Contemplating an SS-7 - Newbie builder

[alandbrooks]

I am about to pull the trigger on a SS7...more likely when I am at Oshkosh. I hear there are some new goodies coming from Kitfox (read that somewhere here)

I have read the manual cover to cover...really like the way it has been laid out and it does make sense.

I have a few questions.
1. Looking for any SS7 builders in the Toronto, ON area.
2. Electrical is my weakest skill, somehow I find it overwhelming at best..yes I have read the Aeroelectric book - sort of makes sense.


I am planning on using Vertical Power VPX-Pro purely for the reason of going to 24V - any suggestions if this would be overkill

3. Covering - here I will be going with Stewart Systems


Can you mix the systems...ie use Stewart Systems and Polyfibre paint for example.

4. Engine will be either UL Power or Rotax 915 - going to put floats on -any recommendations?

Looking forward to the hearing from the group on whether I have lost my mind or not.

[DesertFox4]

Congrats Alan on your decision to build a Kitfox 7SS.:)

It will make a wonderful float plane. The 915 should be a great choice for that mission. I'm sure as with new releases of Rotax engines in the past, Kitfox Aircraft will be on the forefront of fitting it to this air frame and designing a comprehensive firewall forward kit for it.

Vertical Power's simplest offering was considered for my build but I ended up unable to justify the price for the less than complicated electrical system I plan to use. Not sure what benefits of running a 24 volt system brings. You may have studied that way more than I.

Paul from Wisc. has discussed in great detail float options and you may want to do a search for float threads and posts. He is our resident float guru and we much appreciate his experience and generosity on our forums. I would love to try a 7SS on floats. Alas, not much water in the desert S.W. for sea planes to play on.
I went with Poly Fiber so no help on the Stewart System. It is gaining in popularity here.

Have you decided on options?
I went with the Laker Leading Edge and the quick build wing options.

Best of luck on your build. Enjoy AirVenture 2016 as I'm sure this one will stand out over all the others for you.
Congrats again Alan.

[jiott]

Electrical was also my weakest skill, but it sounds like you are a reader and you can learn all you need for these simple electrical systems by reading and gathering info from knowledgeable sources. Yes, I think 24v would be overkill; why complicate a basic proven system. Personally I have heard more bad than good about the UL Power engine. The Rotax 912ULS powers many Kitfox float planes, and the turbo 914 or new 915 would surely be a real performer. Definitely don't mix systems in the covering process; however when it comes to just the final color finish coats I am not sure so will let others answer on this.

[Danzer1]

The only advantage I see to 24v is reduced wire sizes and that would be hardly worth it for the amount of wiring in these aircraft. The upside to 12v is availability of tons of EXP instruments & avionics that you probably can't find near as many wide choices in 24v. The other advantage to 12v is many things (if in a bind) can be found off the shelf in an auto parts store.


Personally I have heard more bad than good about the UL Power engine.

Jim, if you have any references, I'd love to hear/see/read them. I've been following the UL 350 series now for years and after you understand their paper ratings (it's the ONLY complaint I've heard and had myself) seems everyone one I've talked to and read about, has been more than satisfied with them. There were some early issues with alternator fans and a recent one for fuel line replacements - no known incidents resulted and all addressed by UL. I've not heard of one complaint about performance/service/reliability. There are more than a few running now particularly on Zeniths and Just Aircraft. Please advise.

Greg

[Av8r_Sed]

Don't sweat the electrical stuff. There are enough of us here to guide you through it. I woulldn't go with the Vertical Power system unless you really needed a dual bus redundant system. With LED lighting and modern avionics you should be able to go with a 12 volt sytem without compromise. These are still basically VFR machines so simplicity tends to rule.

If you're a first time builder, I would tend to favor the majority proven powerplant. Each Rotax engine that's rolled out has had it's own issues to resolve. You don't want to be a pioneer unless you favor tinkering over flying.

[inzersv]

Alan, I am using the VP-X Sport with the 12 volt setup. So far I love it. Electrical scared the heck out of me, but my hangar neighbor (building an RV-14, full IFR) is an electrical guru and helped me get on the right track. He is using the VP-X Pro with 24 volt setup. I agree with the others, use the 12 volt setup for the SS7. I used the Stewart System and loved it except for the painting. I had never painted before, so it was probably just me. The Stewart brothers are great to work with. Back on the VP-X, go to their website and you can print out all of the installation help that you will need. I installed the Dynon SkyView and it works well with the VP-X Sport. Chad Jensen with Vertical Power can answer any questions you may have. His phone is 1(920)216-3699. Good luck with your build.

