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View Full Version : Move over Super Cub for Super fox?



Esser
01-31-2012, 05:53 PM
Hey guys, I was just reading about UL Powers 6 cylinder engine they are going to be releasing before Oshkosh. In a nut shell depending on what compression and stroke you order it will be tops a 220lbs engine making 200HP.

Now the IO-233 is 210lbs. So whats 10 more lbs? I realize the engine mount would have to be much closer to the firewall to keep it balanced because of how far forward the extra cylinder bank would put you plus a heavier prop. But it's fun to dream.

Impractical especially since you would over speed in level flight I would imagine but practicality went out the window when we got airplanes in the first place. Would the Kitfox have enough rudder for p factor and torque?

3149

HighWing
02-02-2012, 03:26 PM
This looks very interesting. I have a friend who put the UL 350 in his homebuilt - supposedly one of the first in the US and had a world of issues with the engine. Not little stuff like rocker arms, but bearing issues and other heavy case splitting issues. His latest report that the knocking sounds while hand turning the prop after warm up are not significant and he will fly as is. I am sure that after the growing pains it will be a fine engine, but new, right out of the engineers cubicle - I would wait and see.
Lowell

szicree
02-02-2012, 04:39 PM
It's worth noting that these engines make their rated horsepower at something like 3300 rpm, thus requiring a smaller than normal prop. The truth is that to get 100 horsepower out of a lightweight, small displacement engine you gotta spin it fast. This is why Rotax, Rotec and others have gears.

I've seen lots of folks online comment on how "pretty" the UL engine is, but shiny CNC machine work means nothing to me. There are hundreds of engineering details that make an engine robust and bling just ain't one of em.

Esser
02-02-2012, 05:58 PM
It's worth noting that these engines make their rated horsepower at something like 3300 rpm, thus requiring a smaller than normal prop. The truth is that to get 100 horsepower out of a lightweight, small displacement engine you gotta spin it fast. This is why Rotax, Rotec and others have gears.

I've seen lots of folks online comment on how "pretty" the UL engine is, but shiny CNC machine work means nothing to me. There are hundreds of engineering details that make an engine robust and bling just ain't one of em.

First off the engine makes its peak torque at 2400 and makes 120hp at 2700rpm.

You can't really compare a rotax to this engine. Sure the UL350is and the rotax are both close to 165lbs but thats where the comparison really stops. The 100hp 912 is 82.5 cubic inches while the UL350 is 213.77. So over 2.5 times the displacement. A better comparison is the IO-233. Both are direct drive boxer engines that are fuel injected making 130HP. The IO-233 has 233.3 cubic inches and it is direct drive as well. You should care about shiny CNC work. That means that they spent time on all of the details. All those fancy cooling fins were designed to evenly cool the cylinders to avoid shock cooling. I don't mean to come across as aggressive or anything but so many people in the aviation community scoff on new ideas or technology. Why can't we just say, "Hey this is a new engine that looks promising, i hope it works out and they iron out the bugs for the sake of everybody who likes light engines." As for the hundreds of engineers details, before they invested millions I'm sure they thought of them. They do after all know that engines dont sell on bling.

rockwoodrv9a
02-02-2012, 07:28 PM
Why can't we just say, "Hey this is a new engine that looks promising, i hope it works out and they iron out the bugs for the sake of everybody who likes light engines."

Esser,
What a great quote. You are so right. So many times when something new is tried, some take it as a personal attack on what they chose. Im thankful for the first builders that Experimented with the Rotex. I have read many stories of troubles with the first engines they brought over here. I wonder if the same people posted the same thing about the Rotex.

For reference, this is nothing like over at the RV site and discussions about the RV 12. To build it as ELSA, you can't even tint the canopy and if you suggest doing just that, be ready for flames. I guess people forget the Experimental past of the planes we are building and flying. In the end, the experimental part is why we didn't just buy a Cessna or Maule.

Av8r3400
02-02-2012, 08:26 PM
Per the UL website (http://www.ulpower.com/), the peak torques are at 2400 rpm, but peak horsepower (what matters in an airplane) is at 3300 rpm.

