larrysogla

We hear a lot about static thrust....but isn't the propeller in full cavitation at static thrust.........and so just how useful is it?????
Larry

bogbeagle

I guess that, if you didn't have any, your aeroplane wouldn't move.

ThumbSkull

My Feedback: (57)

It is not very useful for a speed plane, but for 3D, it helps you to know how much the engine/prop combo can lift in a hover.

Sport_Pilot

Air propellers do not cavitate.  And they are not in a stall condition at static either.  The propeller moves air and the moving air column prevents the prop from stalling.  Static thrust is really only good for information on 3D, take off, and climb. 

MajorTomski

Well there are two conditions we're looking at:

A very low pitch prop will be operating in its optimal angles of attack and the static thrust you see will be closely equivilent to the flying thrust. As stated above it does become a factor in 3D and the hover because in a hover all you have is static thrust!


BUT for a high speed plane, with a very high pitched prop there is the possiblity that the prop, with the engine at full throttle and the plane not moving, it operating at such a high AOA that it is running very inefficiently. In this case the static thrust comes no where close to the thrust of the prop when the plane is flying at speed.

Find the videos of the Supermarine S-5 and S-6 racers and watch them take off. They had very very high-pitch fixed pitch-props to achieve high speed. As a result the props are very inefficient in the take off speed ranges. Watch how long it take these planse to get up to speed.

Sport_Pilot

Most of the props we are using will have a higher static thrust than flying thust unless hanging from the prop.  For very high speed the static trust will be lower than the midrange thrust, and for a very sleek plane the thrust at top speed will likely (not always) be lower than the static thrust.  Because of the moving air column you would have to have pitchs to diameter ration much greater than 1 to 1 to be even close to stalling.  But stalling could occure when the throttle is opened up quickly.

freakingfast

My Feedback: (3)

Well, one prop that I know of that seems to stall at static run up is an APC 10X10. Ive ran this prop on several engines and the results are the same. When you tach the engine, your at first pleasantly surprised at the great numbers, but when you take off and you start to gain speed you can here the engine lode down as the prop starts get a good clean bite.

To simplify, the slower your intended top flight speed is and the shallower the prop pitch is the more useful static thrust is. 3D plane yes, speed plane....useless.

Sport_Pilot

By lode down the engine is picking up speed?

Lnewqban

Larry,

Just in case you are asking about a boat propeller:

"Cavitation is the formation and then immediate implosion of cavities in a liquid – i.e. small liquid-free zones ("bubbles") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low.
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.
As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapour pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades."

http://en.wikipedia.org/wiki/Cavitation

earlwb

A far as I remember, no one used static thrust numbers with internal combustion engine powered planes. That seems to be unique to the electric motor people, where they are interested in static thrust as they use static thrust, RPMs, prop size and pitch, along with voltage and current drawn to decide what props to use on their airplanes.  But with glow engines or gasoline engines static thrust just wasn't of interest to the flyers. More recently though the 3D people with their hovering and vertical pullouts had become more interested in static thrust.

But with some of the engine power spreadsheets out there, one can enter the rpms, and prop size and pitch and get a fairly close representation of how their engine is performing with different props, etc. Granted it isn't perfect, but it works.

pe reivers

Let's not discuss cavitation here. Air is not in a liquid state. Only propellers in a liquid can cavitate.
When the true pitch of a prop is smaller than the diameter, the blades will not stall in static tests, and thrust can safely be assumed to reduce from static to full speed in a quite linear manner.
I talk about true pitch. Pitch like on our props is not true pitch because it is measured along the bottom of the prop airfoil. True pitch is using the airfoil neutral line for reference.
So the APC 10x10 that showed some stalling of the blades had at least a true pitch of 11" with pitch measured using the airfoil neutral line. With such high pitches, the prop wash vortex that provides incoming air speed is insufficient to keep the effective angle of attack below 10°, and this can be noticed by decreased lift/drag ratios and increased noise. (not yet stalled!). Stall sets in at about 13° AoA, when the max thrust level decreases again, unless plane speed is added to the equation.

Sport_Pilot

I have run props greater that 1 to 1 pitch with no stalling.  However I have had stalling when jamming the throttle foward.  The engine will not get to peak speed, the prop tips vibrate and the propwash buffets the wing and tail more violently.   Advance the trottle slowly and there is no proplom.  A plane with this condition will take a long take off run.  Because of this you would hold the plane and advance the thottle slowly to full throttle  and if it sounds like it should then let go.

Turbine fanjet engines have pitch ratios much much larger than 1 to 1.  But because they are ducted, and the engine cannot spool up as quickly they normally have no issue with this.

pe reivers

makes perfect sense.
In turbines, like in biplanes, stalling is prevented by the adjacent blades.