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#### Atomic_Sheep

##### Well-Known Member
Hi,

I'm reading some material in relation to propeller design and don't understand the theory.

I don't understand is the vector V, why is it going from bottom to top? Shouldn't it be coming from the top of figure 1.8?
Same for 2 pi r n, shouldn't rotational wind be coming from right to left not right to left?

Figure 2-2 makes complete sense.

Also:

How does Figure 2-2 translate into figure 3.6? If you have a linear rotational wind and a constant forward wind, adding those two vectors together would result in a shift of the linear rotational wind vectors but not cause a curve (quadratic function) as seen in figure 3.5??? You're adding a fixed quantity to a linear function and getting a quadratic???

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#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
I don't understand is the vector V, why is it going from bottom to top? Shouldn't it be coming from the top of figure 1.8?
Same for 2 pi r n, shouldn't rotational wind be coming from right to left not right to left?
Those vectors represent the direction of travel of the propeller, not the air.

How does Figure 2-2 translate into figure 3.6? If you have a linear rotational wind and a constant forward wind, adding those two vectors together would result in a shift of the linear rotational wind vectors but not cause a curve (quadratic function) as seen in figure 3.5??? You're adding a fixed quantity to a linear function and getting a quadratic???
Notice the equation in Figure 3-6. Vp is not Vr + V. It is the square root of the sum of Vr2 and V2. (It's the equation for the length of the hypotenuse of the triangle in Fig. 2-1.)

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Lift of any little piece of a foil is dL = rho/2*v^2*Cl*dA. Drag is similar. See where v is squared. So, with speed increasing linearly with radius, lift will increase with radius squared. It has to look parabolic.

There are things decreasing lift outboard too. Chord tends to be tapered and there are always elliptical looking losses toward the tip, modifying the form of the lift curve.

Billski

#### Jan Carlsson

##### Well-Known Member
There is no Theory in propeller design, it is all black magic

#### BJC

##### Well-Known Member
HBA Supporter
There is no Theory in propeller design, it is all black magic
Anyone that can interpret those graphs on Jan's forehead can easily design and build a propeller.

BJC

#### Jan Carlsson

##### Well-Known Member
looking at the Pictures in OP, I say it is a good start to understand what a propeller do, but there is more to it when you have a fuselage on one or the other side of the propeller.
far behind the propeller the air have increased in speed by not so Little, half of that increase will be at the prop disk.

And the fuselage have changed the airspeed, so it will be different along the radius of the prop, (and all around the prop circle) and probably mostly slower then the Aircraft speed. if the airplane is flying without a propeller that slowed down speed will be the nominal Wake or hull effect, then the propeller suck air in and speed this up, and will easier speed up the slower nominal speed, than the faster, this new Wake speed will be the effectiv Wake or hull effect speed.

The faster the airplane go the smaller the increase in airspeed far behind the prop disk will be, the prop wash will be max at max Power and zero forward speed, at flying speeds it might be 10-25% of forward speed, it just depends on Everything

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
...it just depends on Everything
That pretty well sums up aircraft design! Well said.

Thanks guys.

HBA Supporter