Prop with constant chord length?

[Edwinh]

Hello,

I'm quite new to the forum, Rick allready helped me with some quiestions, but it's nice to share ideas with other people as well.
We are going to race in the Frisian Solar Challenge. I tried designing a prop using Javaprop, which looks quite nice now.
But if I look at other props from other teams I see props with a constant chord length. Sort of like choosing shrouded rotor in Javaprop.

If I chose shrouded rotor the efficiency looks better, but is this true if I use a prop without the shroud. If I see the helpfile one speaks about tiploss. I can't quite figure aout the bads or the benefits.

Can it be interesting to have a constand chord lenght, or will this only add too much drag at the tips?

I hope someone can provide some insight.

Edwin

[jehardiman]

The only reason to use constant cord to the tip is if you are using a nozzle. In this case the clearence between the prop tip and the nozzle should be as small as possible , <0.5% D.

On the other hand, maximum efficency of a prop in a nozzle is always less than an open water propeller. It is possible to design a prop with constant cord to the tip, but as you surmize it is less efficient due to the induced drag of the tip losses.

SUBHUMAN II's (the initial HPV sub record holder) props were designed with constant cord up to about the 85% r point, then the cord line was swept forward, the cord was tapered elliptically, and the pitch was twisted off to give an AoA at design speed of ~0. These wheels are a wake adapted CR set with a D of ~0.7m a pitch of ~ 1.0m, rpm of ~180 and a constant cord of ~62mm. They are ~85% efficient open water at about 5.5 knts with ~3/4 hp in. However, when I designed SUBHUMAN III's wheels, I began the elliptical taper and forward sweep/skew at the root and ended up with a slightly more efficient ( ~87%) wheel set at a slightly higher speed. Most modern hpv props are similiar with elliptical or leaf shaped blades and varying sweep/skew with hubs that are too small IMHO.

Note that these prop sets are large and slow turning for thier loading. Your situation may require a smaller, faster turning, higher loaded wheel just from arrangement considerations.

J, What is wrong with too small of a hub?

[jehardiman]

Originally Posted by mark775

J, What is wrong with too small of a hub?

If the hub is too small, geometric pitch angle becomes near 90 and blade speed approaches zero especially if you consider the body wake. The blade area becomes inneffective in generating thrust in relation to it's drag and torque, so just get rid of it as the skin friction on the hub is less than the induced form drag of the blade.

[Edwinh]

Hi Jehardiman,

Thanks for the info. This is kind of what I expected.

Even if I do make a prop with constant chord length I end up with quite a thin blade (short chord) which starts looking fragile. So if I want to make a prop with constand chords, I have to think about a smaller diameter. This allready has less efficiancy, even without considering tiploss.
So mechanicaly it already gives an extra challenge.

The prop I "made" in Javaprop has the "standard" leave shaped blades, but nearing the hub I use the longest chord found in the design to have some strength in base the blades.
I use a diameter of 400mm, at 600rpm I want to generate 180N thrust.
I used E193 re300.000 foils for the outer sections, near the hub I use MH126 re500.000 to get a thicker stronger profile.

The image looks quite nice, but since local compatition uses props with constant chord I started to wonder if I should change the design.
I guess I better keep using the original design.

Edwin

It looks somewhat like this: