Rc sailrocket

[Glueandcoffee]

Sanding the foil and added a fence to stop ventilation

 

[AlexanderSahlin]

Looking forward to see your model sailing! As I commented earlier in this tread, your foil is probably a little oversize, and also seems to have quite much camber in the section. This is a good foil to start testing with. When you want to go faster you can reduce foil-area and probably also camber. When I make anti-ventilation fences, I use to make them extend around the leading-edge, 2 nose-radii or so. But I suggest that you start with the foil as you have built it.

 

[revintage]

Hi,
Nice project! In addition to what Alexander says you have to consider cavitation numbers, except for ventilation. In Tom Speers curves below, you might get a grip on what you need to go for(50kmh=27kn). About lifting forces you need to use CL3, that might be half or less of the CL2 numbers you have used. Let´s say you follow Alexanders advice to go for CL2=0.4 and hypothetical calcuations give CL3=0.2, you are down to one third of the force your previous calculations show. As you have a slanted foil the forces can be calculated for both horizontal and vertical lift. Curious, what profile are you using? These are my thoughts, but Alexander is the expert here.

 

[Glueandcoffee]

Hi,
Nice project! In addition to what Alexander says you have to consider cavitation numbers, except for ventilation. In Tom Speers curves below, you might get a grip on what you need to go for(50kmh=27kn). About lifting forces you need to use CL3, that might be half or less of the CL2 numbers you have used. Let´s say you follow Alexanders advice to go for CL2=0.4 and hypothetical calcuations give CL3=0.2, you are down to one third of the force your previous calculations show. As you have a slanted foil the forces can be calculated for both horizontal and vertical lift. Curious, what profile are you using? These are my thoughts, but Alexander is the expert here.
View attachment 165949

Thanks
Could you further explain cavitation numbers and the graph below. What do you mean by CL2 and CL3.
I do have a slanted foil but I'm not breaking the forces up into horizontal and vertical components as the lower section of the foil is parallel with the kant of the wing sail and the forces from each are resolved through the center of effort for each of them. Ie wing force = foil force . The upper part of the L foil will just lift the rear float clear of the water as it accelerates. the faster it goes the higher the ride height and the less area of the upper foil is in the water. Am I understanding your point correctly.
I'm also curious as to what profile I'm using. The first foil I made was cut and sanded from a piece of thick pvc pipe which was then heated and bent into shape. I had no design parameters in mind from the get go and it was very much done by eye. The second foil is not much better as I simply used the first one as a mold for the carbon fiber. Below the radius of the foil, the root chord is 80mm , thickness 12 mm , position of max thickness is about 25 mm from leading edge and I'm unsure how much camber there is but not much. I'll see what kind of naca profile I get with these dimensions. Keep in mind the profile is far from uniform along the span.

 

[Glueandcoffee]

NACA 2315 or something thereabouts. I know it's far too thick as well. To my knowledge at this scale I should be thinking something along the lines of 6 to 8% thickness rather than 15% .

 

[Glueandcoffee]

Also there is an orange wind warning tonight and tomorrow so I'm afraid there won't be any test runs on Thursday.

 

[AlexanderSahlin]

Hi Revintage and Glueandcoffee,
Design for avoiding cavitation and the graphs from Tom Speer make sense when you have passed 30 knots. What you see in the graph that Revintage posted is that you have to make a traditional hydrofoil-section with sharp trailing edge unpractically thin to avoid cavitation when going faster than some 50 knots. If you can fly the foil deeper in the water, where the static pressure is higher, you can increase the cavitation-speed a little. The AC-75:s seem to have foils with sharp trailing edges and will probably cavitate if they go faster than their current top-speed, 53 knots.
One solution if you want to go substantially faster than 50 knots is a base-ventilated section. The idea is that you have attached flow on both sides and an air-bubble behind the thick trailing edge. Of course you get some pressure-drag, but at high speed and moderate submergence, the pressure-difference between the air-pressure in that ventilation-bubble and the free-stream pressure in the water at the foil's depth is very small compared to the dynamic pressure from the water. For the base-ventilated section I designed for the Paravane Speed-sailer I estimated the pressure-drag coefficient to some .001, i.e. below 1 % of cl at 60 knots.
In my attached sketch is a sketch of a base-ventilated section for cl around 0.2. The graph below the section is the pressure-coefficient distribution for such a section at such cl. I have the coordinates for my base-ventilated sections somewhere in my old computer, I can look for them when you are ready to build a 50 knot+ foil.
What you win with such a base-ventilated section is that you can get a surface-loading near 1 bar for speed around 60 knots for some 10% thickness instead of the very thin section that is required if you have a thin trailing edge.
I am quite sure that also the succesful Sailrocket 2 used a base-ventilated section during the 65 knot run.

