Cruise Speeds

[gammaxy]

This does come up every few years. It would be cool if someone flying an Aerovee with the Sensenich propeller could meet those numbers at 9,400 feet density altitude. I can dig up plenty of evidence of not achieving them :-) I suspect I'm a good 20mph slower.

I averaged 135mph (as measured with an official stopwatch divided by the race distance) at the 2015 Big Muddy air race down low at WOT--my experience has always been I get more power and fly faster down low. http://sportairrace.org/sarl/node/2200

Sonex published a webinar that shows some of their raw numbers that are very close to what I see in practice: https://youtu.be/HQzHaQ-Pk7I?t=5164. The fastest they show seems to be 141mph true at ~2500ft and 3250 rpm. This seems pretty realistic for a clean airplane with fairings installed. I don't normally achieve it, but I'm confident it's possible with some care.

I just don't understand the physics that would allow me to fly 10mph faster at a lower rpm while breathing air that is ~20% less dense at 9400ft. Best case scenario, the reduction of drag will cancel out with the reduction of power and I'd be flying the same speed. But, I also need to increase my angle of attack to compensate for the reduction of lift (the equivalent of throwing an extra 180 pound passenger in the airplane), so I always end up slower.

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!

I think your logic is mostly sound and a fantastic sanity-check, but I don't think multiplying by 85% is fair. Different manufacturers and designs define pitch differently, but one common definition is relative to the bottom surface of the airfoil which should always work out to a lower number (by several degrees) than the pitch of the airfoil chord line. Spot checking the POH for a Cessna 172 and what I see in my experience with the Sonex, I think using 95-100% is actually pretty close at cruise speed. EDIT: I agree that 85% is a reasonable guess for propeller efficiency, but disagree that it can be applied to this equation. The pitch of the propeller is a somewhat arbitrary physical measurement often based on the bottom surface of the airfoil ~75% out from the hub. I think this convention already "bakes in" the efficiency reduction you're trying to apply.

[GraemeSmith]

Well don't forget that the P-Tip prop claims to repitch itself by up to 4" as speed increases.

Prince P-tip props are the only wood / composite anti-vortex droop tipped propeller in the world. This propeller is a wood, fixed pitch propeller that relies on the forward pull of the propeller to automatically provide pitch change proportional to the amount of force being applied to the propeller disk. When a propeller operates at a slower airspeed than its maximum capabilities, it has a proportional pull in relation to the velocity of forward motion. At takeoff and climbing condition the propeller has its largest forward pull, due to the scimitar shape. The tip of the propeller cones forward, as the coning angle changes the propeller will lessen pitch providing shorter takeoffs and higher rates of climb. As the propeller increases in forward speed the disk pressure is reduced, forcing the propeller to increase pitch and top speed. This change in pitch is approximately 4“ from takeoff to cruise. The droop P-TIP delays the vortices. The propeller then has smooth air to provide better thrust and a pronounced reduction in propeller noise. Additional benefit of the P-TIP design is the volume of air the propeller produces. When air flows through a standard tip propeller design the airflow at the tip will flow over the propeller tip, as soon as it passes the tip it will tuck down behind the propeller blade giving a cone of air from the propeller smaller than the propeller diameter. A 68” P-TIP will give the same volume of air as a standard tip 72” propeller. By reducing propeller diameter there is less frontal area or flat plate drag of the aircraft, higher top speeds are the result without sacrificing takeoff or climb.

Though it is not clear if this means the 44" prop repitches to 48". Or if the loaded 44" prop pitches down to 40". Assuming it pitches up to 48" that would account for better speeds than OP's original calculation suggests and Mike Smith (and my) experience shows:


AEROVEE - 44" pitch prop. This prop moves forward 44 48 inches for every rotation.

At 85% efficiency.
48 in x 3200 rpm = 153,600
153,600 / 63360 = 2.4242 miles / min
2.4242 x 60 = 145.45 x .85 = 124mph Which would be 108knots.

