Cruise Speed Difference at 8000 ft, why?

[Whiskey Hammer]

At sea level both the 80hp VW engine and the 120hp Jabiru engine cruise at about the same speed (130mph vs 135mph*).

At 8000ft the VW engine gains a respectable 20 mph boost to 150mph TAS. That makes sense to me. The air is thinner there so you get less oxygen to burn in your engine and less air for your prop to grab but you also get less drag. The balance of which nets you a moderate gain in speed, i.e. the reduced drag adds more speed at max continuous output than the thinner air takes away from your engine and prop.

At 8000ft the Jabiru engine get's a whopping 170 mph, a jump of about 40mph. Double the jump of the VW engine That's what I don't get. How can two powerplant and prop combinations that are so similar at sea level produce such wildly different results at altitude? If the Jabiru were turbo normalized that would make sense, but to the best of my knowledge it isn't.

If the VW engine flys faster at altitude, it's because the drag reduction is greater than the thrust loss. The drag reduction at 8000ft should be the same for each plane regardless of engine choice. That suggests that the Jabiru is better at maintaining it's thrust at altitude.
- Is the 6 cylinder less susceptible to thin air than a 4 cylinder?
- Is the prop really that different?
- If it is the prop, what general design elements of the prop allow it to retain so much more thrust at 8000ft than the prop on the weaker engine? Is it as simple as pitch, where the Jabiru uses a bigger bite that's draggy at high densities but decent in thinner air?


*Edit: Corrected a typo. The Jabiru's sea level cruise speed was written as 150mph. The actual speed is 135mph.

[Direct C51]

I would imagine it is because they are using the continuous output of the Jabiru for the sea level speed, which is 107 HP or about 89%, yet the VW is likely using all 80HP, 100% available. So at sea level the VW is 80HP vs Jabiru 107HP, so 75% of the Jabiru.

At 8000' lets say they are making 60% power. That's 48HP for the VW and 72HP for the Jabiru. So now the VW is only making 67% of the power as the Jabiru.

Also the Jabiru numbers at sea level might be advertised a little low so they can stay within LSA requirements.

[gammaxy]

In my opinion, you are thinking about physics when you should be thinking about the LSA rules.

Sonex lists the performance as 130mph for Aerovee and 135mph for Jabiru 3300 at sea level. The LSA rule is 138mph at maximum continuous power at sea level.

Sonex provides this insight:

The most common question on these speeds involves the use of the 3300 Jabiru. According to the Jabiru Factory, the maximum continuous power rating of the 3300L Jabiru is 2850 rpm. At this rpm and under the specified conditions, the Sonex and Waiex cruise at 135 mph.

If the LSA rules didn't exist, the jump would go away and better match what you'd expect.

I'm easily faster than 130mph TAS with my Aerovee at low altitudes, but don't see any increase in true airspeed at high altitudes. I also don't see 150mph TAS at 8000 feet and suspect most Aerovees don't.

[fastj22]

My jab will TAS at 8500ft full throttle at around 170-180MPH turning 3100 RPM, Prince P-tip. Pulled back to 2800RPM, its 150 TAS burning around 6 GPH. I'm fully dressed with wheel pants and intersection fairings.

[Paul Johnson]

The quoted speed of 135mph at sea level is for the Jabiru 3300L engine which is limited to 2800rpm and qualifies as an LSA. At 8500 feet a normally aspirated engine will produce approximately 75% power. 2800rpm on the Jabiru 3300 is still achievable at higher altitudes as it is a rpm restriction and not power but the normally aspirated Aero Vee will not be able to maintain its sea level rpm and hence power.

[DCASonex]

When I pulled my Jabiru 3300L and replaced it with a CAMit 3300 (which had no L series), local FAA inspector agreed that since it is in the operating manual not to exceed 2,850 RPM for cruise and I am using same prop, it would not matter what engine, or team of over-caffeinated hamsters, was turning the prop, air speed would be the same, and still LSA legal. Also, FWIW: Shortly before CAMit went belly up, Ian reported his 3300 engines making 127 HP.

David A.

[fastj22]

I don't think my Jab 3300 waiex could exceed 135mph at sea level, unless measured at Death Valley. Then, maybe. I'll avoid doing full throttle low approaches in Death Valley.

[Whiskey Hammer]

So far it looks like the theories are:

-The reported sea level cruise speed for the VW engine is at 100% engine power and not max continuous power. This means that the actual cruise speed is lower than 135 and the jump in speed from sea level to 8000ft is therefore larger than the reported numbers indicate and much more consistent with the Jab's jump.

-The Jabiru sea level cruise speeds are conservative estimates to meet LSA requirements and the high altitude cruise speeds are... less conservative. Like the previous theory, this means that the actual performance jump for the Jab is smaller than reported and that the jumps are more consistent with each other.

@Paul Johnson, can you describe the effect in more detail? The max continuous output is listed as 2800ish RPMs at 107hp, and that implies to me that the engine's max RPM is limited by the horsepower it can produce and not an internal rev limiter with lots of underused hp that offsets the 25% power loss at 8000ft. This might be a novice question but is that the reason why you see 500hp engine in high flying planes? So that they can loose a fair bit of power at altitude and still turn the prop at peak RPM?

