min
158,400 / 63360 = 2.5 miles / min
2.5 x 60 = 150 mph
Now, there is no way an airplane will operate at 100% efficiency. Drag and prop slippage means we will see about 85% efficiency. So a more realistic number might be 113mph at 3200 RPM!
44 in x 3200 rpm = 140,800
140,800 / 63360 = 2.2222 miles / min
2.2222 x 60 = 133 x .85 = 113mph

JABIRU - 64" pitch prop. To get the 170 mph cruise Sonex claims at 100% efficiency (not possible) would mean 2800 RPM. The max cruise RPM for a Jabiru is 2750. At a realistic 85% efficiency, and using the max cruise 2750 RPM, the cruise speed is realistically 142 mph.
64 in x 2750 rpm = 176,000
176,000 / 63360 = 2.7778
2.7778 x 60 = 166.6667 x .85 = 142 mph

Let's not even get started on the 687 mile "conservative, low altitude 130mph TAS" range...

[mike.smith]

Different conditions will give different results, but in level flight with my AeroVee I can cruise WOT at 130 kts (burning a LOT of fuel). That's about 149.5 mph. I don't generally run like that except for short bursts, or I feel like I'm whipping the "horse" to death. I generally cruise at max 100 kts or 115 mph, at just under 3,000 rpm. My taildragger weighs 673 lbs empty, and I'm running a Prince P-Tip prop. Math or no math, those are the numbers I've gotten over the last 6 years and nearly 500 hours.

[peter anson]

According to the Jabiru installation manual, Jabiru 3300 cruise speed is 2750 to 3100 rpm. I do normally cruise at around 2750 rpm and speed is around 120 to 125 knots - 138 -144 mph. 3100 rpm should push the speed up to around 140 knots but fuel consumption would be pretty high. For range, I don't think I have ever tried for more than 280 nautical miles, about 320 miles.

Peter

[Direct C51]

Different conditions will give different results, but in level flight with my AeroVee I can cruise WOT at 130 kts (burning a LOT of fuel). That's about 149.5 mph. I don't generally run like that except for short bursts, or I feel like I'm whipping the "horse" to death. I generally cruise at max 100 kts or 115 mph, at just under 3,000 rpm. My taildragger weighs 673 lbs empty, and I'm running a Prince P-Tip prop. Math or no math, those are the numbers I've gotten over the last 6 years and nearly 500 hours.

Mike, what pitch prop do you have? I would imagine the Prince is closer to 46” in cruise. What RPM are you at when flying 149.5mph? 3800? I’m not trying to be condescending, but do you find you more often have a headwind when compared to your GPS ground speed?

According to the Jabiru installation manual, Jabiru 3300 cruise speed is 2750 to 3100 rpm. I do normally cruise at around 2750 rpm and speed is around 120 to 125 knots - 138 -144 mph. 3100 rpm should push the speed up to around 140 knots but fuel consumption would be pretty high. For range, I don't think I have ever tried for more than 280 nautical miles, about 320 miles.

Peter

Peter, looks like we are in agreeance and your real world experience proves my math. I predicted 142mph at 2750 RPM and you confirmed 138-144 mph. I agree, 3100 RPM should give you 140 kts/160mph. Is that a normal cruise RPM for a Jab? Still 10 mph less than published cruise numbers.

[mike.smith]

Different conditions will give different results, but in level flight with my AeroVee I can cruise WOT at 130 kts (burning a LOT of fuel). That's about 149.5 mph. I don't generally run like that except for short bursts, or I feel like I'm whipping the "horse" to death. I generally cruise at max 100 kts or 115 mph, at just under 3,000 rpm. My taildragger weighs 673 lbs empty, and I'm running a Prince P-Tip prop. Math or no math, those are the numbers I've gotten over the last 6 years and nearly 500 hours.

Mike, what pitch prop do you have? I would imagine the Prince is closer to 46” in cruise. What RPM are you at when flying 149.5mph? 3800? I’m not trying to be condescending, but do you find you more often have a headwind when compared to your GPS ground speed?

54" diameter, 44" pitch. At about 3370 rpm is about all I can get in level flight.

I went back and looked at my flight tests between my Sensenich and my Prince props. The Sensenich only got up to 110 kts at 3,250 rpm. The Prince got up to 125 kts at 3370 rpm. Test OAT was around 76 deg F for both tests. Flying out to OSH last time I was getting 130 kts at times (according to my knee board numbers I recorded), but I didn't record the rpms.

