RE: Engine for SAE heavy lift Competition

RE: Engine for SAE heavy lift Competition
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RE: Engine for SAE heavy lift Competition - 7/30/2008 8:02 AM

Bone

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How about a large capacity Saito (within the size limit) with a gasket between the crankcase & the base of the cylinder to reduce compression so that you can swing an extra large prop. Then design the aircraft around the engine performance characteristics - large wingspan with high lift aerofoil section capable of low speed flight.

My $0.02c worth

< Message edited by Bone -- 7/30/2008 8:04 AM >

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RE: Engine for SAE heavy lift Competition - 7/30/2008 9:01 AM

XJet

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You wouldn't want to reduce the compression ratio of a Saito any more than it already is.

Reducing the CR and using a larger prop will simply get you less power and besides, the largest Saito you could use would be the 56, which isn't a powerhouse to start with, having far less HP than a .60 2-stroke and capable of producing far less thrust.

If I were doing this challenge (and I've seen most of the videos so I know how much fun it would be) I'd focus on keeping the wing-loading low and stability fairly high.

Do you have a maximum wing area rule?

I'd also use a 2-stroke engine with a tuned pipe and a fairly large diameter low-pitched prop (to give plenty of static thrust) to help get things up to above-stall speed ASAP.

A high aspect ratio wing will be more efficient but will create structural issues and lessen the amount of control available so a medium aspect ratio would be a better idea.

In the end it may be the little things (such as prop selection even down to the brand/model as well as diameter and pitch), tuning and piloting that determine whether you succeed or fail.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 9:16 AM

Lou Crane

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Erico,

By now, it should be evident that "mere" horsepower isn't necessarily the most suitable criterion. Consider:

When Gran Prix Formula 1 had a 1.5 liter displacement limit, engines could develop well over 400 BHP, at ~10,000 RPM. An American-LaFrance fire engine was equipped with a 100 year old-type 4-cylinder (separate blocks per cylinder!) of less than 100 HP at ~1000 RPM... Which would you rather have to climb a steep grade with a heavy load? The fire engine was equipped to move a heavy load at a modest speed; the GP engine ONLY produced great power at extreme (for the time) RPM.

Which raises another point implied in the discussion to date: You will need to "gear" your power system wisely. Unless you go for complicated and heavy mechanical gearing, the way to "gear" our engines is by choice of propellor. Propellors provide a single "ratio" gearing system, tuned to the engine's ability to reach particular RPM on a given diameter and pitch. (And that may vary according to the different brands of propellor nominally of the same diameter and pitch...)

You'll need ample static thrust to accelerate your package to flying speed within that limited takeoff run distance, but you will also need enough RPM bandwidth to maintain flying speed once off the ground. You'll need to avoid a problem similar to building a dragster with too low a final gear - great initial acceleration, but RPM-limited terminal speed...

The general scale of your overall product suggests that 'winding' a high horsepower 2-cycle engine to its peak RPM range will require too small a prop to be usable for the acceleration to Vmin condition, and that even if it were usable, you'd be wound out to or past RPM peak by lift-off. Generally, our diesel engines maintain higher torque production across a wider RPM band than glows. Glow plug engines produce impressive horsepower numbers because they can increase RPM faster than torque output declines, up to their peak HP RPM. Still, with glow engines, torque - the form of force you most need to turn a large prop well - DOES decline steadily past about 10,000 RPM, and more rapidly than occurs with "diesels." ...Generally...

On the aerodynamic side of the equation: Lift varies as the square of speed. If you can attain sufficient speed to get off the ground in the available space with your intended weight, you may find acceleration quicker with a cleaner, slightly smaller airframe - less aerodynamic drag. The greater V may carry the weight more efficiently in a cleaner, more easily accelerated airframe...

You have an interesting challenge, and study of successful approaches in past years can suggest what does, and more importantly, what doesn't, work well. You may find that a merely good, but consistent, package can succeed better than one that might, occasionally, unpredictably, do far better - but most often doesn't. Work toward consistent, repeatable performance of the machinery and the human team. As that comes together, detail refinements can add significantly to consistent performance...

