MJD

My Feedback: (1)

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