stulwilson

I have built a 4 foot wide flying wing out of 2mm red coroplast, and then filled it with insulating triple expanding foam. (touch 'n' foam) I need to weigh if after the foaming, which was done yesterday. I have just completed it, and am very curious, what I should expect thrust-wise out of my new enigine. I have a graupner 7.2v 480L left handed motor in pusher configuration(box says 17,000 rpm no load speed, 5A current drain max efficency??). I have a graupner 5.5 x 4.3 carbon fibre competition prop on it right now, in preparation for it's first flight. I have 2 other props to try, a 5.5 x4.5, and a 5 x 5 carbon fibre prop. I have a 9 cell cp 1300scr battery pack, and an 8 cell pack (same type of cells). I am also running a 35 amp castle creations pegasus speed controller. I have no idea what the prop designations mean, or what kind of thrust to expect. My pane should (fingers crossed) weigh about 38-40 oz, with about 480 sq inches of wing. I would like to know how to calculate this type of thing, what the prop #'s mean, and any general guidelines. especially any guidelines regarding how much thrust is enough ( other than "you can never have enough"). I know I am asking alot at once, but this is my first electric coroplast plane built from scratch, and I just bought the stuff the model shop guys said was good for that motor
Thanks for any help you can give

"Lets do her!!!" Steve Martin, The Out of Towners

Ollie

The numbers in the prop designation are diameter in inches for the first number and pitch in inches for the second number. Pitch is the distance the prop advances in one revolution without any slippage.

Static thrust (100% slippage) is easier to measure than to calculate. Put the wing on dowel rollers on a smooth table with the prop over hanging the edge. Make a string yoke that attaches to the wing near the trailing edge, with prop clearance on either side. Hook a fish scale like the Zebco Deliar to the yoke. Hold the fish scale so the yoke is taut and the prop can't advance into the edge of the table. Start the motor and get the static thrust reading off the spring scale.

The dynamic thrust when flying will be somewhat greater than the static thrust. If the static thrust is equal to about 1/10 the weight of the model, the plane will just be able to maintain altitude without climbing if it is in perfect trim. If the static thrust is equal to the weight of the model, it will be able to climb vertically. If the static thrust is between about 1/10 the weight and the full weight of the model the plane will be able to climb. The closer the thrust is to the weight of the model the more vigorously it will climb.

stulwilson

Thanks very much. I'll give that a shot. Also thanks for separating that static and dynamic thrust, and indicating how climb rates will be affected. However, I am still curious about this slippage you mentioned. I am not quite sure what that means. If you have an easy definition to give me an idea of how the slippage distance (2nd #) affects how the prop will perform. Thanks again.

Ollie

Think of the prop as screwing its way through the air. If the pitch is 5 inches then, with no slippage, the prop will advance 5 inches for every revolution. If the 5 inch pitch prop only advances 4.5 inches instead of five, then the slippage is 1/10 or 10%. When the prop slips, it operates at a higher angle of attack and produces more thrust, up to the point where it stalls.

stulwilson

So let me see if I get this right. for the 5.5 x 4.5 prop, the slippage would be (5.5-4.5)/5.5 = 18.2%. and for the 5.5 x 4.3, the slippage would be (5.5-4.3)/5.5 = 21.8%. and thus the 5.5x4.3 prop would produce more thrust? what about the 5x5 carbon fibre prop, that would be (5-5=0)/5 = 0 slippage??? is that correct? does that mean that the 5x5 would be a much less effective prop? or am I misinterpreting this information? I guess it could just be a matter of trial and error to determine which is best for the motor/esc/battery combo I have. some props will have less drag and thrust, and be easier on the motor, and some will have more, and be more taxing on the motor. If that is the case, then any tips as to how to tell which prop is right for the motor would be great. Thanks for all the help guys. I have only been in this hobby for 3 months, and have been kind of winging it the whole time. I am trying to get a bit of solid knowledge to go with the sturctural engineering experience gained from frequent crashes and repairs to my airframes. hehe, a good solid nose dive that terminates at the ground can point out the weak spots in a plane real quick.