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RE: Engine for SAE heavy lift Competition - 7/30/2008 1:00 PM

gkamysz

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quote:

ORIGINAL: Lou Crane

Erico,

By now, it should be evident that "mere" horsepower isn't necessarily the most suitable criterion. Consider:

When Gran Prix Formula 1 had a 1.5 liter displacement limit, engines could develop well over 400 BHP, at ~10,000 RPM. An American-LaFrance fire engine was equipped with a 100 year old-type 4-cylinder (separate blocks per cylinder!) of less than 100 HP at ~1000 RPM... Which would you rather have to climb a steep grade with a heavy load? The fire engine was equipped to move a heavy load at a modest speed; the GP engine ONLY produced great power at extreme (for the time) RPM.

This argument is always brought up by modelers, but it is fundamentally wrong in this discussion. The GP engine would get you to the top of the mountain faster every time. However, one must realize the difference in application. The GP engine would need a rebuild at the top, while the fire engine would do it over and over for 100,000 miles. Power is the key in the case presented by Erico. The load doesn't care what kind of engine is used to move it. It only moves as fast as physical laws allow and the only thing that matters is force. To get force (thrust) from a prop, power is required to be input. Since we are not talking about really talking about 20kRPM vs. 4-5krpm. We are talking about choosing the optimum prop and selecting a drive train to run it. With just under 10CC we could have up to 5HP available in a highly specialized two stroke glow engine. With proper gear reduction (another engineering challenge in itself) we could have 95% of that available for the prop.

In any class where displacement is limited, running at high RPM is going to make the most power. This is simple. If there were points awarded for fuel economy or system cost, there might be other considerations. If for some reason gear reductions are not allowed this will drastically change the picture, but a .60 two stroke with a tuned pipe will still win. Commercial aircraft use gearboxes on a regular basis and several years back even turbofan engines began using gear reduction from the engine to the fan.

The PAW's sound good but in reality their ability to turn large props does not mean they make a lot of power. The .60 is rated at 1.2HP and that will be at the top of the RPM range. When loaded down with the maximum 16-18 inch prop RPM will be 6-7kRPM and power will be much less than the rated 1.2HP.

Erico's team has probably taken enough classes in airframe design and propulsion that they know what they are doing there. He came to use for recommendations of engine because unlike aerospace engine manufacturers model engines and props are not supplied with spec sheets giving the information they require to make an informed decision.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 1:58 PM

Snare_TS

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Hello all,

I am also part of Erico5050 team and I thank you for all the advice you have provided.

To clear up any misconceptions, we are building this aircraft with the intent that it will fly with a maximum payload. This generally means we are looking at a very stable airframe and a relativly slow speed of between 30-45mph. This is inherent in our airframe design.
Thus, our propulsion choices need to reflect this and from my research and from what has been stated here, we are looking for a motor or motors that will provide the highest power without regard to fast climbs, fast cruise or any other cool 3d acrobatics. We simply need to maximize our power for the given 0.60 displacement restriction.

There are no restrictions on flight times, so therefore there is no need for fast speeds. Stability and power are our priority.

As for fuel conversions, there doesn't seem to be any restriction, so I am intrigued by a deisel engine, though I don't believe we could have a practicle one in the end. Generally or stock fuel is 10% nitro while we have tested with 30% nitro, which I believe will probably be the fuel we use in the end.

As far as our engine choices, I still see that, as far as power is concerened comes within reach of two Wankels. While they are heavy, I do believe we can reduce wieght in other parts of the aircraft, plus it helps in aircraft balancing.
Our fuel is of course limited and these are thrsty buggers, but since our flight won't last longer than 3-4min (assuing we won't tip stall of the roll this time) it should not matter.

I have not been able to find data on the Jett 60L so I have difficulty comparing it. If anybody has anything on this that would be very helpful.

I don't know how to use th quote system on this forum but I'll try to answer some specific questions:

Do you have a maximum wing area rule?
-No, but our wing design will be developed internally as we move on with our design and manufacturing process

Are you in compentition with US schools?
-Indeed we are! As well as teams from other countries.

Erico's team has probably taken enough classes in airframe design and propulsion that they know what they are doing there
-While this may be true for the rest of our team, alas I am not sure anyone would classify me as knowing what I am doing.

By the way, money is in no way a restriction so feel free to suggest anything that may prove to be the most effective propulsion system.

Thanks again!

