Can a jet drive stator stator fully counteract the motor torque reaction?

Thanks Rick: You beat me to it! I was not happy with my most recent post and chose to edit it this morning. Your reply just came in! Just out of curiosity, what would my top speed be at 32W if I can achieve 10mph at 7.2W?

 

If the motor will spin up to 18,000rpm then you will get 15.6mph at 32W on the shaft. The velocity ratio is 1.23. This is for the 50sq.in case. Power very close to cube relationship with speed.

Rick W.

 

Sounds great to me!
:?: There are a couple of questions I'd like to ask about the impeller design though. What clearance would you recommend between the rotor tips and the I.D. of the shroud? Logic indicates that reducing this to a practical minimum on the order of 0.005" would be beneficial. However, at this small size, I expect that the hydrodynamics of the setup may have a few surprises in store. Also. I'm wondering about projected area. What would you recommend? I chose a three bladed design to avoid setting up harmonics with the stator so a large projected area (APR 70%) means a wide blade chord given only three blades. How would a large projected area effect efficiency?
By the way, How does one go about inserting images into one's posts? I've tried but the site requests an image URL. I don' have a web site but I DO have quite an extensive collection of documentation that I would like to be able to share with members from time to time. The current situation is a prime example. A few pix of the intended torpedo/impeller designs would sure help.

 

Clearance - I don't think the water molecules will mind being squeezed. On the other hand the induced drag is a global effect of the order of a couple of blade chords at least so there is no need to have minute clearances.

Blade Aspect or EAR - I used JavaProp to do the analysis. I selected a 6% asymmetric foil as this one had the lowest Re#. The blade maximum chord for the high power case was 4.3mm. I have not worked out the EAR but it will be quite low. A thicker foil section would have even smaller chord. The efficiency of the prop is related to blade aspect. The higher the aspect ratio the more efficient for an unshrouded prop. With shrouding it is not as critical but using more blade area than needed means you have to drag more material through the water at high speed.

The blade size will more depend on the required strength. Altering size a bit will affect efficiency but not that much. If you are buying a prop then I would go for the lowest EAR available. The prop loading is only medium. Most boat props are heavily loaded. Going to 2 blades the chord goes up to 6.4mm but I agree that the vane passing could be an issue. When you think about it the loss of area created by the vanes probably adjusts for the velocity gain so necking is of doubtful benefit.

One other aspect I should point out is that my drag calculation is for a submerged body. That means it needs to be at least 3D under the surface to avoid wave drag. I was wondering how you will control the level. I expect you would lose radio contact if it goes too deep.

Rick W.

 

Thanks Rick: I am quite surprised at the efficiency figure (82%+)you obtained for the projected 1:20 scale prop (impeller). How confident are you in it? I only question the results because small propellers are significantly less efficient than larger ones all things being equal and an efficiency of 82%+ is good even for the prototype prop!

I calculate the maximum projected area to be only 21.86% of the disc (hub = 9.97%, blades = 11.89%). At a diameter of only 19mm with a max chord of 4.3mm and a foil thickness of 6%, the hydrodynamics are significantly altered by the so called "scale effect". Relative to the hydrofoil, the water molecules are 20x larger that they would be for a full sized foil. I wonder whether the program that you used takes this scale effect into consideration.

The torpedo will be launched at a minimum depth of 4". It will be ballasted to be just BARELY positively buoyant so I would expect that it will take a quite a few seconds to drift up to the surface especially at high speed even if I did not have RC. However, I will have control of both the direction and depth. :idea: If the depth control becomes too much to handle manually, I have the option of installing an APC-4 unit. This is a micro sized solid state gyro that automatically controls the pitch angle of model submarines under water. It can be adapted to do the same for the torpedo.

 

The little prop is just that - little. But how many big props do you see doing 16,000rpm. The end result is that the Reynolds number is up around 50,000. The foil data I used was based on this.

The actual foil modelled was a cambered 6% foil - see attached. Given the short chord this is quite thin. So to some degree it depends on how well the foil is shaped. A deburred piece cut from a tin can would drop about 10% in efficiency. So might do say 1mph slower. There is benefit from the ducting as well. It is a while ago now and I cannot remember the detail.

I expect a bit of deburred tinned can twisted and soldered on to a hub would get about 60% if it was unducted. Ducting and a nice cambered foil as shown will get over 80%. Depends on how accurate the prop is manufactured. If you buy one you can try as is and then see if you can improve performance by shaping.

If you have not shaped a blade before, remember the lifting face is toward the front.

Going up to a 10% foil with a flat back side will not alter things much and will be noticeably stronger. I think working with anything under 0.5mm would be difficult. On the other hand you can get really carried away with perfection here for very little benefit. The power requirement goes up with the cube of speed so dropping from 80% efficiency to 60% will reduce top speed by 10%.

I am interested in your results as always but I would be surprised if my estimates are not in the ball park. I would go looking for what you have wrong before I question the numbers.

Rick

 

The props I run on pedal boats and electric drive have an EAR of around 12%. The blades are lightly loaded so very efficient. Having more area than needed is just more area to drag through the water.

