Prop Blade Twist

The blades of aircraft propellors are intentionally twisted so the the blade has the same angle of attack at the tip as at the hub. From the pleasure boats I've seen, this is not done. Is this a cost saving measure, or is it not really necessary in boats? Are there any manufacturers that do make propellors with twisted blades? I've seen a few tug boat propellors that looked like they had a twist to the blade.

 

Not sure what you are looking at when you say boat propellers are not twisted. Most are made with quite precise pitch angle.

Propellers on airplanes are not necessary set with a constant angle of attack for all radial positions either. The section usually changes being thick at the hub and thinner at the tip. Some also have adjustable pitch so the AoA must vary depending on pitch setting. For a fixed prop the AoA can be optimised over the length of blade to get best efficiency and this does not result in constant AoA across the blade to get the best pressure profile.

Here is a reference covering boat propellers:
http://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/propeller_geometry.pdf

Also compare the pitch angle of the blade at the hub compared with the angle at the tip for this prop:
http://www.deepblueyachtsupply.com/images/productimages/apollo_propeller.jpg

Rick W.

 

Also remember that an aircrafts prop is much larger and therefore the tip is moving at a much higher speed then the hub, so to offset this you have a finer pitch at the tip and a much coarser pitch and deeper chord closer to the hub, this is an attempt to even out as much as possible the efficiency of the prop across it's entire span.
As a boats prop is of a much smaller diameter when compared to an aircrafts then there is less need to offset for tip speed effect.
Aircraft with variable pitch or constant speed props still have reduced pitch/chord further out to the tip..When you alter the pitch on a variable pitch prop on an aircraft you alter it at the hub.

Mychael

Mychael

 

I guess my comment is more targeted at pleasure boats. I swear some of the blades I'd seen on ski boats had absolutely no twist to them. Maybe it's just my eyes playing tricks on me.

 

If you are looking at a directly driven small diameter, low pitch prop with a relatively large hub it will not have much twist. The pitch may only be 10". There are formulas in those links provided that you can use to work out the pitch angle at any radial position.

Rick W

 

The purpose of the twist is not to maintain the same AOA, the twist has the effect of having each section along the blade with a different AOA. the twist is to maintain the same amount of thrust from the hub to the tip so you are having the fluid within the propeller disk area all being accelerated aft at the same speed. it is more efficient that way, hence the reason for the twist. Props without twist also work, just not as well. Where the foil at the hub is moving at relatively lower speed it needs a lot more AOA than the tip, which is moving faster. On a high aspect ratio prop blade the twist is very noticeable, on a small boat prop blade with not much blade span, the twist is just not as noticeable. But both well designed boat props and aircraft props will have twist, both operate in a "fluid".

The thicker foil at the root is only a practical issue, there is no fluid mechanical advantage to it. If you could make any prop out of supper strong material it would be beneficial to have the root foil as thin as the tip. But because we live in a real world with cost considerations, props always have thicker foil sections at the root.

There are some fairly strait forward equations to determine what the AOA of any given section should be along the length of the blade. You should be able to find them from lecture notes off of some University website. I have them somewhere in a box of my ancient notes from engineering school.

 

Petros
I believe you will find you are confusing pitch angle and AoA. The pitch angle for zero angle of attack aligns the blade to the flow vector at all radial positions when the prop is advancing the nominated pitch during each revolution. If this is the case then there is zero angle of attack for the flow over the prop at all radial positions.

One of the papers I linked to discusses this in detail. It gets very confusing with cambered blades because these still have lift at zero angle of attack. So even with zero angle of attack the blade still provides thrust.

Rick

Rick W

 

Both boat and aircraft propellers all maintain the same pitch angle over the full length of the blade. This requires the the blades to twist in order to maintain the same pitch and therefore the same degree of slip along the length.

At low pitch and diameter, it may not look like there is twist but if you actually measure it, it will be there.

Petros is correct about the thickening of the chord toward the blade root. This is much more pronounced in a wooden prop than aluminum or other material because of strength and stiffness requirements. I recently made a wooden prop for an experimental aircraft with a pitch of approximately 55. I say approximately because it is very difficult to determine the chord angle on a thick blade, especially near the blade root.