[jiott]

Danzer, I don't mean to slam the UL Power engine, its just that in my small world I have HEARD more bad than good about it. The bad things had to do with actual power being 10+% lower than advertised, various technical issues which the company is addressing, lack of props available for their somewhat higher than normal direct drive speeds, and more than usual single point failure possibilities. All these issues are argued both ways by various people on several forums and I am not prepared to argue one way or another. IMHO I would be leery because where there is smoke there is quite often fire. Just my opinion/recommendation which is what alandbrooks asked for.

[alandbrooks]

Thank you so much for the word's an encouragement...it is great to know there are members out there willing to assist and express opinions.

I am sure my decision is the right one....

Alan

[Danzer1]

The bad things had to do with actual power being 10+% lower than advertised

To be objective - the only independent dyno test I have ever heard of on a 350is, is this one by Scott Ehni: http://www.zenith.aero/video/ul350is-dyno-test

Anything other than a dyno test is totally speculation as there are way to many other variables to consider to even be worthwhile.

Cheers

[Esser]

Everyone heard bad things about rotaxes when they first came out too...now everyone loves them. Unless you have personal experience with something, I don't think it helps to give hearsay advice.

[jmodguy]

Alan,
Definitely go fot the KF! The 24v electrical system is really not needs on this aircraft unless you just want to spend a lot of $$$..
Batteries cost more, anything electric will cost more. Does Rotax even make an engine with a 24v system? I don't know that's why I ask.
Most EFIS systems today will run on 12 or 28v but the wire size won't change.
VPX is overkill.
Wiring is not complicated but can be intimidating. The one thing you will need is the proper tooling, I.e. wire strippers and crimpers. Do NOT use hardware store strippers and crimpers! These are OK for house or car wiring but they do damage the wire.
Regards
Jeff

[mr bill]

12 volts won't shock you, but you sure will feel 24 volts.

[Danzer1]

Alan,

A few thoughts on the VP-X:

I agree that it could be considered overkill if you are building a simple mostly analogue panel. I to am seriously considering a VP-X for the following reasons:

I am going to be doing full glass dual Dynon panels, each with their own battery backup and lots of other goodies (not in a Kitfox), it integrates very well with Dynon.

I hate fuses! They require some kind of annunciation to determine what is amiss. Pulling fuses to see if burned through while flying an aircraft that has whatever blew, disabling something, is not in my plan.

So the next logical step is breakers. Can see if tripped, but with needing at least 12 to 15 for my config, that takes up more panel space than I'm willing to give up.

So, electronic breakers make perfect sense to me.

Many have reported that the VP-X has been exceedingly fast and simple to wire, setup and diagnose. There have been some niggles mostly with flap/trim wiring & configuration - I believe that has been worked out.

There are many on the Vans Airforce VAF forums that have/are using it, so if you haven't read over there, it may be well worth your while.

Everyone has a different plan, panel idea, mission, so there really is no right or wrong. As a retired engineer though - data (facts), 1st hand knowledge, education, controlled testing, evaluation, real world field results - trumps hearsay every time.

Go with what works for you, your mission, goals and budget.

Greg

[av8rps]

You will really be happy with a Kitfox. And the Kitfox makes an awesome float plane. The biggest thing is to keep it light if you want an amphib as you'll probably be adding 200-250 lbs of weight to the aircraft for the floats.

If I were building a new Kitfox and intended it to be an amphib I would look for a lightweight covering process that still looks good and is easy to work with. (I like Stewarts and Air-tech, polyfiber is too labor intensive and heavy, unless you use only their polytone product).

I would instrument it with modern electronics over steam gauges for weight savings, as well as for all the additional benefits of more information to the pilot (like synthetic vision).

For an engine? I'd go with Rotax as it is proven again and again the best way to end up with a light empty weight Kitfox. And while the UL engines make decent horsepower, they need to turn a shorter prop to use that power as their rpm's are too high for a long prop to work efficiently (once prop tips get close to going supersonic efficiency plummets...). And since seaplanes always use long props turning slower to make them perform better, the gear reduction on the Rotax is premium. A 914 or the new 915 would be awesome, but a 912uls with a big bore Zipper kit might be nearly as good for a lot less money and complexity.

I too would see no need for a 24 volt system. I also would avoid the extra alternator so many*add to their 912 as it is just more added weight and complexity you can avoid by utilizing led lighting and low draw mfd's in the panel.

