ckombo

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Hi,

I am looking for the perfect match for my new Moki 2.10. I need SPEED! I was looking at the Formula 1 Sundowner, but many in the forum are saying its not THAT fast.

Any suggestions?

LGM Graphix

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The Moki 2.10 isn't really a speed engine. I think the Sundowner is probably it's best fit. If you want FAST, you need an engine designed for that. Or, a huge engine with the HP to turn the steeply pitched props like the unlimited engines.
Having said that, stuff that Moki in a 90 sized mustang and it will probably move out pretty good

MJD

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Speed is good, but how fast do you want to fly, IOW how fast do you expect that Moki can pull something? Potential speed is directly proportional to pitch and rpm, without those all the horsepower in the world isn't much good. The highest pitch I have seen in APC's lineup is 14". At 10,000 rpm, 1000rpm over the mfg rating for the engine, that is good for 133mph. That is about how fast a low cost .46 with a tuned muffler will pull a delta or Q500 airframe.

So you need to decide what you actually are looking for in terms of speed. This is indeed the issue with the bigger engines - they are not generally designed for much higher rpm than that, being set up for sport, aerobatic and scale use, with the attendant manners and broad torque range. So couple such an engine with the highest pitch prop you can find that it will spin, and you can still be whupped by a sport .46 on a $75 airplane. Now, I'm not saying that 130mph or so isn't fast, it is pretty quick - just don't expect to start trouncing the ARF quickie 500's at the local field anytime soon with that engine. My guess is you will have to work to exceed 120-125mph or thereabouts. Unless you start hand-carving 12x16 props or something.

MJD

Sessholvlaru

Any idea what rpm's those custom gas twins spin in those giant scale Unlimited pylon racers? Last I read they are up near 200 mph.

Sessholvlaru

Answered my own question! http://www.starrair.com/pages/aircraft.html Nice stuff there! Too big for the Moki though.

MJD

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Lots of pitch on those big props!

MJD

mk1spitfire

I like the f1 sundowner, pretty model, fast landings,little bit of down flapperon/up elevator mix help landings. Your 2.10 should pull her with authority.

Heres a little vid of my f1 but with 1,80 moki,straight castor, About 132mph with 16x12 apc

[youtube]http://www.youtube.com/watch?v=15stDdjUuIQ&feature=player_embedded[/youtube]

dhal22

one of their props is a 22" x 40. [X(] that's how you get some speed.

ckombo

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Speed only depends on pitch and RPM? I think you forgot to take into account drag, weight etc... If I put the Moki on a Mud Duck, getting more than 5mph would be a gift. I am looking for a slick and fast plane, or a plane designed for speed with the right engine and prop.

combatpigg

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.....in MJD's reply to you, I don't think he figured you'd need a complete course on aerodynamics, it looks like he just wanted to point out that to get anything powered by this engine going real fast you're going to need a really specialized prop...or choice of props.

MJD

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You didn't read what I posted literally - I said "directly proportional to", not "entirely dependent on" or "equal to" - and there is a significant difference. No matter how slick you make the airplane this engine is bolted to, you are still hampered by the fact that there aren't any props out there beyond 12-14" pitch. Your car's top speed potential is directly proportional to the drive ratio gear ratio, tire diameter, and limitiing engine rpm. Whether or not it has the power to get there is another matter yet.

So yes, you can fly briskly with the Moki 2.10, and apparently you can get to 132mph on the Sundowner. That's pretty close to where I was talking about in terms of speed. Might be able to coax something to 140 or so I'd guess. Again, this is all with stock propellors of 12-14" pitch. I'm not saying that isn't fast - it's decent enough. I guess what I am saying big picture is don't expect blinding speeds due to the fact that this is a big ol' 2.10 cu in 2-stroke producing close to 5 hp. It sure is, but that's only the power generating station, not the drivetrain.

I don't know what your expectations are. If 130mph or so is speedy enough for you, then you'll be happy. If you are getting bored of Q500's with .46's on them, then you won't. On an extreme speed prop plane forum you said you wanted SPEED in capitals. 130-135mph is FUN as all heck (of course it is!), but it doesn't rank as speed flying. Plus, big airplanes don't look as fast so you have those visual aspects too.