--

I tend to cruise at 2,900

So working it again:

48 in x 2900 rpm = 139,200
139,200 / 63360 = 2.1969 miles / min
2.1969 x 60 = 131.8 x .85 = 112mph Which would be 97.5 knots. And that is AWFUL CLOSE to the 98/99 knots I get at 2,900rpm.

So I'll assume the P-Tips are improving the prop efficiency slightly out towards the tips as claimed.

--

To someone else's comment about efficiency. The McCauley fixed pitch metal prop efficiency figure I am used to using is 0.8 - a sort of "assume the worst" figure.

If you really want to get into it:

https://www.sciencedirect.com/topics/en ... efficiency

[gammaxy]

Though it is not clear if this means the 44" prop repitches to 48". Or if the loaded 44" prop pitches down to 40". Assuming it pitches up to 48" that would account for better speeds than OP's original calculation suggests and Mike Smith (and my) experience shows:

It means it pitches down to 40". The scimitar shape causes the pressure to be farther back near the tips relative to the rest of the blade and will tend to reduce the pitch. Put your finger out near the tip of the propeller and push in the direction of thrust and you'll see which direction the blade should twist due to this effect. This pitch change will be most pronounced when the propeller is the most heavily loaded which will be at full throttle at slow speeds (during takeoff). This should allow the engine to turn a little faster and generate a little more power than otherwise.

There's also a centrifugal twisting moment that tends to want to decrease blade pitch, but it is purely a function of RPM and not loading.

A lot of propellers have some backward sweep near the tips that gives a similar, but probably less pronounced effect. It would be interesting for someone with a high speed camera to actually measure how it compares to the Sensenich. I own both a Sensenich and P-Tip and think they're both great propellers, but I think a lot of details of the P-Tip are more about marketing. It would be very difficult to set up a test to conclusively show its benefit. In my case, the P-Tip is 46" and the 44" Sensenich allows higher RPMs, so I prefer that one.

To someone else's comment about efficiency. The McCauley fixed pitch metal prop efficiency figure I am used to using is 0.8 - a sort of "assume the worst" figure.

I agree that propeller efficiency is in that range, but applying it to the math like in this thread is incorrect for the same reason that estimating glide ratio from angle of attack of the bottom surface of the wing of an arbitrary airplane would be incorrect. If you agree with me that your propeller is not actually increasing pitch to 48", you'll see for yourself that .85 is no longer the right number to get the math to work out.

[markschaible]

Possible cruise speed dilemma?

You need to be flying a Jabiru 3300L engine: "L" for "Light Sport," not the Jabiru 3300A if it was manufactured after a certain date. There is an addendum to the Jabiru 3300 manual for "L" engines. Contact Jabiru USA (now Arion Aircraft) for the most up-to-date information about this. This only became an issue after Jabiru increased it's max continuous RPM specification a number of years ago. Spec used to be 2850 and they increased it to 3300 RPM if memory serves me correctly. We certainly could not understand the logic of increasing their Max Continuous RPM specification to match their Red Line specification -- they were just really proud of the engine! When Sport Pilot was released, we conducted low altitude flight tests at 2850 RPM (was the current max. continuous spec of the engine at that time). When correcting the data for sea level, standard day we ended-up about 3 mph under the "speed limit."

Keep in-mind that Sport Pilot is not a radar gun speed limit. The specification is very specific to sea level, standard day conditions in level flight. There is nothing prohibiting a Sport Pilot from climbing to a more efficient altitude to take advantage of better TAS and efficiency, or from exceeding the maximum speed in a descent. Some monstrously powered bush planes out there prescribe time limits to climb power and have extremely low max continuous RPM specifications (lower than normal for the engine used) specifically for Sport Pilot compliance. A Jabiru 3300 powered Sonex is on much more solid ground with regard to compliance, in our view, than some of the games being played with other aircraft types. The FAA has agreed with our interpretation of the Sport Pilot rules when it comes to compliance of the Jabiru 3300 powered Sonex when it has been questioned in the past.

