Stupid newbie questions about hp requirements

[hoog76]

I've been reading many websites about cozys for a few years now. This has got to be one of the best resources yet.

 

I'm very ignorant when it comes to the details of hp, thrust, props, rpms etc so bear with me if this seems extreme.

 

One of my biggest interests beside a glass cockpit is engines.

 

What I'm confused about is the requirement for larger hp engines. Isn't the main goal to get the prop turning at a specific rpm for maximum thrust?

 

So wouldn't a prop driven by a 60 hp engine turning at 2400 rpm give the same thrust as a prop turning at 2400 rpm by a 200 hp engine?

 

Is a larger hp required since turning a prop at x rpm's has to much resistance for lower hp engines?

 

Would a different prop design with a smaller hp engine work to create the same thrust as a larger hp with a less efficient prop design or would the resistance of the prop be too much for a lower hp engine?

 

Like I said, my knowledge is really limited and I see the formulas shooting across but it might as well be in a foreign language.

 

The reason that I ask is because I'm really interested in diesels. I see a lot of smaller air cooled diesel engines in the 30-60 hp range. Kohler, honda, ford, cat, deere, BS, etc. The engines are fairly low cost,light weight and have a high mtbf. Most have peak power at the 2200-3000 rpm range and could be direct drive. Some of the engines are small enough that it seems that if you could take through clutches or one way gears and combine the power of two engines to a single prop it would also provide some redundancy if one engine were to develop problems.

 

I'm not suggesting to throw lawnmower engines into a cozy but engines that are designed for long term mid range use. Seems that most of the issues/failures and costs are associated with complex engine setups that are in a lot of cases unproven and untested.

 

Just curious as to what the thoughts are about my ramblings.

 

Thanks,

 

Hoog

[kumaros]

I've been reading many websites about cozys for a few years now. This has got to be one of the best resources yet.

 

I'm very ignorant when it comes to the details of hp, thrust, props, rpms etc so bear with me if this seems extreme.

 

One of my biggest interests beside a glass cockpit is engines.

 

What I'm confused about is the requirement for larger hp engines. Isn't the main goal to get the prop turning at a specific rpm for maximum thrust?

 

So wouldn't a prop driven by a 60 hp engine turning at 2400 rpm give the same thrust as a prop turning at 2400 rpm by a 200 hp engine?

 

Is a larger hp required since turning a prop at x rpm's has to much resistance for lower hp engines?

 

Would a different prop design with a smaller hp engine work to create the same thrust as a larger hp with a less efficient prop design or would the resistance of the prop be too much for a lower hp engine?

 

Like I said, my knowledge is really limited and I see the formulas shooting across but it might as well be in a foreign language.

 

The reason that I ask is because I'm really interested in diesels. I see a lot of smaller air cooled diesel engines in the 30-60 hp range. Kohler, honda, ford, cat, deere, BS, etc. The engines are fairly low cost,light weight and have a high mtbf. Most have peak power at the 2200-3000 rpm range and could be direct drive. Some of the engines are small enough that it seems that if you could take through clutches or one way gears and combine the power of two engines to a single prop it would also provide some redundancy if one engine were to develop problems.

 

I'm not suggesting to throw lawnmower engines into a cozy but engines that are designed for long term mid range use. Seems that most of the issues/failures and costs are associated with complex engine setups that are in a lot of cases unproven and untested.

 

Just curious as to what the thoughts are about my ramblings.

 

Thanks,

 

Hoog

Hoog,

the root of your reasoning is sound, as the future of aviation is in diesels. You seem to forget, however, that the recommended engine for a Cozy is the O or IO-360 Lycoming with about 180 HP. You would need to put together quite a few of your 30 HP diesels to make it work.

What I'm planning to do, is use one of the new V6 turbodiesels currently being used in most modern cars in Europe,

like the VW 3.0 liters V6, or the Mercedes Benz 3.0 liters V6, etc.

In stock form they put out 225 HP at about 4000 RPM. Slightly derated to 205 HP at 2800 RPM they would both make superb aircraft engines.

Keep thinking outside the envelope, that's what experimental aviation is for.

Kumaros

It's all Greek to me

[hoog76]

Hoog,

the root of your reasoning is sound, as the future of aviation is in diesels. You seem to forget, however, that the recommended engine for a Cozy is the O or IO-360 Lycoming with about 180 HP. You would need to put together quite a few of your 30 HP diesels to make it work.

What I'm planning to do, is use one of the new V6 turbodiesels currently being used in most modern cars in Europe,

like the VW 3.0 liters V6, or the Mercedes Benz 3.0 liters V6, etc.

In stock form they put out 225 HP at about 4000 RPM. Slightly derated to 205 HP at 2800 RPM they would both make superb aircraft engines.

Keep thinking outside the envelope, that's what experimental aviation is for.

Kumaros

It's all Greek to me

Found this cool site for new and used diesel engines while browsing.

 

http://www.usdieselengines.com/diesel_engine_classifieds.asp

 

Yeah, I know that it's a pipe dream to have an open roomy engine area with a couple of small easily replaceable engines in there but you never what technology will bring in the future. I still like the thought of being able to heave out a 60 hp engine and throw in a new one whenever you need to. To minimize the complexity to a few sensors, throttle, and gas lines would be sweet but probably to simplistic to be attractive.