Ollie

The first number is the DIAMETER! The second number is the geometric pitch. The slippage depends on a whole lot of things in the operating conditions of the prop and is generally not know precisely. It varies from moment to moment during a flight. It depends on the pitch of the prop in relation to the airspeed, torque, RPM, acceleration, drag, inertia, temperature, altitude, humidity, etc. etc.

Almost everyone picks their props emperically by flight testing. Stick with the emperical approach.

BTW there are easy to use programs like Moto Calc available to help you match prop, motor and battery combinations to a particular airframe.

stulwilson

ahh. gotcha. I was way off on the first #. My bad. However, the second number denotes the pitch, and so I guess, what I need to understand is how the second number relates to how the prop behaves. I am curious to know how the geometric pitch is measured on a prop. I will try to look it up. One quick question though, would the higher second number i.e. 4.3 vs. 4.5 vs 5 indicate that the pitch is more aggressive? Without knowing how the #'s are derived, I dont know if they might go the other way. thanks again for all your help. I'll get all this one day.

stulwilson

I went back and looked at your earlier post about slippage. and pitch. you already indicated how it is measured. " how far the prop would travel in one rotation without slippage". If there is no slippage, then essentially the prop with a pitch of 5 moves forward 5 inches. If there is no slippage, then would that be like a very slow moving prop, like a submarine prop, sliding through the water, but not cavitating? except in the air? sorry, I don't have the technical language for this down yet. One other thing I think i am getting from this. at the stall point, slippage equals 100%, and there is no forward motion? at that point, I guess at that point, the direction of the plane changes, and slippage would return to less than 100%? interesting. does thrust increase in a linear fashion with slippage? or is it a exponential type curve (J curve) or is it some sort of bell or parabola with a peak at a certain percentage of slippage? I guess the graph would be different for each prop and pitch. I hope I am beginning to get this straight, and not just confusing myself and everyone else further. Thanks again.

Ollie

Actually the prop is not slipping 100% during static thrust testing. My former statement was in error. The air is rushing by the prop during the static test. If the pitch of the prop is 5 inches and 5 inches of air flowed past the prop per revolution, the slippage would be zero but less than 5 inches of air flows past the 5 inch pitch prop, so there is some slippage. How much depends on the velocity of the flow. The coefficient of lift curve is linear with angle of attack up to the stall. The thrust is proportional to the angle of attack and the square of the speed of flow past the airfoil. The speed of flow past the airfoil depends on how far each airfoil segment is from the center of rotation of the prop. So, the center of the prop disc, which is turning at a lower speed, produces much less lift than the prop tip, which is moving much faster.

See:
http://naca.larc.nasa.gov/reports/1922/naca-tm-79/
for the theory of propellers.

For a less technical description see chapter 14 of Model Aircraft Aerodynamics by Martin Simons.

BruceDana

Question for Ollie...

I read a post on this forum by a Diamond Dust flyer that his Dustwas faster than the theoretical speed (pitch x rpm). I read with skeptism as that would imply no drag from the airframe, and a 100% =+ efficient prop.

I did a little (translate to 10 minutes) reseach on some prominent pylon racing sites and the only references I found to he most efficient prop (in terms of pitch) was about 80% of pitch.

Was the person who posted that his Dust was flying faster (about 25% faster) than the theoretical pitch/rpm full of crap?

Ollie

Let us assume the person measuring the speed was honest but not a very critical thinker.

It's conceivable the measurement instrumentation had a plus 2% (assign your own number) error. It's also conceivable that there was a tail wind of 8 MPH resulting in a 4% error (assign your own numbers).

The stated pitch of props is usually from a reference line tangent to the bottom of the prop airfoil because it is convenient to measure. The geometric pitch should be measured to the nose of the prop airfoil. I'm guessing this gives about a 3 degree error depending on the airfoil. The cambered airfoil of the prop has a zero lift angle of attack, perhaps as much as minus 3 or 4 degrees. It's conceivable that these angular errors and effects made a 10 inch pitch prop behave more like an 11 inch pitch prop as assumed by the pitch and RPM to speed calculation for a net 10% error.

How was the RPM in the air measured? If the RPM were measured on the ground and the engine unloaded in the air, it is another possible source of error in the RPM and prop pitch to speed calculation.

Maybe there was as much as a 4% error in the measurement of the prop pitch in addition to the above mentioned errors.

I've contrived these numbers to add up to 20 %. The point is that without an error analysis the claim can't be taken seriously. That's why speed records must have two way runs and the measuring error question must be addressed in the application to the FAI. Back around 1980 the Austrians claimed a speed record near 180 MPH for a slope soarer. The record was granted but a subsequent error analysis of the measurement system resulted in the record being rescended.

BruceDana

Thanks for the insight, Ollie! That is why the only serious consideration for speed should be times measured over a calibrated course, or with a certified radar gun. The course should be traversed both ways, to compensate for headwind/tailwind.

Those claims of 250+ mph on 8" to 9" pitch propellers would require extraordinary RPM; and I believe no prop is 100% efficient and no aircraft has zero drag (except for those imaginary ones, that go those imaginary speeds).

Unless my math is wrong, a 9" pitch prop (effective), if it were 100% efficient (like it were turning in clay), and there was zero drag from the aircraft would have to be turning 29,333 RPM to go 250 MPH. Very few engines are capable of this (23,466 RPM to go 200 MPH, still a feat).

Thanks!

Ollie

In the case of a radar gun, the calibration error isn't the only possible error. There will also be an error due to the radar gun not being directly in the line of flight.

In the case of timing along a measured course, the are possible errors in the measurement of the length of the course and errors in the time measurements. Let's say you are trying to measure a plane going 180 MPH through a 50 foot long course with stop watches which have a resolution of 0.1 seconds. At 264 feet per second the course will be traversed in 0.189 seconds. In this case the resolution of the watch is only 52.8% of the true time. This is obviously not good enough. Either the course has to be greatly lengthened or the resolution of the time measurement has to be greatly improved or both.

No measurement should be considered complete without a probable error attached to it.

A credible experimental design starts with an error budget and portions of the error budget are allocated to every possible source of error.

rctt

My Feedback: (1)

There,s a link to a thrust calculater on www.rcshowcase.com
It simple to use and has the major prop manufactures listed .You just need to know rpms . I take it IT compensates
for slippage in different manufactures ...
HOPE THIS HELPS RCTT

Jeremy Sebens

Actually, the speed you're referring to is not the theoretical max for the propeller. It is actually what is referred to as the "pitch speed" The advance number (the second number in the prop's designaton) is determined by the chord line of the propeller airfoil (as though that chord line fell on a screw surface), since this is an easy line to measure. This means that at the pitch speed (pitch times RPM) the airfoil is at zero angle of attack all along the propeller blade. If the prop's airfoil were symmetric, this would mean zero thrust, but our propeller airfoils are in fact heavily cambered (APC uses a modified NACA 4412 with a dropped leading edge - LOTS of camber), so the propeller is actually still developing a reasonable amound of thrust at the pitch speed. Depending on the airfoil in use on the prop, the actual zero thrust speed may be anywhere from 110%-180% of the pitch speed. With APC props that I've tested in our wind tunnel here at school, I've found it to usually be around 150%-160% of pitch speed.

Most props do in fact reach their max efficiency somewhere near the pitch speed, so designing the model to fly near that speed, or choosing the prop that most closely matches your model's speed is a good idea for speeding up a plane.

Hope this helps...

BobbyRichardson

Ollie

When you say full weight of the model you are including the model with engine right? So if my scale measures 10 pounds of (thrust) and my plane weights 5 pounds (model & motor) I would have a 2 to1 thrust vs weight ?

Jimbob-RCU

Keeping it as light as you can and buy the best electric motor you can afford. My solution to electric success.

Ollie

Bobby,

A plane with a two to one thrust to weight ratio will be lively indeed. It will climb vertically with an acceleration of about 32 feet per second, per second.

Some hotliners may approach that kind of performance.