UL260i - 97hp @ 3300, 82hp @ 2800
UL260iS - 107 hp @ 3300, not given for 2800
UL260iSA - 107 hp @ 3300 - Aerobatic, not given for 2800
UL350i - 118 hp @ 3300, not given for 2800
UL350iS - 130 hp @ 3300, not given for 2800

Personally, I'm excited for this company to do well with their engines. Competition is good. However, like the VW and Corvair auto conversions, they are meant to be higher rpm motors.

Ever hear a 185 with a seaplane prop hollering on a takeoff run? The tips have gone supersonic and what you hear is the continuous sonic boom coming from them. At 3300 rpm a 68" prop is supersonic (and losing efficiency), a 64" prop is transonic.

Personally, I hope to hear that these motors will run well at lower rpm ranges (at a lower hp level - like the 130 hp motor making 100 at 2500 or something). That will make them a much more interesting option for our application.

Esser
02-03-2012, 04:19 PM
I think your statement is a little general without looking at proper propeller selction. I took a lot of my info on prop size and efficiency from this web site http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm (http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm)
It has a lot of formulas and references for figuring out some interesting things. Let’s take a look at the UL350is First thing we need to do is look at the ideal prop size for this engine using the following formulas.
Two blade diameter d=22*forth root of HP (Forth root being the square root of the square root)
Three blade diameter d=18*forth root of HP
Ref: Aircraft Design a Conceptual Approach. By Daniel p. Raymer, 1989 Pg. 219
Now I know the UL is a higher RPM engine so I am going to choose the 3 blade prop so that I have a smaller radius. The engine is also 130hp so the formula will be:
d=18*forth root of 130
d=60.77 inches
So let’s say a 60” three blade prop. Next we need to figure out our tip speed in feet per second.
Vr=RPM*prop di*3.1416/(12*60)
For the UL350is with a 60 inch prop it would be:
Vr=3300*60*3.1416/(12*60)
Vr=863.94 fps
Now you need to figure out the velocity of the AC in feet per second(Vt).
Vt=KTAS*6076/3600
The Kitfox Vne is 140mph so let’s take a worst case scenario and say the fastest true airspeed we would get is 140KTAS. Obviously on takeoff this number would be much lower but I want to test the limits for my prop to know its good in all ranges
Vt=140*6076/3600
Vt=236.28 fps
We need to combine the tip speed with the forward speed of the aircraft to get the helical tip speed of the propeller. (Vht)
Vht=Sqrt(Vr.Sq+Vt.Sq)
Vht=Sqrt(863.94.Sq+236.28Sq)
Vht=895.66 fps
Our prop will lose efficiency at around 0.85Mach so we need to find out what the speed of sound is at a given altitude. This is based purely off of air temperature. So let’s find the speed of sound (Vs) for -20F.I don’t think many of us will fly that high or that cold but in rare times. We need to use “T” which is temperate in Fahrenheit + 460. So -20 + 460 = 420 T=440 for this equation.
Vs=Sqrt(1.4*32.17*53.34*T)
Vs=1028 fps
To make it simple for us let’s find the Mach number for our prop for easy comparison
Mach of our prop= Vht/Vs1
Mach = 895.66/1028
Mach=0.87
So in our worst case scenario, high up and full throttle at 3300 rpm we aren’t coming close to breaking the speed of sound. Granted I’ll give you that we are losing a bit of prop efficiency but this is at situation you would hardly find yourself in. So I still maintain it is easy to trash a new idea but please, for the sake of keeping pertinent and valuable information on this forum , let's keep an open mind. Also there are some famous planes that tips break the speed of sound. Notably the T-6. In World War II a lot of planes started with 2 and ended up with up to 5 blade props for all the reasons above.

Av8r3400
02-03-2012, 04:56 PM
Excellent research. You've proven my point for me.

A 60" diameter prop will be mostly blanked by the cowling and fuselage of a Kitfox, especially at the low speeds we travel and use in STOL operations. If you are looking for speed and x-country performance a small prop turning high rpm (Sonex with a VW for example) is very efficient.

Generalization: Small props make poor static (low speed) thrust, therefore they are a poor choice for STOL operations.