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RE: Engine for SAE heavy lift Competition - 7/30/2008 2:21 PM

Sport_Pilot

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The wankels would be a very good choice. They use a lot of fuel but I don't think that is your concern. I suggest you buy one and compair. More HP will indeed result in more thrust, you just need to select the right prop. It's not as efficient as a low RPM high torque engine, but for your use they should be ideal. It is simple to set up a thurust stand, I think someone sells one, but can't remember who. You could always simply put the engines on a plane and measure thrust with a rope and fish scale.

I watched the load lift competition in Cobb Co two years ago. I have flown full scale and models. Most modelers don't know how to fly at the hairy edge of a stall with a plane that won't climb. That was reflected to a lesser extent with the event. Just after takeoff and just above ground effect you need to level the plane and let it accelerate at level flight before continuing the climb. Also many planes will not climb in the hot thin air just over the asphalt runway. So you need to let it accelerate at level flight and start the climb just after it passes the edge of the runway. I saw many who simply kept the nose up the whole time wondering why it wouldn't climb.

< Message edited by Sport_Pilot -- 7/30/2008 2:41 PM >

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RE: Engine for SAE heavy lift Competition - 7/30/2008 2:27 PM

Erico5050

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Hey there Snare_TS

Thanks for answereing those questions.

Someone mentioned using a gear reduction, ie a gearbox or belt drive to swing a huge prop

Where can we get such a thing? Or would we have to custom make one ourselves.

By the way, thanks for all the input, i was not expecting so many replies

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RE: Engine for SAE heavy lift Competition - 7/30/2008 2:30 PM

Snare_TS

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Gear reduction wouldn't be too hard to build, and we could buy the pre-fabricated gears to make things simpler.

The only concern I have with this is there is a greater chance of part failure with such a system, so we would have to make it very sound, something that might be difficult if we run into vibrations or other unforseen problems.

I'll look up on his and mention it at our meet tomorrow.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 2:35 PM

Sport_Pilot

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quote:

Someone mentioned using a gear reduction, ie a gearbox or belt drive to swing a huge prop

Friction losses may make a large low pitch prop a better choice. They used them in the 60's when engine and prop sizes were somewhat limited, but larger lower reving engines were a better choice.

It's a shame you are limited to a .60, there are a lot of great .61's out there. Be sure the Jett is not over .60. Sometimes they round down.

< Message edited by Sport_Pilot -- 7/30/2008 2:40 PM >

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RE: Engine for SAE heavy lift Competition - 7/30/2008 2:37 PM

Snare_TS

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Prop size will indeed need to be re-evaluated since the last compition we used either the OS61FX or the OS25.
But this is something we'll work on once we figure out the engin platfor iteself.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 2:51 PM

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I just recieved and email from Jett Engines and they said they could put together a Jett 60L-H with a tuned pipe and it would turn
a 14X4 prop at 11000rpm. Based on two online thrust calculators this would be theoretically 13.5lbs of thrust. I was also assured that this setup would easily outperform a OS 55AX

How accurate do you think this thrust calculation is?

Also about the wankel 0.30, it says it is 4.97cc which is actaully 0.303 cubic inches meaning two of them wouldbe too much displacement, i have sent OS Engines an email to clarify the actual displacement.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 3:10 PM

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OS Engines have just emailed me confirming the wankel has a displacement of 0.303ci
I think this rules out two wankels
I think our best bet is the Jett 60L-H with 0.595ci

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RE: Engine for SAE heavy lift Competition - 7/30/2008 3:14 PM

Snare_TS

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I'll have to run the calcs myself later when I'm not at work and sneaking onto the PC every 2 minutes.
As far as it technically being 0.303, the OS61FX is technically 0.607 and they, as far as I know are permitted.

I'll look into this.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 3:48 PM

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Here is a radical but IMO valid idea:

you know the V-22 Osprey aircraft?

point of inspiration - use two 30 size locked cyclic heli mechanics with two 25-30 size heli engines running two contra-rotating huge blades with controllabe positive pitch at 1600-1800rpm?

massive lift!

