Help me design a fast CC ship
I am a rc pilot since '93. I have gotten into FPV (first person view) flying a couple years ago, and since this technology is picking up so fast the Long Range record has gone up every year. Now the current record is 71 KM+71KM back and it was achieved by Roberto Montiel of Spain with a 3,6m F3J glider with a mh32 section.
It took him close to 5 hours to complete the flight.
There is no way at this moment to break that record with li-po batteries,and I realyze that a soaring ship is what's needed, and a fast one too, since I don't want to be stuck with a 50-60kmh gliding speed for best glide ratio.
I was wondering what is the performance of your category of planes, what is your glide ratio like 15:1 or more, and what is the gliding speed at this nice glide ratios.
At this point I can add that I have designed several planes in the past years, and the latest one is a motor glider with a wide 12.5" chord but only 1.8meter and with this one I can easily soar in the summer.
Now I would love to hear from you guys, what your perfect wingspan is , what your weights, your best performing profiles, and wing chords.
With fpv I am not limited to visibility, so going up and above 3000 meters is not a problem, and then gliding down from that height at high speed to make some serious ground.
Thank you for your help if you are willing to give it, and sorry for the long first post in this section.
The two are in the 6.6M span range. Root chord for the MXC is ~14" and ~12" for the SBXC.
Max L/D is around 30-32:1 for the SBXC @ 35MPH; not sure what the MXC has. Find more info at Dean's site: http://xcsoaring.com
Weight for us is limited to FAI limit of 5KG.
All the tasks at Montague are racing; normal cruise is ~35-40 MPH; final glide to the finish line approaches +90 MPH.
71KM is 44.11 miles. We have done several tasks where the entire flight is 44 miles about a course not directly out and back. I don't have the times available but the winners are typically doing 18-20 MPH for average speed around the course. You can see some of the results in old threads here: there's one on 'Montague 2011'. Doing your 142+KM at 20 MPH would require close to 4.5 hrs.
Your flying site has a lot to do with able to fly well. Montague is unigue in that way. Many recent full scale National Sailplane meets have occured there.
- Bob -
I'll give you the advice JW gave me. First model the tasks and then the design work becomes easier. What are the specs on the plane, mass, area, etc? Do you have autopilot to prevent high velocity? Is the task pure distance or is there a speed component? The planes I have z designed will way outrun an mh32 f3j type ship. I suggest you read through my threads In reverse order 10, 9, 8 to get an idea of the design process. Let me know if you want copies of my course models.
140Km in 5 hours is 28kph average.
The reason that speed is well below the best L/D speed for that plane is the time spent circling in thermals which isn't covering distance (this is an approximation). So to have a shorter flight, spend less time circling.
A plane which flies faster may actually slow you down if the lift is weak and it climbs more slowly than the F3J plane you are trying to beat. I used to fly full-size cross-country and have fond memories of being regularly beaten by a pair of wily old instructors in a glider with a smaller L/D and a lower best speed than the plane I was flying.
Increasing speed-over-the-ground or VMG (velocity made good) in soaring flight is a non-trivial task. Picking the weather and using the available lift more efficiently will be far more important than the performance characteristics of the plane.
This is the video of the record flight, and the telemetry explained
Of course I can't spend 2K for such a risky endeavour so I am looking at building my own with cheap materials, of course I am allowed motor+prop and as many batteries as I want/can carry without disrupting the Glide ratio.
I also have to consider structural problems, as my poor materials will not be as strong as a composite wing. I would probably use blue sterofoam for wing cores laminated with balsa and white glue, so I am limited as far as wingspan etc.
I am looking at 3-3.5 meters, and some parameters that I'd need from you experts is the fuse length vs w/s , don't want it too be too short coupled or have un unnecessarily long fuse. Also what the chord should I chose to allow it to be efficient yet carry enough weight in cells, we are talking about 2kg or so of batteries. Looking for a 4kg total max weight.
Flight would be done using a combination of battery and thermal hunting, at very high altitudes, between 2000-3000 meters altitude.
My 180cm ship with a 12.5" chord and rg-15 section thermals decently at 2.7kg weight, but with 1kg of battery it tops out at 60-80km round trip with no thermal activity. It would be nice to be able to run 160km round trip even without thermal activity.
The other design consideration is that I could employ also some solar cells in the form of 80-90W - 15v 5.4A under full sun.
As a first cut, I would say you don't need camber change or ailerons, since you will not be flying low. Also visibility is not a big concern because you have a camera in the plane. I would look at a smaller version of my #8 as long as there is a decent landing area. You can build it sparless out of the 6 oz uni from Thayercraft and blue foam. The wing will be much cleaner than if you use balsa. That construction method makes a tough, strong wing at very low cost and labor.
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