The constraint with you blades will be to get adequate strength at only 6%. I do not think this is practical so you can go to 10% and say 5mm chord. These would have a curved front face and flat back face. Somewhere between 4 to 5% camber. Something more like the Eppler attached. The differences with these things are no more than 1 to 2% in efficiency.

Rick W

 

Sounds good. Love the under-cambered foil. I had expected a flat pressure face. Obviously, the foil thickness will have to increase from the tip to the root to maintain the structural integrity of the blade. :!: I am certainly not going to fabricate the blades from bits of a tin can!. If I decide to make the blades out of brass, the best way would be to fabricate an accurate pattern in resin. A rubber mould taken from this would allow me to cast up a bunch of wax blade patterns. I might even be able to skip the wax! There is a special two part resin called Duralay which is used to make direct patterns for dental inlays. Duralay burns out just like wax. The blades could then be cast in brass via a lost wax process. I just took a look at JavaProp. Looks like a great applet though it's going to take me some time to learn to use it.

 

I can make a prop out of steel flatbar that will get better than 85% efficiency. A milled prop gets at best 1% better efficiency. By comparison most outboard props are lucky to get 60% but then they are usually more heavily loaded.

If you use JavaProp you can compare a flat plate with the other foils. It will surprise you how well it performs. So the tin can is not too bad. Might need a rib of solder to get the strength near the hub but it is a fast means to test the whole thing out.

Rick W

 

I am VERY surprised! If by a flat plate, you mean that you are dispensing with the helicoidal axial blade twist as well as the cambered suction surface.
That would mean that you would have a variable pitch going from 27mm at the blade tip to 8.53mm at the blade root. If my assumptions are correct, the effective pitch would be about 19.1mm at 0.707r. With a flat suction surface the thrust would be developed purely by the backward acceleration of H2O by virtue of the blade attack angle according to Newton's third law. You are also dispensing with the hydrodynamic pressure differential between the suction and pressure surfaces generated by the camber according to Bernoulli's principle. This generates most of the thrust at higher speeds! This CAN'T be over 85% efficient!! It violates just about everything I know about propeller design.

 

The flat shape refers to the chord. It is one of the options available in JavaProp. You have misread my post. What I said is that you would lose about 10% going to a flat plate. You can try it yourself as it is the default profile in JavaProp. The flat plate is either very thin or has a rounded nose and tapered tail.

Even a flat plate will fly it just has a narrow operating range.

The steel flatbar props I make have about 2 hours of hand grinding to profile them:
http://boatdesign.net/forums/attachment.php?attachmentid=23371&d=1215983491

Rick W

 

Thanks Rick, I think I understand what you mean. Sounds a lot like the flat X-sect blades that were used in the earliest propeller designs. I notice that the helicoidal blade twist is featured in the example that you attached. It has occurred to me that cancellation of the torque reaction with a single stator will, under the best of circumstances, be very much like walking a tightrope! :idea: Even though two contra-rotating props will be a pain to set up at this small size, I'm beginning to think that this may be the most viable solution after all! The designers of full sized torpedoes knew what they were doing. I know that the aft prop has to have an slightly smaller diameter, opposite hand and have a higher pitch than the fwd prop. :?: Is there a rule of thumb relating the aft to forward prop pitch? The torpedo would have the aft prop set just behind the forward one. I have the specs for a system used on the USS Albacore. In her case, the spacing between propellers was about half the forward prop diameter. The aft propeller pitch was approximately 1.2 x the fwd propeller pitch.

 

You need to recalculate the prop using JavaProp at half the thrust. The flow plot will give the velocity ratio. Set the velocity for the second prop at the design velocity times the velocity ratio for the first prop. From memory the ratio was quite low. Much smaller than 1.2 but I would need to go back over what I did. If you select a pitch that is too high only one prop will work anyhow.

I expect it will be unnecessary complexity given the low torque. As originally stated I expect it would be quite easy to ballast the hull to counter the moderate torque.

I noticed in recent scanning of human powered subs that most prefer high efficiency unducted single props. Just use ballast and control surfaces to counter the torque.

Rick W

 

Thanks Rick: I agree that the best and most efficient drive arrangement will probably be the single prop and stator. I think that the contra-rotating props could be made to work but the system is inherently inefficient as power is wasted in the reversing gear and the coaxial shaft bearings and seals. I asked a retired submariner friend why WWII torpedo designers used contra-rotating props as opposed to a single prop and stator. He thinks that they never thought of applying axial turbine technology (stator) to a marine final drive system.
:idea: Each of the AGSS-569 Albacore's contra-rotating props was driven by a separate motor as opposed to a reversing gear. As such, speed of each shaft could be varied. This might account for the seemingly great disparity in pitch ratios (APR 1.2) between the fwd and aft.props.

 

Those props indicate very high velocity ratio compared with what I like to achieve.

If you think about your torpedo you are running a prop similar in diameter to the actual hull. So by comparison with Albacore's prop it is a monster.

If the Albacore prop was the same diameter as the hull then it would have been more efficient but it would have been difficult to make the blade root strong enough.

It is a great photo. It just shows how clean that whole design is back aft. only compromise on that boat was the sail.

When are we going to see the first test run of the torpedo?

Rick W