AeroFinn

Hi Pe and others

Thank you for great attempt to create a usefull thrust calculator

What am I doing wrong? Is there some input values I'v missed?

When I try to use your calculator:

Mejzlik 20 X 6 (prop Constant 1,18; RPM 8800; fuselage drag estimator 3; altitude air pressure calculator 100 meters; local air temperature 20deg Celsius)

I get 22,19lbs not 19,47 lbs (ref. what you seem to get) ??

BTW for a Great Planes Yak 54 25%: Is the drag estimator 3 pretty much correct?

Danke viel best regards,

Artto

pe reivers

You did nothing wrong, My mistake.
I forgot to set altitude back to sea level.

PS
I'll go back and correct the values.

Umran

No problem Pe... I will get the data slowly....


To all.
Another main use of this estimator is the power required readings. For example, running a 23 x 8 - 2 bladed prop at 7000rpm will require 5.04HP. If somehow our engine can deliver this power so that it can be run at 7000 rpm (typically it can by using most of the 55cc engine in the market today), now for a nice scale look we decided to change our prop to a 3 bladed. What size do we purchase if we want to maintain similar flying speed and performance?

Under this scenario, with the same spreadsheet sheet, we simply change the number of blade to '3' - the result for power required is 6.9HP. Mmmm... given the engine that we have, we know for sure it can't deliver. Therefore we reduce the diameter to 22 inch, recheck... 5.88HP, still higher than 2 bladed prop. We reduce a bit more say 21inch diameter, re-check... the result comes out with 4.97HP which is very close to original 5.04HP with 23 x 8 2 bladed prop.

Now with confident we can obtain similar performance if we change the prop 23 x 8 - 2 bladed to 21 x 8 - 3 bladed prop.... The available thrust reading also similar i.e. 12.89 kg vs 13.03 kg for 3 bladed.

Those scale modellers can now smile!!! don't have to purchase a few sizes to suit their engines....

pe reivers

Umran,
I updated my previous post where I stated thrusts @V0
I have been playing around with your spreadsheet. Some items however are not quite clear yet.
Please consider:
Pitch:
do you mean true pitch, measured from the neutral line? Or Tangent pitch.
Do you compensate for propeller airfoil camber? (zero lift angle Alpha-0) some data:
  Clark-YMOD (15%) Alpha0=-2.4°
  NACA3415 Alpha0=-3° (thicker foil for wood propellers)
  Aquila Alpha0=-3.2° (foil much like used in rev-up propellers)
If you add up the zero-lift angle to the true pitch angle, true pitch needs a large correction of up to 20%. This may be even 30% higher than the pitch dedicated on the propeller, which is foil/tangent pitch. That is why some makes have rediculously high prop factors. (Graupner?)

In flight rpm:
Standard procedure is to add 1000 rpm at top speed (thumb rule, debatable) With electric motors, this means less drive power. With Gas engines the power gained/lost depends on muffler system tuning rpm Vs power graph. Top speed however is calculated using the 1000 rpm+ rule. In your calculator there is room to integrate this IMHO.

In my thumb rule calculator I accounted for these items (along with blade shape as manufactured) using the propeller constant. Can you integrate that in your calculations? That would be good!

So far, using the above mentioned arguments, I found insufficient differences to change anything.

Umran

Pe,
The provided spreadsheet is an example of how to use the solution method drafted by me for my thesis work.

Let us discuss it in detail a bit more.
BEM method was the integration of Blade Element with Momentum theorems. It was in place for quite sometimes now to predict the behavior of the propeller. Some Aerodynamic Scholars have proposed a close loop solution (refer to Aerodynamics, Aeronautics, and Flight Mechanics by Barnes W. McCormick). The close loop solutions even though easy to accomplish, the results were a bit far from actual by say around 10 to 15%. The reason for this is, the assumptions that they made in order to close the loop.

Due to this, some recent Scholars have suggested iteration technique, the paper can be found as link in my first post. Like i said before, i put the suggestion for practical usage, unfortunately it is very difficult to converge to usable values.

Looking at this, after a few months of working on the derivations the equations, finally i managed to make it converge almost most of the time and the results are useful enough with margin of error less than 10%.

The Concept of Solutions
Each propeller blade will be divided into smaller elements (in the spreadsheet, 20 of them - not limited to this number, any number also can but the more it is slightly more accurate it will be but the time will be longer to iterate). Forces acting on those elements will be evaluated individually based on established aerodynamic theorem and counter checked with conservation of momentum. These processes require the introductions of interlink variables between the two concepts. Finally summations of all those forces from each element will make up for total thrust, torque and power required.

When you asked about the pitch, yes, in the example there are 2 cell (F7, F8) that depict the individual element pitch, one is in radian and another is in degree. The calculation for each element angle based on the position of the element along the spanwise direction from the center. It has been modeled as such that anyone of the element, if being subjected to a rotation will move forward by the same amount. Say the first element which is located closes to the root, the angle will be large in order to arrive forward at the same time as the element that is at the tip which off course will be having smaller pitch angle.

When you say about airfoil chamber, No, it is not in. Like i mentioned before, i just use 'clark y' airfoil data with flat bottom as the initial ref. The technique is to plot the Cl and Cd based on physical testing data of it, add a curve fitting technique and produce a function of Cl = Cl(a) and Cd = Cd(a). the variable 'a' here is the actual angle of attack on each element. Now if we want to use some other airfoil, perhaps with certain degree of chamber, then we search out for the actual data for that airfoil, chances are, some professors somewhere have test it in windtunnel and produced the result. Once we have those data, we plot again to produce Cl and Cd functions based on angle of attack.

When we talk about speed, yes we can model it directly. Say our engine can rotate 23 x 8 prop at 7000 rpm at full throttle statically. Based on the spreadsheet the amount of power required is ~ 5HP. Now we add some forward speed, say 35mph... with the same 7000rpm, we can see a reduction in power required, i.e. it reads only 3.83HP. From first scenario we know our engine can produce 5HP at WOT, this clearly shows that the rpm will climb to match the power avail if we keep the throttle at full open. How much increment? Then we need to feed in the rpm number so that it match the on ground scenario on power required - give a little compensation as the higher you're in the air the less density the air is...(you may add the flying altitude for compensation) Say in this example, i found out that the rpm will climb to 7600, i.e. 600 rpm extra. This effectively increases the pitch speed from 53mph to 57.6mph.

The main thing here is, all other parameter can be changed as required, say your CL, CD, rpm, prop diameter, prop pitch, chord profile and lots more. However the derivation towards the solution is the key here! And you my friend are the lucky one having to look at this even before my University Senate catch hold of it.

pe reivers

Thank you Umran. I appreciate that this work is for students thesis.

Now that I know that you use true pitch (foil centerline pitch) and translate it to angles I can check my graphs, not by iterations, but by plotting single conditions, and manage my rule of thumb formulae to closely resemble these plots. Bear in mind, that my work is more of a practical nature than scientific. It is the results that count, not the road toward those results. I guess that for you, the road is more important?

I will try to make a single speed simulation with known engine (rpm/power curve available) from static to max speed using a Clark-Y modified foil. Give me some time to work on it.

PS
for airfoil simulations I use the Profili computer program by Stefano Duranti. It is worth every penny I spent on it, and he is always very helpful when I need a new computer installation code. www.profili2.com

Umran

Yes Pe,
I just pave the way so that user can use it as per their requirements. But do not forget, alpha is actual angle of attack i.e. it itself is a function of induce velocity in axial and radial direction.