 

[revintage]

Hi Alexander and Glueandcoffee,
We are talking 27 knots here, so anything better than 4412 will probably do. If going for four digit NACAs I would go for 2412. It´s max speed before cavitation is a just under 35knots with CL2=0.3, with a 1% thick trailing edge a few knots lower. So there is an ample reserve. Did a very simple static Cpcrit calculation of NACA2315 with 1% thick trailing edge and CL2=0.3, giving cavitation at just below 30knots. As I understand it the rear foil is surface piercing, so it will not run very deep, ventilation might be the bigger problem?

Glueandcoffe, an OVER-simplified way of calculating CL3 is CL3=0.9*CL2*(AR/(AR+2)), where AR=span^2/projected area. Note this is very coarse, but it will anyway give you an idea of what to expect. About homemade sections, I have designed a suboptimal one around building material I had in my stash and am now making surface piercing V-foils(12% at 27%, 2.2 camber at 50%) with, for building reasons, a NACA0012 nose for the first 27%, followed by a completely flat bottom. XFoil simulations shows cavitation numbers of just under 40 knots with CL2=0.18 and 35kn with CL2=0.35. My goal is to sail +30knots with them. Definitely not as advanced as your Sailrocket design, though.

Alexander, are you referring to CL3 when discussing "cl" and how is the airbubble of the base ventilated section achieved practically?

 

[AlexanderSahlin]

By "cl" I mean the lift per unit area divided by the free-stream dynamic pressure at one section somewhere along the foil's span. By "CL" with capital letters I mean the total hydrodynamic lift of the foil divided by the foil's projected under-water area and free-stream dynamic pressure. I think those definitions are quite common. For my high-speed applications I designed the paravane just for having the best lift/drag at some 60 knots and avoiding cavitation. The configuration, with a stabilizer controlling the angle of attack, ensured that CL was always near that design-point. I recall CL used to be in the 0.14 range, with a local cl a little higher below the fence and much lower above the fence.
In my application of base-ventilated sections I had a 1 m deep surface-piercing foil with an anti-ventilation fence some 20 cm down. Above the fence I used a section designed for very low cl with a little thicker trailing edge, to ensure air-supply from the atmosphere down the trailing edge.
I think the foil for a Sailrocket will operate in a way quite similar as for the Paravane-speedsailer, but on a hydrofoil, where the hydrodynamic lift must always just support the combined weight and sail-force of the boat (like your sailing foiler), you have to design for a very wide range of cl. In this case you have to consider both cavitation in the leading-edge suction-peak at high cl and cavitation at high speed and low cl.
Glueandcoffee: as I wrote before, I recommend that you start sailing with your present oversize and over-camber foil.
When the craft can take-off and do maybe some 20 knots, you can consider a foil for higher speed. After the initial testing you will have a better idea about what you need.
Since the sail-force is not limited by the weight of the boat here, a model Sailrocket should be capable to sail as fast as the full-scale version, so you shall not be happy with just 30 or 40 knots!

 

[revintage]

Thanks for clarifying, Alexander. So your cl is what we less educated usually call 2D CL and your CL is 3D CL? Have never seen that distinction before, though.

Really interesting with the base-ventilated foils. Have to digest your info for a while.

About my V-foiler I will try altering incidence ie. cl/CL of the main foils wrt to speed in a very simple electronic system with a GPS sensor, MPC and linear actuators controlling the foils to get optimal depth at any speed. If it doesn´t work as expected, I will anyhow have potentiometer-controlled incidence adjustment with the GPS removed.

 

[Glueandcoffee]

Glueandcoffe, an OVER-simplified way of calculating CL3 is CL3=0.9*CL2*(AR/(AR+2)), where AR=span^2/projected area. Note this is very coarse, but it will anyway give you an idea of what to expect.
Definitely not as advanced as your Sailrocket design, though.


