you could have good working carburettor on each cylinder with very efficient plenum and induction system, but you will only address full power, you will never have the possibility to tune the A/F ratio.
So no, it cannot be better than an injection system on the same level of refinement, and god know that you need a carburetor wizard to make a 4 Weber-engine working.
Of course you can tune multi carb systems to run properly at all power levels. On a motorcycle this is something done all the time with pilot, needle and main jet changes to make the engine A/F ratio correct across the full speed and throttle opening range, as well as making sure the carbs each have the same air flow. The end result often works better than fuel injection on a motorcycle engine, particularly in terms of smooth throttle response. As I mentioned the problems with EFI (like abrupt throttle response in a vehicle with high power/weight ratio) have been significantly mitigated over the last 20 years but the resulting fuel efficiency is generally no better than when using modern (meaning 1990s) Japanese carbs.
However I was not suggesting that multiple carbs are practical for an aircraft. As noted, I used the comparison between EFI and multi carb motorcycle engines to make the point that it’s metering to individual cylinders that provides almost all the performance advantages for fuel injection over a single carb on an aircraft engine. It is not injection itself, except that this provides good start-up behavior and with no float bowls minimizes the issues with alcohol and water.
My additional point was that for me and I think for a lot of others, a single carb is the best solution for an aircraft engine given the simplicity of ownership and operation, limited number of hours flown and limited additional fuel cost for a small 150 HP engine. For others, fuel injection is better but that is far from universal, with either mechanical injection or EFI.
Anyway, there is no problem finding props with higher than 2800 rpm, except perhaps for constant speed. This is a bit chicken and egg kind of problem due to existing “as is” rather than engineering/physics.
Like all design, propeller is the process of finding the optimum based on multiple constraints. In this case one constraint is tip speed (as you pointed out), others are blade efficiency resulting from length and aspect ratio, blade stiffness in multiple directions, blade static stress, fatigue stress for materials where that applies and so on. At high rpm with low strength materials like wood, the design is limited to a diameter and blade shape that is not as efficient as it might be at lower rpm and higher strength materials. Wood does have the advantage of infinite fatigue life, but that goes along with low static strength.
The Onex airframe layout appears to be designed around a 3400 rpm wood prop, which is not a very efficient prop, so to the extent a small diameter prop offers other advantages in the aircraft configuration it would be a good idea to take advantage of them.
Interesting post @Silvaire thank you.
I was not aware of the fact that some people actually prefer carburetors! Or that motorcycle carbs were so refined.
As far as I’m concerned fuel injection is a must have ever since I had a choice , due to the safety of flight risk I see in carb heat operations.
While it seems that carb icing never was a real killer in (my) real-world, I did see enough near misses due to carb heat remaining on.
I have been a vocal opponent of carburetor technology ever since my instructor explained to me the insane business of having to pull on this (and not on that) in short final and then push it back again before going around. I look forward to a heated exchange after letting this out !
Another thing that struck me in this very lively thread (thank you guys for the contributions) is how vastly different our view of the use case “flying” is. Most people (?)e think in terms of relatively low altitude and/or optimum cost of ownership over years.
It must be my affair with Galatea that ruined my sense of value, but I tend to look for the best possible technical solution I can afford (you may keep that TBM, thanks)
And also, my flying sweet spot is 9000 ft above sea level (which would be ground hugging in Switzerland) ;) in an unpressurized plane and more like FL200 in a pressurized one.
In this context, I see major benefits in turbocharging and this is why I thought the Rotax 916 was a major step forward.
Meanwhile I really like the UL520T, which is news to me.
I think both engines deserve that a plane be designed around them. Doing so would maximize the benefit of their light weight and very high critical altitude.
A lightweight Lancair ES lookalike with a 520T ? Sweeeeet
While it seems that carb icing never was a real killer in (my) real-world, I did see enough near misses due to carb heat remaining on.
I get it… 
I once flew behind little Continentals and they are renowned ‘ice makers’
However, be aware that in (my) real world I’ve been flying my carburated O-320 for 13 years and had carb ice just once, on the ground, and never in flight. I don’t use carb heat on final approach, the recommendation for my type is “use as required”
My cruise altitudes are 7500 and 8500 ft and as long as I have good initial takeoff performance and don’t go much below 500 fpm in reaching those altitudes I’m happy. My service ceiling is high enough that I’ve never been there, 14,000 ft or more. Maybe I should try it just to find out but for my normal flying a turbo would just be another thing to maintain.
Silvaire wrote:
The Onex airframe layout appears to be designed around a 3400 rpm wood prop, which is not a very efficient prop, so to the extent a small diameter prop offers other advantages in the aircraft configuration it would be a good idea to take advantage of them.
The Onex is designed for engines of max 130 or 140 HP and max 200 lbs weight. No other considerations exists as far as I know. Sonex offers Aerovee (VW), Rotax and ULPower. The reason I need a small diameter prop is ground clearance. If it was a STOL aircraft it would have longer legs
On the Piper Pawnee (O-540) we used to tow gliders before the WT9, we had the two blade large diameter metal prop removed and replaced it with a small diameter 4 blade wooden Hoffman prop. This was done due to noise. I sometimes also flew another Pawnee that had the original metal prop. Our Pawnee had better climb and better acceleration (yet same cruise speed), so go figure
Props, especially fixed pitch props, are surprisingly and rather insanely difficult, and few people and manufacturers actually know props. Hoffman obviously does, and so do Duc and Prince. Sensenich not so much. Two well known benefits of wooden props are vibrational damping and light weight (low inertia).