You probably don't want to beat on people for pointing out simple mechanical limitations to posted goals, when this is a technical forum and you said you wanted to go fast. The limitations I pointed out due to engine rpm potential and available prop choices are real, tangible, practical "today" issues that stand in your way. Those are the things that require attention to improve the potential above bolting a stock engine to a stock airplane with stock props and flying the same speed as everyone else.

MJD

ckombo

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Ok, so in a perfect world, what prop size would you design for this engine?

ckombo

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And also, if i cant find a prop with the pitch you are talking about, why not just augment that with a bigger prop? I recently heard from an Auronatical Engineer at Goodrich (from work) that it is more efficient to swing a bigger prop slower than a smaller one faster. Therefore, with greater efficiency, why not swing a bigger than normal prop with less of a pitch?

I am trying to understand the physics here..

Mike Connor

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There is some truth to that statement but what you are looking for is the best balance between diameter and pitch for your airframe. A larger diameter may be better suited for dragging your workbench accross the garage but not suited for max speed in a fairly clean airframe. In your case a large diameter prop may be like being stuck in 2nd gear. Your engine size and airframe is not included in my experience so I will leave a recommendation to others.

ckombo

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It has to all depend on max thrust. The greater the force the greater the speed. That is just basic physics.  I read online where a user measured the thrust from 10 different propellers on his .46 and found a 12x4 to "pull" the hardest.

combatpigg

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The 11x7 has a 20% higher load than the 12x4. If the engine was lugging the 11x7 but able to turn the 12x4 more effectively, this is what you might have experienced.

On a good day with a .45-.50 sport engine, a 12x4 propped plane might get you 55 mph if it could be turned 14-15,000 rpm.

On a bad day with a 11x7 prop and the same engine...all it would take is [a sickly] 10,000 rpm to do 65 mph.

Sound effects with the 12x4 will be more impressive.......

ckombo

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I actually clocked my plane at 94mph using the 12x4...

combatpigg

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A quick check shows that it would have taken 25,000 rpm and about 9 HP.

Prop tip speed of 892 mph.

ckombo

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how are you figuring that?

combatpigg

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... a calculator designed to give thrust and HP results after you plug in prop and rpm numbers.

MJD

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Well, it depends on thrust, sure. But not thrust on the ground, though it has plenty to do with how much thrust the propellor develops at a particular airspeed. Static thrust readings are useless for anything except determining how well your aircraft can hover, or accelerate from a standstill. No different than putting your car in low gear, measuring how heavy a trailer it can pull, then determining how fast the car can go from that.. if that worked, there would be a lot of tractors at Daytona. So while it might be good to know that a .46 makes lots of thrust on a 10-4, that info is only of practical use to a free flighter (vertical climb at modeerate speed) or a 3D hover nut, but it has little universal value.

There are numerous airplanes that would demonstrate what you have observed flying faster on a 12-4 than a 11-7. And there will be specific reasons why this is, most likely one or both of:

- engine may rev higher, producing more power
- aircraft flies too slowly for the 11-7 to work efficiently

Don't think along the lines that smaller diameter is "better", because that is too broad. Smaller diameter is quite the opposite, not better for prop efficiency. But what else are you going to trade off to compensate for higher pitch when the goal is to present the same shaft load at a given rpm? If you have a given engine that produces maximum power of X horsepower at rpm Y and that is where you want to run it, then when you are manipulating prop choices to find the combination that works best, you need to trade pitch for diameter to maintain load factor that allows the engine to run at those rpm. If this engine is running at Y rpm with the throttle fully open, it is producing that much power whether it is spinning a square stick of wood or a racing prop.

That's all it is - the fact that for a given engine you need to reduce diameter to maintain rpm with higher pitch. Definitely not that "smaller is better". That's one of the counterintuitive issues that messes people up sometimes - "why" you might reduce diameter.

If you want to go fast, and are not already running your engine at optimum rpm for the engine/exhaust setup , then that is job #1 - find the power. And note that I mean power, not energy or thrust, but power - power is the rate at which work can be done, or the rate at which energy is released. It is a function of rpm times torque. Speed comes from power - the faster you move the airplane, the more work is done per second, therefore you need to deliver energy at a higher rate = more power. The preceeding paragraph is for the benefit of those who think that torque is all that matters btw, which is oversimplified poppycock but keeps coming up in these discussions.

So the talk about smaller diameter, higher pitch always comes about when discussing a given engine and operating range, which typically we are.