Esser
02-03-2012, 06:09 PM
I will agree that a slow small diameter prop could be less efficient but why not just add a bit more power to put it in it's efficient range? As for the fuselage blanketing the prop this is something I have long disagreed with since a prop is just an airfoil. It makes thrust by creating low pressure on the front in a tractor configuration. It doesn't move the plane by shooting air back. Maybe that contributes. Either way I haven't found any good reads on this yet. If I find anything supporting either side of the argument i will post it.

HighWing
02-04-2012, 10:31 AM
These discussions have and will go on forever. On the other discussion group there was a note on adding HP and the Model IV in question weighs in at 778 lbs. with the Jabiru 3300. I did a preliminary weight on my soon to fly project and it came in at 635 lbs. and am hoping for around 650 - and I am expecting that my 80 hp Rotax will outperform in most parameters, the Jab 3300 in that plane. Regarding a prop sucking an airplane through the air vs. pushing it. I don't know about that. Interesting things happen at the back (down) side of an airfoil. We lost a couple of souls four years ago when a Bonanza couldn't get out of ground effect on a hot day. Then there is the recommendation that rib lace spacing be tightened a bit in the prop wash area. I suspect there is more than a little effect on the back side of an airfoil. Then again the Jab 2200 has been a very marginal performer in front of the bubble cowl on the early Kitfoxes but flies fine with the tapered cowl of the Sonex. this is just logical to me as with the Kitfox, much of the prop is blanketed by the large round cowl.

Propellers? A whole nuther ballgame. If the math was perfect, then every propeller would work fine. There are some real dogs out there and I suspect each designer did the math. I contacted my friend with the UL 350 and asked specifically if he is still flying with the original prop. The short answer was No. The long answer follows. "I’m currently running a Catto 74X34 and it's great and the best of the fixed pitch I’ve been able to try. My goal was a large diameter fixed pitch prop for short field performance. I originally wanted to get a ground adjustable but none of the prop mfg of ground adjustable props would sell me one because they didn’t know the UL350 and the pulses it created. The other problem was that none of the available ground adjustable props had a diameter greater than 64” and if they did make one longer for a continental or Lycoming they still would not sell it to me. My original prop was a Culver74x34. It was a crappy prop with a lot of vibration and 50 lbs less of thrust compared to my current Catto. I began working with Catto and he first sent me a 72x36 prop which they use on the 0200. It was a great cruise prop but not the short field performance. He then sent me a prop he build for a Jabiru 3300, 74x32. It got me off the ground much faster but I lost 10mph on the top end. Then based on the performance #’s of the 74x32 he said a 74x34 would still give me the good short take off and give me back the 10mph at the top end…….and it did! Not only that it climbed out better than the 73x32, go figure that!!! but that was also what he said it would do. So I’ve settle in with the 74x34 (for now). I also tried a Sensenich 68x36 wood and it didn’t come close to the Catto props in thrust, climb out and vibration. I also have a friend that is working with Whirlwind and they are now looking at building a prop for the UL350. I’ve also been told by Robert Helms(US Distributor for ULPower) that the Turbine Cub guys are also interested in the new 200hp(220lbs) 6 cylinder for their cubs."

We all need to thank the pioneers, and I mean this sincerely. For you guys that feel the pioneering spirit, Go for it, but with your eyes wide open.
Lowell

HighWing
02-05-2012, 10:47 AM
One more thought on props - I know this should have been moved to "Propellers" but.... This morning I was digitizing some old film photos and ran onto this photo. The picture is from the late 80s and I think it was taken at the Reno air races. Notice where the fat part of the prop blades are. The round cowl is taken up mostly by the spinner, but there is relatively little prop in front of the cowl - most of the bite is outboard of that. This is one reason the Powerfin propellor was such a poor performer in the round cowl Kitfoxes - it had the fat part right in front of the cowl.
Lowell

szicree
02-05-2012, 11:30 AM
I will agree that a slow small diameter prop could be less efficient butAs for the fuselage blanketing the prop this is something I have long disagreed with since a prop is just an airfoil. It makes thrust by creating low pressure on the front in a tractor configuration. It doesn't move the plane by shooting air back. Maybe that contributes.

