Contemplating an SS-7 - Newbie builder

[AirFox]

Greg, Steve doesn't fly the UL anymore. Not sure where you got his endorsement from. If the UL was the best choice then there would be lots of them flying in this type of planes. Just saying.

[Danzer1]

Yes Scott, I should have used the past tense, as of last public report (I'm aware of) when he owned it, he was happy with it. Here: http://www.supercub.org/forum/showth...nder-in-Alaska Post #17.

Further - I did not say it was the best choice - it is MY choice.

There are lots more of them flying as time progresses - true of any engine or any product - you always have to start with one! They are garnering a very nice following.

I really don't get why the Rotax fanatics have to push the buttons of anyone that makes a different choice. I'm just trying to dispel misinformation that had been previously posted. Everyone's free to make whatever informed decision they like, that's the beauty of EXP!

Cheers

[av8rps]

In defense of the UL engines, I never heard that Steve Dentz was unhappy with his once he got it dialed in and worked through some initial issues. I think he just wanted to build a Carbon Cub so he sold his UL powered Highlander. I'm guessing if he were asked about the UL today he'd give a really straight answer as he sold the Highlander a while ago. Or you can read his earlier posts about the UL 350IS vs the 912uls here.http://wingsforum.com/viewtopic.php?...tart=15#p32632

I hope what I said about the UL engines didn't come off as bashing it. I was just sharing what I knew from what Steve shared. Personally, I would love to have a good option for the Rotax, as I agree with the issues related to its complexity. The UL is a much simpler engine, and I'm all for that. So more power to those that want to go the UL route. I still marvel that someone hasn't made a replacement for the 912 that is better for less $. The 912 design is now almost a quarter century old, so I really do keep looking and watching to see when a better engine shows up. That should be long over due. Unfortunately, thus far I haven't personally seen one that made me want to give up my Rotax. But I'm gonna keep watching as sooner or later it's gonna happen.

And hey, doing different things with our planes and especially engines is what this sport is all about. So I support anyone that wants to go the road less traveled. I'm sure one day us Rotax guys will be proven wrong, and frankly I'm looking forward to that day. Maybe we just need more UL's in Kitfoxes to convince us.

[Danzer1]

I hope what I said about the UL engines didn't come off as bashing it.

I didn't take it as a bash Paul. I was simply expanding on your statements with my own. And I do agree with the rest of your most recent post as well. The rest I wrote, was to set straight 2 much earlier posts in this thread by another member, that were way off base.

Regards, Greg

[av8rps]

That's cool Greg. Sometimes a lively conversation like this ends up being very educating to all of us, so we need these discussions.

And through all these recent posts I definitely learned something I wasn't aware of, that being the torque range of this engjne. I admit I'm a bit confused how max hp can be applied unless you go to 3300 rpm (according to their power charts). Even though the torque curve of that engine is nothing short of incredible (did i read correctly that it is nearly 3 times the torque of the 912?), it still appears that if you don't go into the 3300 rpm range you are not going to get max power.

I had to really think about that, but after reading Steve's comments about how the UL compares to the 912 at 2700 + rpm, it makes perfect sense. Unless he runs the engine faster he is not going to get max hp.. fortunately the UL engine can pull so hard you can get away with lower rpms lugging the engine without having it explode. So If you want to run a longer prop slower to get max thrust you will hav no choice but to limit the rpm. End result, doing rpm less than 3k puts you in a similar hp range as 912uls.

I'm thinking it is similar to a Porsche I own, it makes gobs of torque from just above idle to the 3500 to 4,000 rpm range. But to really make the car go fast you need to run it up into the upper 7k to low 8k range where max horsepower is avalable, even though max torque is at a much lesser rpm.

Is that a good comparison? Am I understanding that correctly?

[rosslr]

I have missed most of this discussion but it is interesting to see the 'slow' take up of the UL engines in the Kitfox frame. I looked closely at them an found them really impressive - the thing that tipped me in the 912is direction was the well developed FWF pack for the engine that is only in early development and trial for the UL engine - as far as I know. Has the factory fitted one yet? I know there was talk of them doing so but haven't heard anything yet. And there was someone on this forum a few years back who was fitting one but haven't heard anything of the progress or outcomes. I am not sufficiently nor skilled enough to experiment with fitting relatively new engines but I think UL engines, the MW fly engines and the Camit engines (an improved jabiru engine) offer some real alternatives to Rotax. Having said that, although the initial cost hurt, our 912is sport didnt miss a beat on our recent trip.

Looking forward to more on these other engines for those who have more knowledge.

cheers

ross

[alandbrooks]

Ross....I agree there...I am in the same boat as yiu are...where I don't want to be a trail blazer...I like the the UL because of it simplicity...I know at one point Kitfox did have a FWF option for the UL not sure what happened as it is no longer listed as a option on the website...I will speak to Jim as OSH...and see what he says. In doing research on props for the UL is a challenge as well..as I am in the early stage I am sure in a year there might be better options. That being said I am more than happy to experiment if I am able to get some assistance here.

To all who have commented your support and views are appreciated and I am sure we will have much communication in the future

Alan

[Danzer1]

That's cool Greg. Sometimes a lively conversation like this ends up being very educating to all of us, so we need these discussions.

And through all these recent posts I definitely learned something I wasn't aware of, that being the torque range of this engjne. I admit I'm a bit confused how max hp can be applied unless you go to 3300 rpm (according to their power charts). Even though the torque curve of that engine is nothing short of incredible (did i read correctly that it is nearly 3 times the torque of the 912?), it still appears that if you don't go into the 3300 rpm range you are not going to get max power.

I had to really think about that, but after reading Steve's comments about how the UL compares to the 912 at 2700 + rpm, it makes perfect sense. Unless he runs the engine faster he is not going to get max hp.. fortunately the UL engine can pull so hard you can get away with lower rpms lugging the engine without having it explode. So If you want to run a longer prop slower to get max thrust you will hav no choice but to limit the rpm. End result, doing rpm less than 3k puts you in a similar hp range as 912uls.

I'm thinking it is similar to a Porsche I own, it makes gobs of torque from just above idle to the 3500 to 4,000 rpm range. But to really make the car go fast you need to run it up into the upper 7k to low 8k range where max horsepower is avalable, even though max torque is at a much lesser rpm.

Is that a good comparison? Am I understanding that correctly?

Not quite. A dyno measures torque - the twisting energy developed by the engine. On any chart, HP is a simple mathematical equation (conversion) to arrive at the HP number. Nothing measures HP. HP = torque X rpm divided by 5252. IE: 310nm x 2700 rpm/5252 = 159 HP - HP is just math - torque is the energy produced.

In the Porsche and the Rotax you use gearing to multiply the torque up or down to achieve max torque and so many use the HP number to approximate the torque available based on the "transmission" ratio. SO HP is somewhat relevant with a transmission (PSRU).

Conversely, in a direct drive fixed pitch prop combination - HP doesn't matter at all - only peak torque (twisting energy) because you can't "gear it" up or down. If the max torque occurs at 2800 rpm for example - there is no good reason to run it at a higher rpm. And with a wide flat torque curve - you have more "power" available at a wider range of rpm - great for cruise!

There is no free lunch when it comes to power. Hence the reason for "highly modifying" the Rotax - that gains more torque and therefore more HP. It's also the reason you have to run the Rotax at a fairly high rpm all the time - it is a very peaky torque curve that occurs at the higher end of the band. You would have a hard time modifying a FADEC controlled UL engine to gain more torque.

Does that make sense? Greg

[av8rps]

Ok, now I'm really confused. So if we use the Rotax 912uls as an example, it makes 128nm of torque at 5100 rpm, so 128x5100=652,800 which is then divided by 5252 to arrive at 124 hp. But we know thats not right. So when I use 94 ft lbs for the torque number instead of newton meters, I get approx 92 hp. So do we need to use ft pounds instead of nm?

And still using the Rotax as my example, I know that the 912 makes the highest torque at 5100, but in my Kitfox amphib with an IVO IFA, if I were to adjust the prop to full pitch giving me only 5100 rpm at max throttle setting, it would take me forever to get off the water. In fact my original blades I had would only let me get a bit over 5 grand for takeoff, but by switching to a lower pitched blade design I was able to get to 5800 rpm on takeoff, and that cut my takeoff time in half (13 seconds vs 30 seconds with me and a 210 lb passenger). So what does that? Is it the inertia developed at 5800 that makes the difference?

Oh, and climb is very similar. At 5100 max climb rate is minimal (approx 400 fpm depending on the weight), but at 5800 my climb is over 1,000 fpm at average weights. So I'm still not understanding the relationship of max torque rpm to max horsepower? (I know...I probably need to read more about engine theory ).

Even more confusing is when I check my Porsche, as the 300+ hp engine in it makes 310 nm of torque at only 4600 rpm (which shows 199 hp at that rpm), but the UL350is shows 320nm at rpms as low as 1700. So there has to be something wrong with the advertised torque specs of the UL engine, as there is no way it can make the same power (torque) as a 300+ horsepower Porsche engine that makes the car go 170 mph. Or maybe Porsche should be replacing their engines with UL's

What do I not understand?

[Danzer1]

So do we need to use ft pounds instead of nm?

Yes that is for pound feet of torque not newton meters.

And still using the Rotax as my example, I know that the 912 makes the highest torque at 5100, but in my Kitfox amphib with an IVO IFA, if I were to adjust the prop to full pitch giving me only 5100 rpm at max throttle setting, it would take me forever to get off the water. In fact my original blades I had would only let me get a bit over 5 grand for takeoff, but by switching to a lower pitched blade design I was able to get to 5800 rpm on takeoff, and that cut my takeoff time in half (13 seconds vs 30 seconds with me and a 210 lb passenger). So what does that? Is it the inertia developed at 5800 that makes the difference?

Oh, and climb is very similar. At 5100 max climb rate is minimal (approx 400 fpm depending on the weight), but at 5800 my climb is over 1,000 fpm at average weights. So I'm still not understanding the relationship of max torque rpm to max horsepower? (I know...I probably need to read more about engine theory ).

Because your prop is set up for (length primarily) a 5800 rpm engine which equals 2386 rpm at the prop which is about 65% of mach on a 70" prop - inefficient to start with - but very safe for the prop maker. So now run that same engine/prop at 5100 rpm which equals 2100 rpm at the prop and now a 70" prop is only 57% of mach - and even less efficient. So at 2100 rpm you could theoretically use a 100" prop! But you couldn't at 2400 rpm.

Prop manufacturers will not design for less than max engine rpm - even if it's only a 5 minute rating and all err very far on the safe side for lengths. Constant speed props help get over that hump, but longer, more efficient props would certainly help things - but try and find a prop maker to do it!

Prop length, prop design and prop pitch all affect how much bite the prop can take at a given rpm - so to summarize - it's all in your prop! Torque is torque though! Like I said - there is no free lunch!

Even more confusing is when I check my Porsche, as the 300+ hp engine in it makes 310 nm of torque at only 4600 rpm (which shows 199 hp at that rpm), but the UL350is shows 320nm at rpms as low as 1700. So there has to be something wrong with the advertised torque specs of the UL engine, as there is no way it can make the same power (torque) as a 300+ horsepower Porsche engine that makes the car go 170 mph. Or maybe Porsche should be replacing their engines with UL's

Because the Porsche has many more gears than your 2.43 Rotax. It uses multiple gear ratios to optimize torque in each gear to get you to 170 mph. You aren't doing the "pulling" at engine rpm to the wheels, you are multiplying torque to get to 170. Is 310 nm of torque at only 4600 rpm it's max torque? - if not you also have more power (torque) available at a higher rpm. Nothing wrong with the ratings, why would Porsche want an engine that pulls max at 1700 rpm? They want more power higher up to get the speeds you want. It's all about the mission.

Greg