With all that said, I will admit I don't always follow my own advice. My little 80 hp 912 Kitfox 4-1200 on amphibs is actually a bit "piggy" (heavy) having really heavy paint, steam guages, carpeting, plexi glass for everything. But yet it performs great. I attribute that mostly to the efficiency of that lightweight 912 making the most prop thrust per pound and per horsepower, all while keeping the empty weight on amphibs under 800 lbs. Obviously a Kitfox Super Sport on amphib floats will weigh more than mine, but i believe if you power it with a 912 series Rotax with 100+ hp you will have great performance too.

Paul Seehafer

[Danzer1]

A would agree with Paul and his float plane expertise on everything written except:
the new 915 would be awesome,

A 914 weights 166 lbs plus fluids fully operational.It's rating of 120 hp @ 5800 is only good for 5 minutes. Great for takeoff, but does nothing for cruise. It's rating @ 5500 is 100 hp continuous - might as well be a 912! However it's max torque is at 4900 (torque is the true measure of energy to the prop) and that is 128nm (continuous) or 311nm torque at the prop (minus psru eff. loss) at 91hp. So it's "burst power" capability may have some usefulness for takeoff, but otherwise it's just a more expensive, complicated 912.

For instance, the UL 350is is 173lbs wet (including fluids) and produces over 310nm torque in a band from 1700 rpm all the way up to over 2800 rpm. It beats the 914 in every respect in a real world operating rpm band (not the paper rated 3300/130 or the Rotax "burst power") including cost.

The jury is out though on the 915 as Rotax has not produced any dyno results yet. I can tell you though, they have published the dry weight with PSRU at 185 lbs, if adding everything else to that like all their other engines, that would be another 25lbs or so for 210lbs plus fluids. They also have published that 135 hp to be the max at 5800 rpm good only for a whooping 1 MINUTE! Projected price to be near $35k. Not sure the final price and continuous operating specs will justify the 1 minute "burst" gain to me. I will wait to see the dyno torque curve to further assess.

Bear in mind, I'm not doing floats either.

Greg

[foxbait]

Hi Allan, I'm in Niagara Falls, started my SS7 build last April, been a slow process so far but you are welcome to come and check things out.
Mike

[av8rps]

I also agree with most of what Greg just wrote.

No one at this time really knows what the 915 will really do. So that really is an unknown. But I'm guessing Rotax did their homework before dumping millions into a new engine.
However, if that 915 is too much of an unknown, a zipper kitted 912s is readily available.

And while I really like the concept of the UL brand engines, so I agree with Greg on horsepower and torque ratings vs the 912-914-915, the Rotax PSRU makes for a much more efficient prop on a STOL or seaplane aircraft. It takes whatever torque you have and turns it into a much higher thrust number than an engine without a PSRU would have.

My best example and defense would be to talk about a 130 hp UL powered Highlalnder that the builder (Steve Dentz -who in my opinion is a master builder) struggled a lot with his to get it to the point it was working well. Ultimately I believe Steve got it really dialed in and it gave him a really good performing Highlander. But yet Steve Henry (The dead stick takeoff guy and friend of Dentz) uses highly modified (up to 145 hp) Rotax 912 and 914 engines on all of his new airplanes that he also competes with in Valdez and Oshkosh. So I believe if he knew the UL was better than the Rotax, I'm pretty sure he would have tried one on at least one of his many builds. And I know he is open minded and always looking for something better as he just tried a 4 stroke 140 hp snowmobile engine in his latest Highlander. But he recently took that out and replaced it with a 912, stating there is a lot more to do to make it a viable alternative to the tried and true Rotax.

With all that said, I would'nt want to discourage anyone from using the UL engine as I think it will work well if you just follow what Steve Dentz did with his. And I also think the Ul will work as good or maybe better than the Rotax if it is used on a faster airframe that requires use of a short prop. But for a STOL type aircraft, I still think the Rotax will perform better overall.

Now if someone would just come up with a PSRU for the UL (and the Jabiru) then Rotax would have some serious competition...

[alandbrooks]

Team

Thank you for all comments yes my eyes have been opened even wider..its great to see the difference in opinions

For the engine decision will be a little later - have to decide when I rig the wings...the 1 degree forward rule...etc and by that time (1yr from now) a lot can change and I am sure most issues for both UL and Rotax will be sorted out...:)

For the electrical I am still pondering. What I plan to do as far as the panel is concerned is go with a single Dynon EFIS, radios, Com etc basic day/night VFR and pre wire everything before hand to accommodate a second EFIS auto-pilot servo's etc. Thats why I like the Vertical Power...simpler - a few bucks more but less pain IMHO.