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RE: Engine for SAE heavy lift Competition - 7/30/2008 4:04 PM

Erico5050

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Compared to the actual weight of the osprey, it is not lifting that much,
We are hoping to build a plane that lifts more than twice its own weight and reaching the 55lb limit.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 4:20 PM

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I think the rules must have specified a fixed wing or someone would have thought to use a heli. Although using a heli powerplant would be possible. A lot of the high reving two strokes are not made any more, but the heli two strokes are still the high rev type.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 4:28 PM

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A gear driven engine is the way to go. The early quarter scale planes went this route and pulled 20lbs planes all day normally. The setup is not as robust as new gas engines but more than enough for what you need. Try and get the specs on the old Byron or Dubro drives. I think the gearing was about 2-1; dont make it even so the stresses are not in the same place on the belt every time but I think they could pull a 20-10 prop at 4500-5000 rpm. Go with a mousse can muffler rather a than a tuned pipe because although the tuened pipe will make more the tune has to be perfect or it might not pull enough if it goes off. The mousse will allow the engine to have more torque under the HP curve.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 4:30 PM

TFF

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os Wankels are cool but they are not as powerful as the same 2 stroke piston engine.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 5:49 PM

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quote:

ORIGINAL: TFF

A gear driven engine is the way to go. The early quarter scale planes went this route and pulled 20lbs planes all day normally. The setup is not as robust as new gas engines but more than enough for what you need. Try and get the specs on the old Byron or Dubro drives. I think the gearing was about 2-1; dont make it even so the stresses are not in the same place on the belt every time but I think they could pull a 20-10 prop at 4500-5000 rpm. Go with a mousse can muffler rather a than a tuned pipe because although the tuened pipe will make more the tune has to be perfect or it might not pull enough if it goes off. The mousse will allow the engine to have more torque under the HP curve.

Mick Reeves has a belt drive for the Zenoah 62 in production. I'm merely using it as an illustration of what can be done with this technology. Yes it's a pain to make but you have to ask yourself how many m.e. majors in the competition wouldn't just LOVE to design one of these! He claims a 50% thrust increase from 20# to 30# using a 1.75: 1 reduction ratio. Here's the linkhttp://www.mickreevesmodels.co.uk/~mickreev/Access/accessories.htm

Unlike TFF above, I'm inclined toward using a tuned pipe (and high nitro fuel!) to get more thrust and thus, carry more mass aloft. While it will only give improvement at a specific RPM, I believe the most important part of the competition is takeoff thrust. Once airborne, the thrust won't be as critical and climb rate will be the limiting factor. Once the plane has achieved minimum flight duration (or minimum alt.) the pilot will throttle back to descend and falling off the tuned pipe should not be an issue. TFF may ultimately prove right but its a competition and I think you should attempt the most extreme configuration imaginable and back off for consistency/durability only if you have to later.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 6:11 PM

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If you have seen these fly and how many of them are on the edge, I don't think you would recommend a larger higher p-factor prop to deal with. You may make a small gain in thrust, but simply lose it with a crash or stall from handling a heavy barely climbing plane that is yawing way to much. I think its better to keep it simple and simply use a larger flat pitched prop.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 6:30 PM

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I suspect the 11000 rpm figure from Jett is very conservative.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 6:52 PM

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Jett and Fox tend to be conservative with their numbers. Just the opposite of most others. I would have also recommended the Fox .61 as it has both great horsepower and torque, but alas it's a .61. Carb might take too long to learn to tune with all of the other issues though. Something more user friendly may be better.

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RE: Engine for SAE heavy lift Competition - 7/30/2008 7:15 PM

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If it was me, I'd be glad to have Jett's name on the engine. Good people there, good products!

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RE: Engine for SAE heavy lift Competition - 7/30/2008 7:29 PM

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There are guys in the UK who have been researching this for years as there is a similar competition over there. They were restricted to s single engine, Irvine 40 I think, and the theory guys always seemed to favour the geared option.

However as you can see from the reports on the link the successful aircraft were usually ungeared.

Our club's CFI was often asked to fly the model.

Here is a link http://www.bmfa.org/unichallenge/index.html

If I was doing this I would buy a Jett 60 and prop it for max BHP at take off air speed. You could play around with a tuned pipe if you have time but make sure that Dubb Jett knows what you are using it for.

< Message edited by j.duncker -- 7/30/2008 7:38 PM >

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RE: Engine for SAE heavy lift Competition - 7/30/2008 9:14 PM

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quote:

However as you can see from the reports on the link the successful aircraft were usually ungeared.

Do you know if it was a performance issue (didn't climb as well, weighs too much), reliability issue (gear or belt breaks), or learning curve ( too much or too little prop, needs differant plug or fuel, etc). Knowing the issue could help the decision process.

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