Does anyone know of a Jab that actually produces 170mph TAS at 8000 ft max continuous output? Fastj22 has helpfpully mentioned that his only get's 170mph at 8000ft with full power.

I asked the company's tech team and got a non-technical answer: "The Jabiru 3300 has 50% more horsepower than the 80 HP AeroVee." I'll follow up and see if I can get more info.

[Whiskey Hammer]

Sonex got back to me and here's the more detailed explanation:

Both top speeds are TAS, based on running our specified props at WOT at
8000' on a standard day on a fully-faired airframe. In both cases the
engines are producing 75% power.

At Sea Level our published cruise speed for the Jabiru is reduced because of
Jabiru's published limits. At altitude some of the "lost power" of the
Jabiru is gained back by advancing the throttle further.


I did some math and it turns out that everyone in this thread was more or less right. For people like me who want a logical train to follow, here's the logic that shows that the MPH per HP gain of the two engine's is essentially identical; and consequently that the Jabiru can cruise at 100% throttle at 170 mph with the same or less stress of the max continuous output.

Stipulated:
1) Wide Open Throttle (WOT), or 100% throttle, is the maximum amount of fuel that the system is allowed to throw into the engine. At sea level, this produces the maximum power of the engine (generally limited to 2 minutes in duration).
2) At 8000ft, you're producing roughly 75% of whatever power your throttle level would normally produce at sea level.
3) Producing horsepower produces heat. Engine's can only tolerate so much heat. Therefore the max continuous horsepower is the balance point where the engine produces as much heat as it can safely absorb. Since horsepower directly correlates to heat produced, anything below this point is generally safe for continuous operation.

Starting with the AeroVee...
Engine Data
Max Horsepower: 82
Max Continuous Horsepower: 80
Notes: It looks like the AeroVee already operates practically on top of it's HP peak. I didn't expect that, but the numbers are taken directly from the AeroVee website - see the horsepower curve at http://www.aeroconversions.com/products/aerovee/index.html.
Cruise Data:
Cruise at Sea Level: 130 MPH
Cruise at 8000 ft: 150 MPH

Soooo if 80 HP get's you 130 MPH, you're getting roughly 1.63 MPH per HP. At 8000 ft and 100% throttle you're doing 61.5 HP at 150 MPH, so you're getting 2.44 MPH per HP. A gain of roughly 66.63%.

Why doesn't your engine blow up if you hold it at 100% throttle for a an hour or two at 8000 ft? Because you're still only operating a6 61.5 HP and therefore only absorbing 61.5 HP worth of heat. The AeroVee is good to about 80 HP of heat. Well below what you're putting out.

Now for the Jabiru...
Engine Data
Max Horsepower: 120
Max Continuous Horsepower: 107
Notes: Data taken from http://www.jabirupacific.com/specs/3300.htm.
Cruise Data:
Cruise at Sea Level: 135 MPH
Cruise at 8000 ft: 170 MPH

If 107 HP get's you 135 MPH, you're getting roughly 1.26 MPH per HP. At 8000 ft and 100% throttle you're doing 90 HP at 170 MPH, so you're getting 1.89 MPH per HP. A gain of *victourious trumpets* roughly 66.79%. An almost identical performance gain to that of the AeroVee.

As before you're only putting out 90 HP and you're good to about 107 HP.


Conclusion: In terms of MPH per HP you're getting the exact same performance gain between the two engines, the Jabiru just has waaaay more full throttle horsepower to throw at the problem. The AeroVee is pretty much at full power already so maxing out the throttle doesn't get you much more benefit. In any event your air-anemic engine is putting out so few HP that you can cruise at full throttle fine.

[gammaxy]

You put a lot of research into this, but I believe there are still some faulty assumptions that aren't your fault.

This equation that relates air density and true airspeed to power required to overcome drag comes up all the time in aviation:
https://en.wikipedia.org/wiki/Drag_(physics)#Power

Once you understand this equation, you can develop simple rules-of-thumb like:
Going 15% faster requires roughly 150% of the original power.
Flying in air 75% as dense requires roughly 75% of the original power at the same true airspeed.
Combining the two, going 15% faster in air 75% as dense will require 150%*75% = 112% the power.

This is a rough approximation, but reality is worse since you need additional angle of attack (and the resulting drag) to maintain level flight in less dense air.

One way the math works is if the Aerovee's listed sea level cruise speed is actually given at about 65% power. Another way it works is if 150mph at 8000 feet is very generous (I believe Sonex agrees that it may be generous in their video about flight testing with digital instruments). In reality, it's a combination of both, but it's very unlikely true airspeed is greater at altitude if 100% power is used at sea level.

Check out the performance documented by some turbocharged Aerovee guys http://sonexbuilders.net/viewtopic.php?f=7&t=3184. If you interpolate a little, it seems like you'd expect them to fly 150mph at 8000 feet around 32-33" MAP. This corresponds to 7-10% more power than the normally aspirated engine has at sea level. This agrees nicely with my back-of-the-envelope estimate that 112% power is required to get 150mph at 8000 feet. The ~5% difference is likely explained by 130mph at sea level being a little low.