Rick, your statement on AoA and pitch leads me to a question. What is the relationship between AoA and slip? Does that mean that a negative AoA also means a negative slip?

 

Mychael,

What you wrote is untrue, what you are confusing is camber with thickness. While thick airfoils can have a lot of upper surface camber, highly cambered THIN foils still have better performance (L/D) than highly cambered think airfoils.

Rick,

No, I am not confusing pitch and AOA. The pitch angle is dependent on both free stream speed and rotational speed of the prop. In order to generate thrust you need to have an angle of attack above the pitch angle.

And the term "slip" I think is an obsolete misnomer, there is not really such an idea in modern fluid mechanics. The term comes from observed behavior when fluids, both gas and liquid, were not well understood. It tends to hang on amount boaters. For a fluid of any type to generate a force (or lift), you have to curve or accelerate the fluid. The mass of the fluid times the acceleration is the force you generate. When a boat hull "slips" to lee, it is only the angle of attack on the keel and hull you need to counteract the lateral sail force. The boat is still traveling in a strait line, and while it appears to be "slipping" as if you where on a solid surface with wheels pointed strait ahead, it is actually the AOA of the hull you are observing. Occationally this term is applied to propellers, but this is the same issue since a prop is a foil moving around in a circle. The AOA is the amount of precived "slip" you need to generate the thrust. So it is not really "slip" at all.

 

Petros
Pitch angle for a propeller blade is a function of the design pitch (p - distance advance per revolution) and the radial position, r, on the blade. It is a simple inverse tan function. Nothing to do with speeds. I have exctracted the attached image showing the formula from the previously linked paper.

I have also included the figure that defines the other angles of significance regarding the foil and its application to propellers.

It is not unusual to run with negative angles of attack with a highly cambered foil but they are still providing lift. It is also not uncommon to have some variation in pitch with radius over the blade. If this is the case, the pitch value for the prop is usually given at 75% of the full blade radius.

Rick W.

 

Tom
My previous post with the image and pitch definition might have already answered the question.

The reference line on a foil for determining AoA is usually taken from nose to tail as shown in the drawing. With a cambered foil you still get lift if the free stream flow is aligned with the foil nose to tail line. So at zero AoA there is still lift for typical blade foils.

There is another angle of interest regarding the blade and that is the angle that results in zero thrust. As noted in the definitions under the diagram this is referred to as the geometric pitch.

So with reference to the geometric pitch there should be no thrust and the AoA will be negative. As far as I know the pitch of a prop is usually given as the geometric pitch. That is the advance required for each revolution to produce zero thrust. So for a correctly aligned shaft you cannot get negative slip if the prop is providing thrust.

I have seen negative slip with angled shafts in lightly loaded applications. The reason is that the required operating AoA for a horizontal shaft will be very small or even negative. The shaft angle adds to this on the down-going side and subtracts from it on the up-going side so you get extremely high forward thrust on the down-going and some reverse thrust on the up-going. The result is that the blade actually advances more than its geometric pitch each revolution so has negative slip.

I have actually seen this being discussed in terms of the prop being better than 100% efficient because it has been determined to have negative slip. This is untrue. It is simply that they did not allow for the shaft angle. Shaft angles introduce nasty loads on the blades that are more noticeable with high efficiency props designed for low slip. If the slip is high then it is operating at a higher angle of attack and angling the shaft does not cause as much force variation as the blades rotate.

Rick W

 

Yeah, that's the word I was was trying to think of when I wrote it up before. Your getting out of my level of knowledge now talking of highly cambered thin airfoils. I know that a thinner airfoil will produce less drag but I'm still not sure about it's relative efficiency at low air/fluid speeds when compared to highly cambered thick airfoils.
If what you say is correct (and I can only relate it to my aviation experience)
then why for low speed/low stall/high lift wings we see the thick cambered type and not the thin ones? And yes I know this started as props but surely the principle is the same.

Mychael

 

Thanks Rick, I don't deal with angled shafts much and so had not considered the effects of that. It would seem that a cambered blade of zero pitch could produce thrust though.