PROPELLERS

Laminated glued , dowelled propeller block before carving Initial trials The product Imported Power fin

Currently I have three new propellers for teh Hirth F33 engine. A wood two blade 57x34. a 3 blade Ultraprop and a power fin 60x28

Warning:Propellers =Danger. They are like spinning matchets ( panga) at 2400 rpm.

Propellers provide the forward propelling force (thrust) and are spun by the power plant ( engines)of various types. The power plants could be piston engines, turboprop. Small aircraft with gross weight not higher than 1000 kg will almost always be powered by piston engine.Generally the bigger the propeller the better the efficiency given a specific engine power.
An engine RPM denotes Rotation Per Minute.

***************************************
To properly classify an engine one needs to know Power ,Max torque and at what RPM it is attained. a torque, power vs rpm graph from the manufacture provides this info. The reduction ratio is also important.

My small engine Hirth f33 has a reduction ratio of 2.5 max power of 28 hp, at 6200 rpm. Torque is the turning force moment measured in Pound feet (lbft)or Newton meter.(NM)
Most auto engines attain max torque at mid rpms while aircraft engines are peaky, that is max torque RPM is attained near their peak RPms. My small engine attains max torque at 6200 RPM near its peak rpm of 6500.
For a given engine power the slower the speed of the propeller at best torque Rpm, the bigger the diameter it can handle and the better the efficiency of the propeller and hence most engines will have a reduction drive either belt,like my small engine or geared.

Some engines which by default run slowly drive propellers directly. These direct drives will most likely produce best torque at 2500 -3000 which does not make sense to reduce the speed.

Max big diameter even if desirable may be limited by practical reasons since you need to reserve some space from ground to spinning propeller tip. The right brothers with only 18 hp available used two huge wooden propellers by way of chain reduction which made the propellers use most of the available 18 hp.

***************************************
The propeller blade is not just angled but twisted, so tip angle is less that root angle.
A propeller can have 2 ,3,4. etc blades. More blades same diameter take more power even at lower pitch but got more static thrust so it depends on application.

***************************************

Measuring RPM with optical Tacometer Measuring static thrust with two spring balances 60 kg for 3 blade 59 inch prop 2200 rpm on hirth f33

PITCH. Is the bite of the prop into the air measured in inches
Alternatively in degrees at the tip or a specific place along the blade. The desired cruise is the determinant of the pitch, engine power allowing.
A 42 inch propeller will theoretically move an aircraft forward by 42 inches in one rotation of the propeller. Thus the reason why the bite angle is more at the slower root than at the faster tip. Slip is the gap between theoretical pitch and actual.
Props can have fixed pitch or adjustable. Wooden propellers will often be fixed pitch. Adjustable pitch props can be either ground adjustable or in flight adjustable. You need lower pitch at takeoff and higher pitch for cruise. Fixed pitch balances in between. Fixed speed governed props spin at rpm specific speed while pitch is continuously adjusted on need basis.
Over pitched blades can stall especially at the tips which makes them less effective and vibrate.

Tip speeds should also not exceed speed of sound(match 1). STALL is loss of smooth aerodynamic flow on the curved side of the blade. A prop blade is actually a micro wing airo foil.

**********************************************

When pitch in degrees is known or measured at the tip or any other radius along the blade then

Pitch in inches =Radius at the point x2xPixTangent of pitch angle

This is true if you measure angle at most points from half way to tip and remains a constant. Eg 10 degrees pitch angle at the tip of 55 inch diameter prop gives pitch of 30.45 inches within a small margin of error.

This is also helpful for carving own wooden props like I did earlier as above.if one gets hold of a fixed pitch propeller whose pitch is not indicated, this helps determine it quickly.

*******************************************
Prop vibration is more pronounced with 2 blades than 3 blades. Even for for my two types the vibration is evident for the two blade. Propellers need be dynamically balanced just like auto tires due to the high rpm-s they spin with to avoid vibration and failure. Earlier on in this project unbalanced propellers used to break our mounting bolts.

*********************************************
Materials for propellers are often wood, metal alloys, composite or fiberglass. I have one carved wooden, and two fiberglass 3 blade and 2 blade.
Leading edge protection is a strip of metal or tape to protect the blade from damage from hard objects sucked by the prop. prop slip stream is that cylinder of air that's pushed back as the aircraft moves forward. this stream is spinning too in the direction of the prop. During static thrust tests all that gust of air moving back is slipstream and becomes less and less as the craft gains speed.

***************************************************

I have a 60 inch two blade Power fin adjustable with a protractor

A 3 blade ultra prop adjustable with pitch blocks (10-15 deg pitch blocks)

a Tennessee prop 2 blade 57x34

*****************************************************
Even an increase of an inch in diameter or an increase in a degree of pitch greatly increases the power required to turn it at specific RPM. In my small engine an increase of tip angle from 9 to 12 degrees in the adjustable type reduces static peak rpm to 1900 from 2200. Most prop fiber blades diameter can be trimmed slightly to position them. In this video the blades was trimmed from 55 inch to 53 inch in order to take in a specific desired pitch at 2300 RPM. Often you trim and seal tips with two part epoxy according to manufactures instructions. Some you simply chop like the utra prop.

***********************************************
Static thrust is the pull when the engine throttle is at maximum and the frame is static. A good strong Spring balance measures it effectively. Measured in pounds or kgs . Ikg =2.2 pounds. Lower pitch = higher static thrust but lower cruise speed and vice versa. Prop rpm can be measured by rpm sensor in the engine or a remote optical rpm meter. A digital protractor sets the pitch quite well. One needs to ensure that all blades are equally set. If an engine lies a head of the pilot,The prop is a tractor setup. otherwise its a pusher.

************************************************

The direction of hub spin. belt drive does no change the spin direction from that of engine. Gear drive often reverses the spin, hence the RHS or LHS spinning props. Most small props have 6 bolt holes and one inch centering pilot hole.

*************************************************

Feathering

This is setting of prop pitch to 90 degrees i.e the blade is set parallel to the direction of flight.