View attachment 165999

In all honesty I don't have a clue what CL2 and CL3 are. I know what a coefficient lift is and I know what an aspect ratio is. Since I don't know what CL2 and CL3 are, I can't do the calculation. Even if I knew what CL2 was and did the calculation I wouldn't know what to do with CL3 and therefore what I'm expecting is beyond me. I'm also equally none the wiser to cavitation number. I know what cavitation is and it happens when the pressure in the liquid drops below the vapour pressure of said liquid(I think). The rest is like the stars... way over my head.

By "cl" I mean the lift per unit area divided by the free-stream dynamic pressure at one section somewhere along the foil's span. By "CL" with capital letters I mean the total hydrodynamic lift of the foil divided by the foil's projected under-water area and free-stream dynamic pressure. I think those definitions are quite common.

After the initial testing you will have a better idea about what you need.
Since the sail-force is not limited by the weight of the boat here, a model Sailrocket should be capable to sail as fast as the full-scale version, so you shall not be happy with just 30 or 40 knots!

I must have been out the day they explained that in school. Haha.
I agree that I really need to just get out and test it to see what I need. I admire your optimism but really think I should get my first boat wet before I aim to beat the full scale version. Never mind the rc version.

Really interesting with the base-ventilated foils. Have to digest your info for a while.

simple electronic system with a GPS sensor, MPC and linear actuators controlling the foils to get optimal depth at any speed. If it doesn´t work as expected, I will anyhow have potentiometer-controlled incidence adjustment with the GPS removed.

I very briefly considered base ventilated/supercavitating foil for mine but decided against it as its another layer of complexity and as one of you said earlier its only 27 knots so I assumed cavitation wouldn't be a problem for the small boat. I've got a small understanding of their benefits and drawbacks. Its probably not worth it below 40 knots.

Interesting idea with linking a GPS to an actuator for incidence control. My humble opinion would be to just increase the root angle of attack high up on the foil and have the tip(V intersection) angle of attack closer to 0 on the lower part of the foil. Might not be possible in your case but it seems simpler.

 

[revintage]

Interesting idea with linking a GPS to an actuator for incidence control. My humble opinion would be to just increase the root angle of attack high up on the foil and have the tip(V intersection) angle of attack closer to 0 on the lower part of the foil. Might not be possible in your case but it seems simpler.

Do you really think it is simple(or cheap) to make a twisted foil? Mayfly used that configuration with success in the 70´s, with twisted foils milled from solid aluminium.

Your opinion might hold for a vessel like yours, that will be sailed without heel. But a "conventional" foiler with two surface piercing mainfoils will and must have the same CL wether the leeward foil is deeper or shallower at a certain speed, depending on heel.

But back to your rear foil, use the lift equation and the numbers you used earlier, but substitute your CL2=0.6 with the value you get from the CL3 equation to get a more realistic force figure. But first of all use a CL2 of something like 0.3 as a NACA2315 with CL2=0.6 will cavitate in the ballpark of just under 25knots. So instead of 0.6x500x196x0.0225=1323 we get 0,9*0,3*((0,36^2/0,0225)/(0,36^2/(0,0225+2))*500*196*0,0225=441N with the rear foil completely immersed in water. 0,36m is the 14" span you mention. Curious, is the foil ment to be fully immersed whatever speed?

Might have misunderstood the concept, though.

 

[AlexanderSahlin]

Thanks
Could you further explain cavitation numbers and the graph below. What do you mean by CL2 and CL3.
I do have a slanted foil but I'm not breaking the forces up into horizontal and vertical components as the lower section of the foil is parallel with the kant of the wing sail and the forces from each are resolved through the center of effort for each of them. Ie wing force = foil force . The upper part of the L foil will just lift the rear float clear of the water as it accelerates. the faster it goes the higher the ride height and the less area of the upper foil is in the water. Am I understanding your point correctly.
I'm also curious as to what profile I'm using. The first foil I made was cut and sanded from a piece of thick pvc pipe which was then heated and bent into shape. I had no design parameters in mind from the get go and it was very much done by eye. The second foil is not much better as I simply used the first one as a mold for the carbon fiber. Below the radius of the foil, the root chord is 80mm , thickness 12 mm , position of max thickness is about 25 mm from leading edge and I'm unsure how much camber there is but not much. I'll see what kind of naca profile I get with these dimensions. Keep in mind the profile is far from uniform along the span.

When I read your comment about how the sail-force and the force from the foil a

Do you really think it is simple(or cheap) to make a twisted foil? Mayfly used that configuration with success in the 70´s, with twisted foils milled from solid aluminium.