Anyway, without knowing density altitude, the spec is a little, hmm...can't think of the right word.

Manufacturers always give performance numbers in TAS at specific indicated MSL altitudes (if they give an altitude at-all, you usually have to look at a POH to find that). The example I gave from our archives happened to be a higher density altitude, however, if you plug the same indicated numbers into "standard day" conditions (8,000 ft indicated, 29.92 ALT setting, 15 degrees Celsius, 130 mph indicated) you come-up with essentially the same TAS (151 mph), pressure alt is 8001 ft and Dens Alt is 9816. If it's truly standard temperature at altitude, -9C, then you would be truing out at 144 mph if you're indicating 130 mph, but the colder, more dense air at-altitude will probably allow you to see 135 mph indicated (150 true) without breaking too much of a sweat.

I averaged 135mph (as measured with an official stopwatch divided by the race distance) at the 2015 Big Muddy air race down low at WOT--my experience has always been I get more power and fly faster down low.

Did this race include takeoff, climb, descent and landing, or was the watch started and stopped with the aircraft flying across the line at cruise speed? 135mph is an outstanding average speed for a cross country at low-altitude. Flight planning for a normal cross country would take into account takeoff, climb, descent, pattern and landing so you're never going to plan a normal cross country (time or fuel-wise) using only the cruise speed and fuel burn numbers at your target cruise altitude. Also keep in-mind that you will see a higher indicated airspeed down low, but TAS will be lower.

The fastest they show seems to be 141mph true at ~2500ft and 3250 rpm

This definitely supports our published numbers: 130 mph TAS at 3,000 feet, which is based-on cruising the AeroVee at 3150 RPM.

[builderflyer]

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).

Best Regards,
-Mark

Mark,

Do you have the approximate gross weights at which your speed tests were run? I assume the factory aircraft were not loaded up to max gross for the tests or could have even been lightly loaded. This may partially explain why some of the others differ in their test results if their aircraft were more heavily loaded at the time of their testing.

Thanks,

Art,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[builderflyer]

Possible cruise speed dilemma?

You need to be flying a Jabiru 3300L engine: "L" for "Light Sport," not the Jabiru 3300A if it was manufactured after a certain date. There is an addendum to the Jabiru 3300 manual for "L" engines. Contact Jabiru USA (now Arion Aircraft) for the most up-to-date information about this. This only became an issue after Jabiru increased it's max continuous RPM specification a number of years ago. Spec used to be 2850 and they increased it to 3300 RPM if memory serves me correctly. We certainly could not understand the logic of increasing their Max Continuous RPM specification to match their Red Line specification -- they were just really proud of the engine! When Sport Pilot was released, we conducted low altitude flight tests at 2850 RPM (was the current max. continuous spec of the engine at that time). When correcting the data for sea level, standard day we ended-up about 3 mph under the "speed limit."


Mark,

Thanks for reminding me about the Jabiru 3300L model engine. I believe that engine is placarded at a max continuous rpm of 2850 because of the trouble Jabiru stirred up in the light sport industry when they raised the max continuous rpm on the 3300A model from 2750 to 3300. My early Jabiru engine is a 3300A model. So when Jabiru raised the rpm limit, they effectively removed my Sonex from the light sport catagory. In discussing this matter with Pete Krotje (owner of Jabiru USA at the time), he suggested that I keep the original manual that came with my engine which specifies a max continuous rpm of 2750. Then, if ever questioned by the FAA, as to the legitimacy of my Sonex qualifying under the light sport rules, I could present the manual along with Sonex LLC confirmation of meeting light sport requirements even at 2850 rpm. This combination should, hopefully, satisfy the FAA with regards to my particular aircraft.

My earlier point, however, was that if a Sonex has a 3300A model engine on it and the manual that came with that particular engine (or the only version of the manual that is kept with a specific aircraft's documents) specifies a max continuous rpm of 3300, for example, then that Sonex can't legally be operated as a light sport aircraft. I don't know if there are any Sonex out there that fit into this status.

