What is the most weight a moki 1.8 can pull vertical?

im guessing around 18-20 lbs, thatd be a 1-1 ratio

Maybe even more since my DZ 140 pulls more than 18 pounds tested on the bench. A Moki 180 must be stronger than my YS.

The prop was an APC 17 x 8 and put not enough load to the motor. I don't know if a larger prop turning slower will generate more or less thrust.

My thinking is that you should keep around 1.5 : 1 in power to weight ratio since a 1 : 1 ratio has not unlimited vertical, 1 : 1 ratio will give vertical until drag stops the plane and then should be able to torque roll all day. Full throttle torque rolls are hard to do since a blast of air is sometimes required to "straighten" the plane.

Zak its probably more close to 25-30lbs thrust

If a Moki 1.8 can pull a 25~30 lb plane vertical...that was the question.

the moki 1.8 has an advertised 21 lbs of thrust

sounds right to me, I think that you just need to make the plane as light as possible. Light flies....heavy is a turd

I have a moki 1.8 on a 300XS. It spins the 19x8 Mejzlik prop at 7800 RPM . Using "thrust HP" that calculates to just over 23 lbs of thrust .

Thrust on a stand is not the same as thrust on the plane. Some planes, with fat cowl, will reduce the effective thrust because the area of air that's blowing onto the cowl immediately turns into drag. Here's a little equation I came up with to estimate real thrust:

Find the prop disk area: A = (Diameter/2)^2 * Pi
Calculate the cowl profile area, say C, as viewed from the front.
Effective thrust = Bench thrust * [(A^1.5 - C^1.5) / A^1.5]
The term on the right is the approximate efficiency factor

For example, 19x8 Mejzlik would have A = (19"/2)^2 * Pi = 283.5 sq in.
Estimate the Extra 300 XS' cowl profile area to be a rectangle of 10" * 8" = 80 sq in

Effective thrust = 23 lb * [(283.5^1.5 - 80^1.5)/283.5^1.5] = 23 lb * 0.85 = 19.55 lb

So a 19" prop on a 10"x8" cowl would have efficiency factor of about 85%. Of course, a more streamlined cowl would have higher efficiency than a brick wall cowl, way, the Sukhoi's cowl.

So when we use bench thrust or calculator thrust to predict its performance on a plane, take it with a grain of salt, because we will lose some due to the cowl drag. That's why profile planes and pattern planes tend to require less power than the scale planes to get the same performance-per-weight. Hope this helps.

The above is based on the assumption that
Static Thrust ~ D^3 * RPM ^2 * W * CL
where D = Diameter
W = prop cord width
CL = prop airfoil coefficient of lift

The Thrust HP calculator assumes slightly differently:
Static Thrust ~ D^4 * RPM ^2 * CL
I think they take "W" out of the equation to make it simpler for us users. They assume that W will increase proportionally with D, which is true in some cases. But when you use Thrust HP to calculate a wide-blade prop, you have to adjust the cord width accordingly.

wow pretty impresive.

Hey, I am just trying to apply some aerodynamics I learned in grad school, since I can apply only 0.1% of it at work

i dont think you learned that in grade school

What about the Moki 2.10 Anyone know the kind of thrust that thing can dish out? Thanx.

Thanks, found it!

What if you used some type of scale hooked to the plane. may be a fish scall. I tried this once with a OS 61 and it pulled about 7lbs. DO you think the weight of the plane will affect the reading.

The weight of the plane will not affect the final reading, because the weight vector points down, while the thrust vector points forward. The mass of the airplane will only make the scale reading reach its peak thrust a little slower.

You are sure right I didn't learn that in grad school. What I wrote above is a *very crude* simplification of what I learned in grad school. To accurately calculate the thrust and drag, one needs to integrate the CL & CD with respect to the radius of the prop, for entire 360 deg of rotation with varying pitch, cord width, and airfoil cross section. As you can observe, a prop's angle of attack decreases as it goes toward the tip, and the cord width varies as well. So it becomes a very complex partial differential equation that cannot be integrated by hand.

I just applied the principle I learned in undergrad & grad, then roughly assess the effect of thrust that D, W, RPM, & C have on thrust and drag.

I actually first read about these equations when I was in high school in a book "How They Fly" when I started getting into RC flying.

interested in equations that relate to thrust. where could i find more written info?

http://www.mh-aerotools.de/airfoils/prpstati.htm

This article seems to suggest keeping the prop angle of below 10 deg to optimize static thrust; above 25 deg the prop begins to stall during static flight.

If the tip of the prop is 10 deg, then

Pitch / (Diameter * Pi) = Tan(10) = 0.176

Pitch = Diameter * 0.176 * Pi = 0.554 * Diameter

So if Diameter is 18", pitch should not exceed 10 if you want at least the tip to generate thrust efficiently.

But, come on, we want not only the tip, but also the entire prop to generate static thrust efficiently.

So, wanting the middle of prop blade to be efficnent,

Pitch / (Diameter/2 * Pi) = Tan(10) = 0.176

Also, we want the prop tip to function like helicopter blade, whose optimal angle is 5 deg:

Pitch / (Diameter * Pi) = Tan(5) = 0.275

These two equations lead to this roughly:

Pitch = 0.28 * Diameter

So 18" prop should have Pitch of 5 if we want the tips to act like helicopter and the middle to lift efficiently.

So, props like 18x6, 30x10, 14x4, 12.25x3.75 work great for static thrust, because little or none the prop stalls during static flight.

http://www.mh-aerotools.de/airfoils/propuls3.htm

i can vouch that the 14x4w apc works real good .

Chad

If a moki 1.8 is supposed to pull around 21 lbs., how much would a moki 2.1 pull?? I was thinking of putting that on my midwest extra.

+- 25lbs static thrust or 21.25 lbs effective thrust