Fact is that if the fuse was big enough and flat enough, the plane would actually roll backwards as the prop wash is deflected forward, just like a thrust reverser.

Incidently, that rule of thumb for prop length must be based on some reasonable range of rpm. For example, suppose one engine makes 81 hp at 3000 rpm and another makes 81 hp at 100 rpm. According to the formula, both engines would require 66 inch props. However, the 100 rpm motor is cranking out upwards of 4000 ft-lbs of torque. To absorb all this power, the prop pitch would have to so coarse that the blades would probably snap off at throttle up. Obviously, there's more to this than one simple formula.

I'm certainly not trying to say the UL engine is no good, just that there are tradeoffs when going this route. And regarding the CNC work, I'm sorry but it most definitely does not tell me that the other details have been sorted out.

Dorsal
02-05-2012, 01:16 PM
I will agree that a slow small diameter prop could be less efficient but why not just add a bit more power to put it in it's efficient range? As for the fuselage blanketing the prop this is something I have long disagreed with since a prop is just an airfoil. It makes thrust by creating low pressure on the front in a tractor configuration. It doesn't move the plane by shooting air back. Maybe that contributes. Either way I haven't found any good reads on this yet. If I find anything supporting either side of the argument i will post it.

I think you will find that props make thrust by moving air back as wings make lift by moving air down.

Geowitz
02-05-2012, 03:28 PM
Since we seem to be going deeper here is an interesting perspective I found...

http://amasci.com/wing/airfoil.html

Esser
02-05-2012, 05:54 PM
HighWing I really liked your comments. Geowitz I have seen that website before and haven't had time to go into it in depth but I am interested. I do believe that air forcing backwards does play apart when the airplane is going slow. After all the propeller follows a helix so when the airplane is stationary or going slow, the propeller would direct air backwards. This picture shows that the propeller loses efficiency below the 40ish mile an hour area and I think that is because the tips of the propeller aren't moving forward following their pitch. Either way Geowtiz, that websites theories could prove me completely wrong.
http://www.epi-eng.com/images/Redrives/PropEff-01.gif

Of course this is just me thinking out loud. I am actually quite interested to keep researching this. But as HighWing said, there may not be an exact science. Either way I don't think it is a bad thing to try to learn as much as we can to help each other.

Also, szicree, I see what you are saying with the torque, although I am sure their are exceptions to this rule of prop size because I don't think a prop would do anything at 100 rpm. I realize you are just saying that rpm as an extreme example so I do want to research this more and find out if their is a lower limit to this rule of thumb. As for the CNC work I am just making a point that you can't discount an engine because it looks good and bling doesnt sell engines. Obviously it doesn't mean it's a good engine. But I'm sure if they want to not waste their money they will do their best to create a long lasting engine. Even if it isn't perfect at first.

As with anything with flying there is compromise in everything weather you have a good prop for STOL or good for cruising high rpm. Everyone knows that a big prop wont make you go faster but it will give you more thrust.

Anyhow it is all good information, keep it coming. I am heading to Costa Rica for a week so I wont be commenting for a bit but I'll try to do some reading while I am going.

HighWing
02-05-2012, 11:21 PM
Since we seem to be going deeper here is an interesting perspective I found...

http://amasci.com/wing/airfoil.html

I finally looked at the link and had to chuckle a bit. My grandson - a freshman in a Chandler, AZ high school wanted to do some sort of aviation project for their science fair. He decided to do a lift vs. angle of attack project - Loves aviation. He came here for a visit and what finally developed was a wind tunnel with three airfoil shapes and the means to measure lift in grams. The airfoils were of equal size and consisted of a flat surface, a symmetrical airfoil, and a typical asymmetrical airfoil. He took the data home so I can't give exact results, but the difference between the three was probably less than 10%. As could be expected, at zero angle of attack the asymmetrical airfoil did have some lift, but it was not great. I thought I had pictures of our set-up, but they must be in AZ as the science fair is next Sat. A note - he knew all about bernoulli.
Lowell

BajaCat
02-12-2012, 01:22 PM
I sent an email to John McBean about the UL350is and got a reply. John said they have a firewall, mount and cowlings, but no test plane to put them on. It doesn't sound like he's going to take it much further until that happens. Anybody out there ready to try the UL350is on their SS7? I'd volunteer mine but I won't be to that point for at least another six months.

