High Performance MPX Foil/Self-righting Trimaran-The Test Model

MPX Testing and Development

It's going to be close but I think I'll have the foils finished enough for the test sail on Thursday-Dans last for a while.

 

Mr. Lord,

You appear to have increased the chord of the lower section or your foils significantly (20 to 30%).

Can we assume that your choice to do this is based on the lack of lift that your model demonstrated during your most recent test sail?

Also, I am curious why you did not replace the jib with your genoa sail during this test, when you realized that the winds were very light.

Regards,

-Rosebud

 

MPX Testing and Development-First Flight and Light Air

This is sort of a summary of the two light air videos, starting with the last one first:

And this is the longest of the first two videos in light air:
https://www.youtube.com/watch?v=74KEpcmgCh8

 

MPX Testing and Development----First Flight!

And this is the only video so far in any wind-learned a lot:
See Post 1426, page 96 for more on what was learned technically about how the foils worked.

 

Doug,

I am curious as to what led you to increase the chord width on the upward-pointing portion of your "V" foils by 20-30%???

The increased drag, coupled with the length of your beams has the potential to create some undesirable sailing characteristics.


-Rosebud

 

Rosebud, what would you suggest ?

 

Rosebud. The equation for lift is L = (1/2) d v2 S CL
Where D (density) and V are constant, increasing CL will increase lift at the expense of more induced drag. Increase in S (area), only causes a slight increase in skin friction, but a big increase in L.
Therefore increasing the chord of the foil increases S which is better than increasing LOA to get more Lift.

 

MPX Testing and Development

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Not going sailing tomorrow,but the good news is that Dan is available next Tuesday and Thursday. The foils won't be finished and painted today
and they need at least 3-4 days after painting to be wet sanded. I was going to sail with them unfinished but changed my mind-not worth it.
All the information pertinent to the modified foils is in post 1546 and the previous posts referred to in that post. I think the i-flap may be a real good idea but we'll see next week.

 

MPX Testing and Development

The mods made to the foils are not just to add the i-flap but to increase the span(of both foils) and increase the radius between the more or less vertical portion of the "old" foil. This is important because the effective aspect ratio of this type of foil is based on the whole immersed portion of the foil as I learned from Greg Ketterman in a discussion of the Hobie Trifoiler foils. At first glance, the horizontal portion of the foil appears short and fat-relatively low aspect ratio
for a hydrofoil. But it was explained to me that the radius between the vertical and horizontal portions of the foil allows a high effective aspect ratio. This same reasoning applies to all UptiP foils and to specifically to my UptiP foils with the i-flap.

Pictures,L to R- 1) & 2) 1 shows the port ama foil before the mods done a while ago: the angle between the vertical and horizontal portions was reduced and the radius increased as shown in 2. These changes serve as the basis, on both foils, for the i-flap being added now. 3) & 4)- the Hobie trifoiler foil with large radius. The radius on the ama foils is closer to that of the NACRA FCS 20 foil, picture 5), 6) Storm about to hit the "shop" 1:12EDT today :

 

To the contrary, the skin friction & lift both increase linearly (more-or-less) with area. The skin-friction coefficient may not change much, but the skin-friction force certainly does. It's easy to get confused between coefficients & forces, so when the area changes, it's often better not to use coefficients at all.

Induced drag is another example where this is true. If you look at nondimensional equations, you might think that a change in chord affects the induced drag (since it changes the aspect ratio). However, it also changes the area, and if you write the equation in its dimensional form, the only geometric variable that remains is the span.

 

Nice one

+1 for that

 

He made what I considered a good suggestion that's more in line with the working solution we see in the AC 72's. It was addressing the large bow up/stall component that Doug's model has shown a tendency towards in these early tests. It's some pages back.

 

MPX Testing and Development----First Flight!

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I didn't agree then and I don't agree now that the boat "has a large bow up/stall component". As I have explained before there were two distinct "events" in that video:
1) a mush -up, where the bow rose and then settled back, caused by the combination of wands that were too long and an excessively high angle of incidence setting on the main foil-since corrected and,
2) a radical pitch-up(and pitchpole), clearly caused by the over sensitivity of the rudder and the helmsman's inexperience with the boat. This was discussed earlier and a similar event in a similar sized RC trimaran was described. The difference in the two trimarans is that this one has hydrofoils and an even deeper rudder than the original and that allowed the rudder power to cause the hulls on the Fire Arrow to completely leave the water-an awesome sight by any measure. The pitchpole was a fortuitous event that showed that the boat would recover from a pitchpole. This, too, was directly caused by abrupt rudder movement during an attempted gybe.
The correction, which hasn't been tested in enough wind, was to program the Dual Rate and Exponential functions on the radio which are used in RC aircraft
for exactly the same purpose I used them: to de-sensitize the rudder.
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In addition to the above more problems plagued the boat in those first foiling conditions and are discussed in detail after the first posting of the video. But all in all she handled the conditions well and showed that while the foils weren't working perfectly, by any means, they did create lift that allowed the boat to fly the main hull: if the foils on the ama had not been working the boat would have simply rolled over. And if the main foil hadn't been working the main hull would never have flown since the wind conditions that day-though too much for the rig-were not strong enough to cause the main hull to fly as a result of wind pressure alone. All of this discussed in detail earlier.

Pictures, L to R -1) this was a radical pitchup and is not clearly a rudder induced pitchup. The boat was rapidly gaining speed and it may have been a combination of speed and small rudder movement coupled with the high angle of incidence of the main foil and the long wands. This pitchup occurred about 1 second before the
rudder induced pitchup in 2). 3) this was a screen shot taken by an individual attempting to ridicule the boat. Backfire! There are very few if any RC trimarans that would pitchpole to this angle--and then recover!

 

Is there a relation to the ideal foil size compared to the displacement hull in the AC 72? It seems that a full length hull would more easily transition to foiling mode than a half length hull, and allow for a better high speed foil, as opposed to a foil that has to handle the transition from an overpowered short hull to a fully foiling boat.

 

MPX Testing and Development

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The foil size is related to the main hull in terms of the design takeoff speed. A very long, narrow hull will achieve takeoff speed earlier(theoretically) than a relatively wide main hull like the Fire Arrow. The wide hull begins to develop wavemaking resistance before reaching takeoff speed-theoretically. Any dedicated observation of the main hull in the light air of the last video will show almost zero wake so it may not be the problem I thought it would be. It's also related to the design top speed in that if the foil area is unnecessarily large top end speed will be reduced.
The Fire Arrow is a scale model of a sport foiler and as such has a priority in flying the main hull in relatively light air with no regard to the impact on top end speed. Therefore, "tuning" its behavior will concentrate on improving light air performance. She seems to have remarkable light air performance in the 0-4 mph wind range. So far we haven't sailed in the 5-8 mph wind range where I fully expect her to fly.
And within a few weeks I'll have the rig modified for reefing to allow sailing in up to around a 15 mph breeze or more. There is no calibration between the model and full size sail area since the righting moment of the model is drastically lower that the proportional RM of the full size boat so the model has about 20% less SA than exactly scale and the fullsize boat would be able to sail with the main and jib in the same pressure an F18 can-but the Fire Arrow crew(1-3 people) would be sitting comfortably on the side of the cockpit.