 

hmm, just by searching around a bit there are lot's of different options when using an auto-diesel conversion, thanks for the tip.

 

Anyway, anyone have any guidelines in how you determine optimal hp or thrust that a plane needs?

 

Thanks,

 

Hoog

[Norm M]

The propellor for a 60 hp engine at 2400 rpm is very different than the propellor for a 200 hp engine at 2400 rpm.

 

Here are a couple of basics to consider, for a given airplane.

 

It takes a certain amount of horsepower to make the airplane cruise at a given speed. More horsepower, greater speed. But- It takes a lot more horsepower to get a little more speed. Thus, a 180 hp will cruise faster than a 160 hp, but the 200 hp will be just slightly faster than a 180 hp. To the point where it is difficult to compare because a slight improvement in drag reduction will show greater results than the difference in horsepower. The 180 hp in a Cozy seems to be a very good match.

 

For a given horsepower, the fuel required will be roughly equivalent. There will be arguments about different engines being more or less efficient, and this may be true. But when you here outrageous arguments about 200 horsepower on 10 gallons per hour, you are hearing a sales pitch, and not good data. If you run a 160 horsepower engine at max output, and a 200 hp engine at an equivalent power level, they will burn about the same amount of fuel, and cruise at about the same speed. The 200 hp may weigh slightly more, and thus could actually be slower at an exact same power level.

 

The big difference in power can be seen at takeoff performance. Here we are not looking at the big difference in drag as a function of speed, but simply how much static thrust can we get from an engine. The larger horsepower will win this game every time. (With an appropriate prop, of course).

 

The big thing about commercial diesel engines is the weight. Aircraft engines have many compromises because they are built to deliver a high horsepower to weight ratio. The modern automotive engines would be the obvious solution for a powerplant, if it weren't for the fact that they are at a disadvantage with regards to weight by the time you get everything together that is required for an aircraft engine. This means some type of reduction drive so that the prop can spin approximately 2400 to 3000 rpm, while letting the engine run at 4500 to 6000 rpm. Next, with a water cooled engine you need to accomodate some type of radiator, with a turbocharged engine you usually will need some type of intercooler. Packaging these elements becomes a significant challenge.

 

There are many others with better knowledge of the exact equations behind prop power, cruise drag, and all of these other issues. But perhaps this message will help you understand some of the relationships.

 

Regards-

Norm

[kumaros]

Hoog76,

 

Going back to your original post, which I cursorily replied to before. First of all, no question is stupid, we all learn from each other. As you say, lurking and learning from all these websites of people that have done before us what we are still dreaming of, is an inspiration.

 

As I said in my previous post, the root of your reasoning is sound. While power is important in an aviation engine, torque from low RPMs is essential too, if you want to turn a big, efficient propeller. Gas engines, especially auto-conversions, tend to have peaky torque curves and are thus unable to turn the ideal propeller to the RPMs where they would produce their maximum power. It's like trying to climb a hill in fourth gear. It would help if you'd have an in-flight adjustable prop; that would be like having a gearbox, shifting from take-off pitch to cruise pitch. It's with a fixed pitch prop, however, where diesels come onto their own. Tending to have flat torque curves from 1500 to 3000 RPMs, and peaking in power close to conventional prop turning speeds (like less than 3000 RPM) makes them ideal. What was less than ideal until recently was their weight due to their robust construction, their tendency to knock and vibrate in low RPMs and smoke at high power settings. I went to the URL you gave and saw mostly heavy duty engines from trucks, tractors, earth-moving equipment etc. In my classification of diesel engines: first and second generation of engines.

First generation, plain old clunkers.

Second generation, turbodiesels, common until about 5 years ago.

Third generation, common-rail 4-stroke turbodiesels, currently being used in more than 50% of new cars bought in Europe (the HDI's from PSA, the TDI's from VW, the D4D's from Toyota etc.)

Fourth generation, supercharged and turbocharged common-rail 2-stroke turbodiesels, for the time being in development as future aviation engines, or in miniature form used in K-cars in Japan.

 

First and second generation you already know.

 

For some beautiful examples of the third generation go to:

 

http://www.germancarfans.com/news.cfm/newsid/2041216.002/page/2/lang/eng/mercedes/1.html

 

and from there take a look at the link with the 30 photos of the Mercedes Benz V6.

 

Also:

 

http://www.germancarfans.com/news.cfm/newsid/2041117.004/page/1/lang/eng/volkswagen/1.html

 

Now, to address your point about redundancy using more small diesels, you could take two smaller (like 1,3 to 1,7 liters, producing about 100HP) common-rail turbodiesels and stick them together turning a common or two counter-rotating props, as the brothers Leon have done:

 

http://www.infortel.com/cozy/article_english.htm

 

Daihatsu, a subsidiary of Toyota, has produced some beautiful fourth generation engines. Imagine engines producing a HP per cubic inch (like 50HP from a 40 cubic inch, and 61HP from a 61 cubic inch engine), while burning (in automobile application) less than a gallon per 100km (imagine fuel economy of more than 90 mpg). Take a look at the third one in this engine line-up:

 

http://www.daihatsu.com/motorshow/tokyo02/eco/

 

As for myself, being in the initial stages of home-building, I can begin building the airframe and keep my options open, waiting to see what is available in two or three years time.

 

Kumaros

It's all Greek to me