[peter anson]

Peter, looks like we are in agreeance and your real world experience proves my math. I predicted 142mph at 2750 RPM and you confirmed 138-144 mph. I agree, 3100 RPM should give you 140 kts/160mph. Is that a normal cruise RPM for a Jab? Still 10 mph less than published cruise numbers.

I never cruise using 3100 RPM* but recall being at a fly-in some years ago and talking to a guy who had flown down from Bundaberg in one of the Jabiru 230s from the factory. He said that they were running at 3000 RPM the whole trip. I tend to fly by fuel flow using an indicated 21 to 22 litres per hour. Again, the installation manual says 75% power would use 24 litres per hour. I have tried running at higher RPM but the gain in speed didn't seem to justify the extra fuel consumption.

* I did cruise at up to 140 knots one time when I was worried about reaching my destination by last light but can't recall the fuel rate.

Peter

[sonex1374]

Peter,

Talking to the Jabiru USA team they say the 3300 loves to spin up and run at 3000 rpm in cruise. However, like you, I rarely push the engine that fast. I prefer to run at around 2750-2900 burning somewhere between 5.8 and 6.5 gal per hour. I always get the impression that I can lean a little more and save fuel, but it just seems to purr at those settings, so that's good enough for me. At 2750 down low that will yield around 135 mph, but go up a bit higher or run at 2900 and cruise speeds increase to about 150 mph. The 3300 will do 170 mph but it burns a lot of fuel and just seems hard on the engine so I never do it (probably overly caution, but that's just my opinion).

Jeff

[markschaible]

Hello SonexBuilders.net!

Every few years this topic comes-up, and invariably, folks are dismayed at our published figures because they are looking at indicated airspeeds and/or GPS ground speeds. Please keep in-mind that all of our published cruise speeds are given in TAS (True Airspeed).

Here's an example of typical cruise data on one of our archived flight test cards for an AeroVee powered Sonex:
==============================
Indicated Altitude: 8,000 feet MSL
Altimeter Setting: 30.07
Temperature: 13C
RPM: 3150
Indicated airspeed: 130 mph
True airspeed: 150 mph
Density altitude: 9,425 ft MSL
Pressure altitude: 7,863 ft MSL
==============================

Here's a look at what we typically see with a Jabiru 3300 powered Sonex or Waiex using the same atmospheric conditions to do the TAS conversion:
==============================
Indicated Altitude: 8,000 feet MSL
Altimeter Setting: 30.07
Temperature: 13C
RPM: 2850
Indicated airspeed: 150 mph
True airspeed: 173 mph
Density altitude: 9,425 ft MSL
Pressure altitude: 7,863 ft MSL
==============================

Get out your whiz wheel, digital E6B or find a good online TAS calculator, punch-in your data and see what you get!

Best Regards,
-Mark

[builderflyer]

Possible cruise speed dilemma?

For those flying behind a Jabiru 3300 and wishing to stay within the "light sport" rules (i.e. drivers license medical), it is likely necessary to use a maximum continuous rpm of 2750 at sea level. Fortunately, the manual that came with my older engine specifies 2750 as the maximum continuous rpm but I believe that later manuals call for a higher rpm. Even at 2750 rpm at sea level my Sonex really pushes the CAS limit of 120 kts (138 mph) as specified in the light sport aircraft rules..

The point is that if you have a Sonex (or Waiex) with a Jabiru 3300 and it came with an engine manual that specifies a maximum continuous rpm of something over 2750 rpm, chances are you aren't flying a Sonex that can be legally operated under the light sport rules (depending, perhaps, upon which prop you may be using). Just my non-lawyer opinion after discussing the matter with Pete Krotje (sp?) formerly of Jabiru USA a few years ago. Others may choose to disagree.

Art,,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,Jabiru 3300 #261
(Currently flying under BasicMed but want to keep a driver's license medical available as an option)

[lpaaruule]

I assumed 8000' referred to density altitude, not indicated, but I see on the performance specs it's stated as 8000' MSL. The density altitude in this case is closer to 10,000' than 8000'. The last time I was at 10,000' indicated, the density altitude was closer to 9000', but the OAT was much colder.

Anyway, without knowing density altitude, the spec is a little, hmm...can't think of the right word. [edit --> ambiguous]

Seems like it would take the same fuel flow to get 150mph indicated, no matter the altitude. Other variables besides aerodynamic drag come into play I'm sure.

Interesting discussion though.