The founder of Whirlwind knows how to design a propeller from first principles, does it very well and his hollow CF blades are cleverly manufactured.
Sensenich hasn’t done much new in a long time.
A successful airplane design is a careful balance of engine, prop and airframe to serve a precise mission.
One textbook example is the Risen family.
Despite the choice of a regular Rotax engine, the variable pitch prop was custom designed to match the expected cruise speed.
And when Mr Porta did the Superveloce version, he not only replaced the engine, but redesigned the airframe with shorter wings, smaller tail…
The logic is that for a given climb performance goal, higher power allows shorter wings, which in turn allow less structure and higher cruise speed.
This is also how a lightweight engine such as the 916 can be made to fully shine.
Darkaero are another great example, a spectacularly light weight and high (predicted) performance aircraft built around a certain engine and custom prop.
Defunct DynAero in France also did a textbook job. The venerable MCR 01 still sets the standard in its category.
At the opposite end – and that infuriated me back in my DA40 life – is the Diamond approach: business driven, with ridiculous lack of funding for new iterations.
The mere fact that they designed the DA 20 and DA40 shows the talent of Diamond’s engineers, and it is so frustrating to imagine these poor lads being told to stay away and watch marketing thoroughly demolish their work over the years.
The DA40 family started life with analog avionics, a nice W&B envelope and useful load despite a mostly fiberglass airframe that was built to endure flight school usage,
It had the potential to dominate the market for new 4 seater aircraft. The DA40 should have at the very least sent the PA-28 and C-172 into retirement but the opposite happened…
More of this, showing that the new ownership is not really helping. The DA50:
Its wing is actually the outboard portion of the DA62’s, minus engine nacelle. Clever ? Maybe…
The resulting step in the taper looks awful! It us nicely highlighted by the interruption if the TKS leading edge strip.
One can only wonder how much this kind of compromise contributes to the underwhelming speed and range of this strange aircraft…
Flyingfish wrote:
A successful airplane design is a careful balance of engine, prop and airframe to serve a precise mission.
Perhaps, but look at this video. A Onex with a UL350 taking off, accelerating at low alt:
30 s : 100 mph (87 knots, 161 km/h)
48 s : 150 mph (130 knots, 241 km/h)
1m 47s : 200 mph (174 knots, 322 km/h) Vne is 216 mph.
And it’s all done with an “inefficient” small diameter prop by Prince
It was this guy who convinced me about Prince prop, because he had tried “them all”. The mission for Sonex is, and has always been, fast, fun, maneuverable machines (fully aerobatic) that anyone can afford and build in their garage. It flies nice with a 70-80 HP VW, but won’t get these speeds of course. The next step is the Subsonex jet
Also note the gradual apply of throttle. This is from his mail some time ago:
The power to weight ratio of the 350is powered ONEX is about 5.5 pounds per horsepower. This coupled with the highly pitched Prince propeller combined to produce a large amount of P-factor which requires a large right rudder input. You do not just push the throttle to the firewall and go. If you did that, the aircraft would start to turn to the left and you would run out of right rudder and the aircraft would exit the runway to the left, not good! The proper procedure is to slowly apply throttle with neutral elevator, the tail with start to lift on it’s own at about 35 miles per hour. Maintain the tail high until 60 mph then back stick will rotate for the lift off. At lift-off you can go to full throttle [optional], a lot of right rudder is still required for a centered skid ball. You will now be arriving at 100 mph in climb, resulting in a climb rate of 1,600 feet per minute, which can be held at this rate till you reach 5,000 feet!
Anyway, back on topic. IMO the 916 is probably perfect for extreme STOL aircraft, similar to the HP monsters from Edge Performance (which could be the only reason they made the 916, to offer an alternative to Edge). This is circumstances where you “need” lots of power for short durations, and the ability to turn a big prop.
I don’t think the 916 was made as a reaction to a competitor. really, I believe they are just going up the power ladder on their own initiative.
The Onex video made me scratch my head: what should one learn from this ? I found acceleration weak for such a powerful engine and top speed ok but not more.
A Risen with the basic 100 HP Rotax 912 is will accelerate and cruise faster , while carrying twice the payload in a side by side configuration.
I really like Risen’s approach, because when they installed the 915 engine, they actually redesigned the airframe around it. The result is a masterpiece in aerospace engineering.
In my opinion, this aircraft sets the achievable optimum in its category, against which other aircraft designers should measure their own success. 197 knots cruise at FL090 on 26 liters per hour, and less than 10 minutes to get there? wow…
Flyingfish wrote:
what should one learn from this ?
Well, Listen carefully young man
What you should learn is you can get the same performance with a fixed pitch prop, no retractable, no multimillion CFD analysis, no multimillion carbon fiber design, and at a fraction of the unit cost, and everyone can built it themselves. And it’s not even that difficult.
The Risen is a motorglider with a big’ish engine. Mount a couple of model airplane turbojets on a glider, and you will have better performance (it it has been done). The Blackwing is cooler, better looking, and faster, and more advanced. The Tarragon is the fastest UL (unofficially at high alt with an Edge Performance 917 or whatever it was).
For some real cool and advanced stuff, there is DarkAero with their DarkAero 1. You can follow them on Youtube, and see how it ends up.
The 916 is most certainly made first and foremost for STOL. The Carbon Cub is the first aircraft that will use it.
Well, there’s another minute detail about the Risen.
Risen‘s performance? Good, but certainly not revolutionary. Let’s call it in line with what I’d expect from technological advances nowadays, no more no less.
PS
Ah, the detail? The price.