I am also planning to build as light as possible but not going overboard - as I have access to some real good commercial airplane composite technology and tools...I have some ideas.:)

I am looking forward to the build process..which I am sure will be a lot of fun.

[Esser]

You don't have to rig the wings forward for heavier engines for the most part. There are some good threads about this if you search. I did a bunch of theoretical C of G calculations and it would be very hard to get enough weight forward to put you outside c of g

[Danzer1]

Alan,


I am sure most issues for both UL and Rotax will be sorted out

For the record:

By all user accounts - the 350is performs as advertised. Those that think it performs poorly either can't read a performance chart/manual and/or have a poorly matched prop. I've heard of neither complaint from ACTUAL USERS.

Steve Dentz built his in 2010 and was an early adopter (kudo's). He has been using a Catto 74" fixed pitch since 2013 and is happy with the current performance. I still don't think it's the best match for the engine.

Sebastian Heintz - owner of Zentith has been using one in his CH 650 since 2011 with a 65" Whirlwind ground adjustable propeller and he's been so pleased, UL engines are their engine of choice for Zeniths. I still don't think that's the best prop match for the 350is either.

Since then there are many more propellers available. Some you won't even find yet on the prop manufacturers sites. Airmaster is working with Whirlwind on a cs prop for the 350is. Whirlwind is also working on a STOL prop too. The recommended normal operating range for the engine (in the UL operating manual) is 2200 to 2800 rpm - well within the parameters of most props. The notion that the operating rpms are somewhat higher than normal is hogwash.

I've been in contact with the manufacturer and they are not aware of any current technical issues to be worked out - zero - none - nada! They are however working on improvements mostly in optimization of the FADEC engine management (I'm still hopefull for a "lean of peak" mode).

There are less single point failure possibilities in the 350is than any Rotax. It has dual plugs and dual iginition. It has available dual ECU's, dual fuel pumps and dual alternators. SO you get to choose your tolerance for risk vs budget.

Contrary to most all Rotax's, it doesn't have or need carb heat, carb issues with floats and balancing, mixture control, fluid cooling system, water pump, psru and it's 500hr inspection, 5 year rubber replacements, etc, etc.

The idea that the 350is has more single failure point possibilities is again - hogwash.

It is my "opinion" that most of the "smoke" regarding these "issues" is more smoke and mirrors than truth. IMHO.

I'm fine and good with those that are happy with their Rotax's, I just don't buy into the religion. I'd rather have the simpler (by far) 350is with it's 310nm of torque (or higher) from 1700 rpm all the way up to over 2800 rpm than a Rotax (912uls as example) that peaks at 310 nm torque (at the prop) at 1934 rpm (also at the prop) and drops swiftly on both sides of that peak.

I, like you have about a year to make a final decision, so anything can happen in that time. And I want to see the the CS Whirlwind and STOL before finalizing.

I am for sure going with the VP-X though - the way avionics change, I want the ability to just change some pin locations and simply reassign and reprogram.

Regards, Greg

[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/showthread.php?45341-First-Super-STOL-Highlander-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?f=218&t=20515&hilit=ul350+engine&sid=28d390908ddcd542c5c5c05793072984&start=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 :confused: ).

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

[Danzer1]

I should also add: It's not just the prop manufacturers. A longer prop would likely exceed the MOI of the Rotax gearbox and if you tried to fit a 100" prop to it (or even 80") - you would likely rip it to shreds! So you are also limited to what the gearbox can handle.

Greg

[jiott]

HP = torque X rpm divided by 5252. IE: 310nm x 2700 rpm/5252 = 159 HP - HP is just math - torque is the energy produced.

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!

I've been laying back because I truly don't want to always be the bad guy pointing out errors. I'm glad that av8rps pointed out the incorrect HP formula. It should be torque x rpm/7121 = HP if torque is in nm. So 310nm x 2700rpm/7121 = 117.5hp. Now according to the strange logic above, HP doesn't matter and there is no reason to run at a higher rpm than torque peak, then for the UL350is engine we have been talking about there would be no reason to run higher than 2400rpm which is the 320nm torque peak for that engine. 320nm x 2400rpm/7121 = 109 hp. That would be ridiculous for an engine rated to 3300rpm and 130 hp.

