Contemplating an SS-7 - Newbie builder

[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...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