In order to generate 4.95hp, your Moki specs say it has to run at 9k. For the moment, let's assume that is indeed the rpm at which maximum power - the product of torque and rpm - is maximum. So we might have a choice of three props of equal static load: let's say 22-7, 18-10, and 15-14 that we could bolt to it and get the same Y rpm reading. Let's not even think about engine unloading, that is kind of an additional factor that needs to be allowed for, but for now assume that the engine does not change rpm. This does not affect the relevancy.

The issue here that I think might be giving you headaches, is the fact that as the aircraft starts to move forwards, the angle of attack of the propellor blades changes. The airstream direction the prop blades see, aka the relative wind, is a combination of the rotational velocity of the blades in one direction 90 degrees to the flight path or so, and the forward airspeed. The higher the airspeed the less the angle of attack of the prop to the relative wind. On the ground you will get noticably different thrust readings between the three props listed, because each prop is working at a different angle of attack, and is therefore at some different place along the L/D curve for the airfoil. The higher the angle of attack, the higher the lift, but also the drag. So less power is being wasted as drag with the fine pitch props, which will be working more efficiently (it might be time to say "aha") than the high pitch props.

So.. what this means is that for every prop, for a specific rpm there is a forward speed where the L/D of the prop blades is maximized and thus efficiency the best, and therefore the least power is being wasted as drag generated by the prop blades and, speaking of pure physics, ultimately as heat. Your job as a speed flier is to manipulate the pitch and diameter until you reach the maximum airspeed - this is where the engine system is working most efficiently since the highest % possible of power is generating lift force versus air turbulence.

That 20-7 would be perfect for a draggy biplane, whose top speed is 50mph and needs all the low speed thrust it can get to drag all the wires and bits around. The 15-14 prop likely does not generate enough low speed thrust to get the bipe of fthe ground. The 18-10 might fly it well enough, including having enough low speed thrist to accelerate and take off nicely, but there could still be wasted power due to excessive pitch.

When your plane flys faster on the 12-4, I guarantee it is flying at or below pitch speed of the 4 inch pitch prop, but a higher percentage of the power is generating lift versus drag and thus you get more thrust from the prop. The 7 inch pitch prop is wasting a lot of power as drag because the lower airspeed is causing the blades to operate at a higher and less efficient angle of attack, and therefore you are beating up the air with the prop and generating drag but not generating enough thrust at that airspeed to go any faster.

Now, apart from this, I think most 12-4's will spin faster than an 11-7 on the same engine, so the engine is also possibly a little higher up the power curve. Both factors may be equally at play, or not, don't know the engine or the airplane - but it doesn't matter because the general observations are consistent.

If the airplane was a clean sport design, then it would not fly faster on the 12-4 any more than your Toyota would have higher top speed when you shift from 3rd to 2nd. But it will climb better and pull more load - all at lower roadspeed so that the work done per second is the same. Similarly, if your car pulls to redline in top gear and obviously has more to go yet, then you need a lower top gear ratio - one where the engine produces maximum horsepower at the rpm that matches the road speed that absorbs 100% of the available power.

To put all that claptrap in short form:

- don't care about static thrust
- pitch speed is very important and significant
- power must be sufficient to meet the goal, all inefficiencies accounted for
- the best speed setup is one where you engine runs at peak power rpm, on a prop whose pitch speed is matched to the airframe so that the prop works at best efficiency at that airspeed.
- the Moki is a great engine and makes lots of power, but has some practical limitations that should be taken in to consideration - max rpm, available prop choices mostly.

All I ever wanted to pint out is that despite the engine's size and horsepower, there are factors to consider that will limit your top speed, and maybe more than you think. The state of tune of the engine and prop choices mostly. If you persist in thinking that high static thrust props will let you fly fast, then you will be eternally disappointed. Thrust is everything, yes - but there is a big difference between static thrust readings and thrust generated at airspeed, and you need to look at these factors to make informed choices.

Reaction engines like a rocket motor are handy because they don't care how fast they are moving, they just accelerate until drag+weight = thrust, then go no faster. No darn props to think about.

MJD

MJD

My Feedback: (1)

In the time I typed all that and tried to figure out how many typos there were, there were what, 5 posts..?

ckombo - if you Google "pitch speed calculator" you'll find a utility that does the basic math for you. This math IS correct. What is often different is the true aerodynamic pitch value or the inflight rpm. This utility gives you the maximum speed before the prop stops making thrust.

Mike Connor

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