Of course HP matters. Torque is not the "energy produced" it is only the twisting force. To be usable torque must always be coupled with rpm, which is the definition of HP. HP is by definition the rate of doing work (work is the same as energy in engineering speak). If the logic above were correct, I could take a torque wrench with a long cheater bar and put it on my prop and develop the same 320nm of torque, and fly my plane off the ground because HP (which is derived from torque and rpm) doesn't matter.

Since it is obvious that HP and rpm does matter, you will get the best performance from your airplane by running the prop at the highest HP possible for takeoff. So if you have a nice relatively flat torque curve like the UL engine you will still get more takeoff performance by running the prop as fast as you dare (higher rate of doing work).

This then brings us to the most efficient prop speed, because after all the best engine in the world is worthless if the prop can't turn that HP into thrust. Now I don't claim to know much of anything about prop design, but I do observe that ever since the Wright brothers max prop speed has tended to be limited to around 2600-maybe 2800 rpm to stay below mach 1 and also to be reasonably efficient at turning HP into thrust. I'm open to correction on those rpms, but I believe they are in the ballpark. I'm also not saying that some breakthrough won't occur and maybe some makers have already found a way to efficiently increase rpms somewhat. What this boils down to IMHO is that finding an adequate prop match for the 3300rpm UL engine will be tough to do. Lets say you stay at a more reasonable 2800rpm max for that engine. From their website torque is 310nm x 2800rpm/7121 = 122hp. So it looks to me that 122hp is the max USABLE hp that you can expect from that engine. This is why I made the statement earlier in this thread that I had heard from other folks that the UL350is 130 hp was overstating their power rating by about 10%. I'm not accusing UL of misinformation because the engine does produce 130hp at 3300rpm, its just that I doubt anyone will actually use it at that rpm. Of course all of what I have just tried to explain was labeled "HOGWASH".

[AirFox]

Nicely stated Jim. I love to see an adult jump into a conversation.

[Esser]

Can I just chime in and say that this forum is usually a very civil and informative place? I have gotten great information from all opposite ends of opinions on this site.

However, Greg, without starting a spat between you and me, a lot of times I read your comments and find them very aggressive in nature. In fact, I only see tension between members when your comments are involved. So in the future, and this goes for everyone on here to keep this forum great, can we keep the aggression to a low as much as possible? People have different opinions and insights and having real conversation about those differences we all come out better for it.

Belittling other members opinions effectively shuts down conversation and helps no one. I feel that this goes for all of us to be the bigger person.

Have I been perfect in the past? No. But everything I say on here us to hopefully help someone else and it's never my intent to belittle someone. If I am wrong, or perceived to be wrong I can listen to someone else's RESPECTFUL opinion and decide from there whether I respectfully disagree or agree.

[Floog]

K u m b a y a :)

[Danzer1]

Read this: http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

Why does Lycoming, Continental and any other direct drive engine manufacturer limit rpm to 2700 to 2750 or so? It is not for engine longevity although that is a byproduct. It is because they all produce max power (torque) at or near 2400 rpm. There is simply no reason to run them higher than 2700 rpm or so. They do because the props are inefficient.

Same thing when Ford changed there ratings on the Triton V10 from 362 hp @ 5000 rpm to 320 at 4000 rpm. Max torque for both ratings was still 457 ft lb at 3250 rpm. Everyone got all upset and said Ford detuned the engine and oh my god it doesn't produce as much power anymore. Wrong - Ford simply rpm limited the engine from 5000 to 4000 rpm because it didn't have any reason to run at 5000 in the 1st place. It is exactly the same engine and still produces exactly the same amount of power.

Maybe there's tension because I'm the only one here with the balls to set the record straight and separate opinion from mis-stated "fact". "Adults" would admit when they are wrong! So be it. I'm done here to.

Cheers

[jiott]

Good; I think this is a good place to end it. I joined this forum to make friends, not enemies, and share all kinds of good information. I apologize if I have gotten too aggressive at times.

[rosslr]

Come on you guys - I'm with Esser - you guys have a heap of knowledge that I have benifitted from over the past years - sometimes the topics are beyond my understanding not being from an engineering or technical background - but I really learn a lot when you enter into the conversation and work at creating understanding together - despite coming from different starting points. I can understand you frustration Greg that everyone doesn't instantly understand where you are coming from, but that is the challenge of communicating - the world would be easy if it wasn't for people! Jim, I have always found your explanatory style helpful and accessible.

So I certainly hope neither of you take your bat and ball and go home!

Less frustration and more trying to understand where the other is coming from - and dont loose your sense of humour!

Was that you singing Kumbaya Doug? Blessed are the peacemakers!

r