Your opinion might hold for a vessel like yours, that will be sailed without heel. But a "conventional" foiler with two surface piercing mainfoils will and must have the same CL wether the leeward foil is deeper or shallower at a certain speed, depending on heel.

But back to your rear foil, use the lift equation and the numbers you used earlier, but substitute your CL2=0.6 with the value you get from the CL3 equation to get a more realistic force figure. But first of all use a CL2 of something like 0.3 as a NACA2315 with CL2=0.6 will cavitate in the ballpark of just under 25knots. So instead of 0.6x500x196x0.0225=1323 we get 0,9*0,3*((0,36^2/0,0225)/(0,36^2/(0,0225+2))*500*196*0,0225=441N with the rear foil completely immersed in water. 0,36m is the 14" span you mention. Curious, is the foil ment to be fully immersed whatever speed?

Might have misunderstood the concept, though.

For the Sailrocket-configuration it is very important that you keep the part of the foil below the bend in the water all the time. Ventilation on that part of the foil will result in a flyout and uncontrolled landing.
I watched the video from the nautical-mile record, and saw that the rear hull was flying. Very beautiful indeed. This means that Sailrocket 2 keeps altitude in the way developed by Grunberg and successfully used by many others, with a planing surface in front, and the main-foil trailing behind, with the surface-piercing part adjusting itself at a small α and small lift, while the fully submerged part takes the much larger force from the sail.
When you go testing, be prepared to trim the angle of the rear foil so you fly the rear part of your Sailrocket at the best altitude.

 

[revintage]

Took a closer look at the inital drawing and found I had missed the upper surface piercing part of the foil. This means the lower part takes the force from the sail all the time. Alexander, took a look at the CL=1 of the sail. The numbers in the lift calcuation was with cl=0.6. So when using a more realistic cl=0.3 corresponding to a guesstimate of CL=0.15-0.2, the area of Glueandcoffees rear foil might only be about 3x to large, instead of the 10x indicated earlier.

 

[Glueandcoffee]

Do you really think it is simple(or cheap) to make a twisted foil? Mayfly used that configuration with success in the 70´s, with twisted foil milled from solid aluminium

Nothing in this world is ever cheap. Something I heard someone say a few years ago: if it flies, floats or f*cks... rent it.

But back to your rear foil, use the lift equation and the numbers you used earlier, but substitute your CL2=0.6 with the value you get from the CL3 equation to get a more realistic force figure. But first of all use a CL2 of something like 0.3 as a NACA2315 with CL2=0.6 will cavitate in the ballpark of just under 25knots. So instead of 0.6x500x196x0.0225=1323 we get 0,9*0,3*((0,36^2/0,0225)/(0,36^2/(0,0225+2))*500*196*0,0225=441N with the rear foil completely immersed in water. 0,36m is the 14" span you mention. Curious, is the foil ment to be fully immersed whatever speed?

Might have misunderstood the concept, though.

How does CL differ from Cl2 or CL3. Is there a CL1 or CL4. Imagine you're explaining to someone who googled what a coefficient lift was 5 or 6 weeks ago. I heard about CL2 and CL3 last night and still don't know what it refers to or how its defined. Is it dimensionless, does it have units , vector, scalar, speed ,pressure, force, area ,direction, characteristic. I don't know.
But anyway I have 2 foils made. The pvc one and the carbon one. I'm not talking the surface piercing portion of the foils with these dimensions I'm about to give. Just the lower portion opposing the sail force.

The pvc one is 355mm radius to tip. Radius chord 75mm. Tip chord 50mm. Max thickness approx 8mm. No ventilation fence.

The carbon fiber one is 180mm radius to tip. Radius chord 80mm. Tip chord 65mm. Estimated profile naca2315. Has ventilation fence.

The lower portion of the foil stays fully submerged at all speeds so its force balances sail force. The upper portion of the foil pierces the surface so speed , boat weight and sea state balances forces to effect ride height of the rear float.
Treat the upper portion as a separate entity to the lower portion. Their forces act at right angles to one another. Like a planes rudder and elevator.
This is for pvc foil.
Red line is sail force
Pick line is apparent wind
Green line is true direction
Blue line is water level
Yellow is upper foil force.
Black is 14 inches

Calculation done below is for carbon foil. 150 odd newtons seems ballpark compaired with the sail.