Thanks for your response,

Art,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[markschaible]

Mark,

Do you have the approximate gross weights at which your speed tests were run? I assume the factory aircraft were not loaded up to max gross for the tests or could have even been lightly loaded. This may partially explain why some of the others differ in their test results if their aircraft were more heavily loaded at the time of their testing.

Thanks,

Art,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

We've been able to get these numbers at various loading up-to and including takeoff at max gross.

Bottom Line: if you can't indicate 130 mph with an AeroVee powered Sonex or 150 mph with a 3300 powered Sonex at or around max continuous RPM, then something is wrong: Fairings & wheel pants, rigging, engine/carb tuning, propeller, airspeed indicator calibration, etc.

I'll leave this discussion with this, Kerry's recent article at: https://www.kitplanes.com/the-one-true-airspeed/

[builderflyer]

Mark,

Do you have the approximate gross weights at which your speed tests were run? I assume the factory aircraft were not loaded up to max gross for the tests or could have even been lightly loaded. This may partially explain why some of the others differ in their test results if their aircraft were more heavily loaded at the time of their testing.

Thanks,

Art,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

We've been able to get these numbers at various loading up-to and including takeoff at max gross.

Bottom Line: if you can't indicate 130 mph with an AeroVee powered Sonex or 150 mph with a 3300 powered Sonex at or around max continuous RPM, then something is wrong: Fairings & wheel pants, rigging, engine/carb tuning, propeller, airspeed indicator calibration, etc.

I'll leave this discussion with this, Kerry's recent article at: https://www.kitplanes.com/the-one-true-airspeed/

Thanks Mark,

I have a plans built pitot-static system on my Sonex and as built it provided a rather high, unrealistic IAS. After calibration (it was all static system error), I was able to achieve a difference of only 1 or 2 mph between IAS and CAS from 90 mph to 190 mph. So I feel pretty good about any cruise numbers I may report. One correction on your "bottom line"............for the 3300 powered Sonex you said an "indicated" 150 mph at max continuous rpm whereas I believe you meant to say a TAS of 150 mph. My Sonex is reasonably fast but not that fast.

Ok, I'm done.

Art

[markschaible]

No, I mean indicated for the 3300. your TAS at 8,000 feet should easily reach 170 mph.

[SonexFactoryTech]

Mark brought this thread to my attention and asked me to share my numbers. Numbers are boring, and variable, and not as absolute as everyone wants them to be but I'll dump mine here and leave ya'll to it.

Aircraft: Metal Illnes
Polished finish
Tailwheel configuration
Jabiru 3300-powered (engine s/n 170)
Sonex-recommend Senensich glass-coated prop
Fully faired
Empty weight 654 pounds
Gross weight for both flights; about 936 pounds (Plane, pilot, stuff, and 17.2 gallons* of fuel).

(*A legacy Sonex tank vented straight up on a conventional gear Sonex allows about 17.2 gallons of fuel onboard.)

Date of Flights: Jan 2006

Flight One:
Pressure Altitude 8000 feet (Alt set to 29.92)
WOT = 3110 RPM
GPH = 6.9 (6.7 to 7.0 range)
OAT = 10 degrees F
CHT = 246/249
Oil T/P = 193/53
EGT = 1313/1357
IAS = 163
CAS = 164
TAS = 188.75
Endurance = 2.46 hours
Range = 465 mi
MPG = 27.3

Flight Two:
Pressure Altitude 8000 feet (Alt set to 29.92)
2850 RPM
GPH = 4.9 to 5.0
OAT = 10 F
CHT = 247 / 251
Oil T/P = 181/47
EGT = 1392 / 1428
IAS = 146.5
CAS = 148.7
TAS = 169.94
Endurance = 3.4 hour
Range = 577 mi
MPG = 33.9

I also once flew 486 miles in 3.9 hours on one tank of gas, and flight tested a scenario where flying at 100mph IAS at about 2000 MSL I could lean out to 2.2 GPH, giving a no-reserve range of 780 miles.

Blue Skies....

Kerry Fores