Jim

Dorsal
02-19-2012, 06:29 AM
What I think I know about props

A well seasoned prop guy once told me break a prop into three parts, inner third, middle and outer third. The inner thirds function is to connect the middle third to the hub, the middle thirds function is to connect the outer third to the inner and the outer thirds function is to create thrust. This may seem an over simplification but mathematically it is fairly close to true. This also suggest that we rarely block out props with the plane.

A prop creates thrust by accelerating air backward faster than the plane is going forward, it can never follow it's helix perfectly and create thrust.

Airfoils are not necessary for creating lift/thrust but for doing so efficiently.

To compare engines scale the prop diameter linearly with max rpm (this will hold tip speed roughly constant)

To maximise static thrust turn the prop as slowly as possible and maximize length and yes 100 rpm will work well. Windmills and helicopters are good examples of this (reciprocity does apply whether you are taking energy from moving air or putting energy in to move air). The diameters do get a little unwieldy for airplanes though :)

The down side of a long prop is that while it creates great static thrust it must do so a a very low pitch and will therefore run out of thrust quickly as speed increases. (see point 2)

Disclaimer, the next point is fast math and only roughly correct.
Given thrust increases as the square of diameter and linearly with rpm and restricting this to the types of props we use on our planes I believe the following rough guidelines apply. Holding tip speed constant static thrust is maintained if engine horsepower increases proportional to the prop speed at which it is achieved. This is to say that 20% more HP at a 20% higher RPM will roughly produce the same static thrust. The higher HP engine would achieve this thrust with a shorter prop at a higher pitch and therefore create more thrust at cruise (I have no rule of thumb for impact on cruise speed).

The above is a short summary of what I have learned from selecting/designing props for hovercrafts and airplanes. It is far from complete but should provide a good place to start.

BTW Geowitz love the link.

Esser
02-19-2012, 04:21 PM
Dorsal, great post. I have been researching airfoils more and more and I have come to the conclusion that it's not a cut and dry subject and is controversial. Either way thanks for pointing to some different ideas of thinking. I have convinced myself that propellers do propel the air backwards but airfoil shape makes it more efficient. After all, boat props aren't airfoils and they propel and air is also a fluid. It goes much more in depth than that obviously. But I have looked at a lot of wind tunnel videos (Also controversial due to the tunnel having a top and bottom and an infinite wing) and I do believe that airfoils do cause air to "push up" the wing but at the same time the website that Geowitz posted the air comes down off the trailing edge. After all everything has to have an equal and opposite reaction. As you can see in this diagram. The air comes up to the bottom of the wing and then off the trailing edge continues lower than it started.
http://upload.wikimedia.org/wikipedia/commons/7/74/Werki1.jpg

Also HighWing, I think that plane you posted had the aerodynamic spinner to make the radial less draggy more than position the 'fat part of the prop past the cowl. An opposite extreme would be the geebee which was a very fast plane in its day with most of its prop blanketed.
http://www.airfields-freeman.com/MA/GeeBee_R-2replica_96.jpg


Anyhow interesting discussion.

Av8r3400
02-19-2012, 06:25 PM
I guess I'd disagree with the comment that "most" of the GeeBee's prop is blanked. Nearly half of the length of it's (96" diameter, 7240 sq. inch disc area) prop is outside the cowling (52" over the cylinders, 2123 sq. inch). This view shows what is blanked by the cowl is not blade, but mostly hub.

But, never the less, this is a high speed, racing application, where a long prop, with high static thrust is not advantageous.

http://www.airminded.net/geebee/r1neam_12oc.jpg

Esser
03-03-2012, 03:44 PM
UL Power released the new UL520i and UL520iS at 180hp and 200hp at an installed weight of 242 lbs. 20lbs more than they were hoping for. Expected though. I talked to Robert Helm, the engine is not on their website yet because the production team beat the marketing team. He can get one in 8 weeks. They are priced at 32,000 and 35,000 respectively. The first 10 orders get a 15% discount. Anyhow it will be interesting to track their progress.