XC racer #8 in a series - Page 11 - RC Groups
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Nov 04, 2009, 04:19 PM
Registered User
Steve - So you did not see mine land did you - You should have seen the look on Bill' face when it just settled down so nicely - I would rethink the flaps.
There are two reasons:
1) Flap increased camber for launch - if you can launch that much higher then your starting point will make a difference.
2) If you need more lift you can crack the flaps. Some advantage I've experienced in the past flying Lynsel' / Bamberg special.
I've had no problems over 14 years with this construction so don't worry about it Go for it.
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Nov 04, 2009, 05:23 PM
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TrekBiker's Avatar
I could still go either way, spoilers or flaps, otherwise I dont think an RE racer could take too many of my hot landings, especially at Montague where the terrain is more varied. Greg has a point that it weakens the wing though. Not feasible with his sparless design so I would need a spar. I noticed on the final sled run at Cal Valley the L/D on #3 (E374) started to seriously suffer past 45mph so I'm thinking of thinner airfoils with less camber for speed. The MXC uses a sort of modified RG15 I think which is low camber and thickness for speed. It thermals very well loaded to 16oz and I've always been impressed with how the RG15 Eagle flies in wind. Ellias has said he would like to load his MXC even higher than 16oz but cant due to the 11 lb limit. I was thinking of building to max 11lb wieght and 15oz wingloading with an airfoil around 8% range and around 2% camber, that would be the only way to even have a chance at keeping up with the MXC on speed day (all other factors being equal). I could put that wieght into strength instead of lead and could always do the Wurts-Wiley method of a second set of tips to reduce loading for light days. how to build something strong enough thats bagged, not molded and is so thin with that higher wing loading is the problem. Not a problem if you never lose sight of it but its almost inevitable the day will come that I lose visual and experience the 100+mph high G recovery. We proved the SBXC is capable of well over 100mph without flutter at this last Montague when we lost sight of it at 3400 ft, reacquired visual at around 1500 ft going straight down. The pullout to keep from blowing up took another 900 ft. the poor vario lady could'nt keep up with the altitude change. this situation happens to most XC pilots eventually I think (in my case I never regained visual and totalled Dudleys SBXC a few years back)

weather permitting, Bill and I may make another attempt at the 10K saturday after next following our last monthly SVSS contest at the Davis field. Will #10 be completed?

Steve
Nov 04, 2009, 06:45 PM
Master of the Wind
G Norsworthy's Avatar
It certainly is close. If I push it it will get done. Depends on the other projects.

Properly done flaps with a U-bar and no holes in the wing can still be done sparless. We still have about 2lbs to play with in the current design. Flaps will slow the plane down so you can drop it in a smaller field. Spoilers will just speed it up and make it drop. Good for a Sagitta 600 but not for an 11lb plane. Also if you go almost out of sight and panic you can pull the flaps. The plane instantly goes into a mode where it can't rip itself apart and presents more surfaces to your eye at any given orientation. Just make sure the flap servo is strong enough to pull the flaps at full speed.

Take the AG23, RG15, DS21, and MH32 and lay them over each other. Now cut a foam core with a 28" bow. Now hand sand out the wire sag. You see where this is going. The thing I like about the Drela series is it accounts for the Re change toward the tip. Makes for a very nice handling plane that you can stand on a wingtip.

If you want to go faster with #3 you have to put in the chunk of lead that brings it up to 11 lbs. Different plane but makes the low saves more difficult.
Nov 06, 2009, 12:29 AM
Master of the Wind
G Norsworthy's Avatar

Fuse Mold


I didn't make this mold and there is lots of info already on RCG on how to do it. I do know the plug was turned on a lathe, probably from wood. The nose looks like a half ellipse with half axis 19" and diameter 4.75" at the hatch line. The back part is more or less a cone and the wing saddle area is filled in by hand to look nice. The fin stub was probably made with a bagged piece glued on the plug and blended with filler. To make the wing cutout make a sample center section of the wing you intend to use and cut out the plug until it fits. Use the airfoil polars and how you intend to fly to determine the angle. That is something you can always change after the first flight and before adding the fairing.
Nov 21, 2009, 09:07 PM
Master of the Wind
G Norsworthy's Avatar

final assembly


The third one of these is about ready. It lacks only the battery, receiver, and lead. I will also have to check the range with a disser tail, or put the antenna in the wing. Plan is for next weekend at Curtis, at least a hand toss and maybe a launch after assessing the landing area. Tail fairing comes after maiden flight and final filing of the elevator mount.

The wing fairing was done this time with Accuply as a release film and microballoons with cabosil filler. White will make it easier to paint. This one may get an orange fin for visibility.

In the end the all-glass wing looks pretty good, almost as stiff as the carbon wing.

I got word of a giant Supra in the planning stages. Next year could be very interesting for XC.
Last edited by G Norsworthy; Nov 21, 2009 at 09:12 PM. Reason: fuzzy photo
Nov 26, 2009, 01:21 AM
Registered User

XC wing


Soooo, build the wing with a spar in the inner panels and sparless in the tips. I laid out the last wing Buddy Fox built. 150" span, 1500 sq. in. with flaps. Weighed 4 lbs. ready to fly. It would go faster than my Mazda GLC chase car. The model at 11 lbs was an absolute pussy cat to land.

The joint in the 3 piece wing was about 2/3 of the way out in the main panels. straight joint.
Worked like a charm.

wish I had it now.

JDK
Dec 10, 2009, 04:58 PM
Master of the Wind
G Norsworthy's Avatar

lead nose wt


wish I had it now.

Nothing stopping you. With a bit of local help I built two of these in a rented room and part of a garage.

The lead shot I had did not want to fit in the given space with bags and tape so I had to mold it to fit. Started with a piece of blue foam shaped to the open area. Pressed it into plaster of paris. Make sure the plug is tapered so the lead will release later. Chip out the foam and let the plaster dry completely or the lead will spit. Melt some lead in a metal can and pour it into the mold. Let it cool and pop it out. That's it, ready to use.

Next time I will either pour shot into the nose epoxy during joining or mold another solid piece and embed it into the nose epoxy. Just don't want to go over and have to chip it out.

Copy 3 is now ready to fly and compare with the carbon version. Should be no difference. A few details are left after maiden and adjustment of the stab incidence.
Feb 20, 2010, 12:20 AM
"Flying is my life!"
FlyVA's Avatar
Hello Greg...how about detailing how you bag dihedral breaks. How do you support the core beds? How do you "shape" the mylars at the break?
Feb 20, 2010, 09:37 PM
Master of the Wind
G Norsworthy's Avatar
This plane did not have any dihedral breaks bagged in. The center section is flat and the tips are bagged flat and cut after.

I have bagged in dihedral breaks. We made an MDF table on top of the flat table and spent probably an hour shimming with a level to ensure it was still straight. If I remember correctly it was hinged with silicone. We then stuck the beds on with a light shot of 3m77.

The mylars were done by cut and fit. The bottom is not important and the top was made as close as possible with a dry fit then joined with electrical tape. You can put the tape on the top and get a slight ridge in the layup or put the tape on the bottom and get a slight depression where the tape is. Most of the centerline is covered by the hatches anyway. After that it is pretty much like any other wing, just bigger pieces.
Mar 21, 2010, 03:37 AM
Registered User
Kai@UCSB's Avatar
It flies !!!
And the first day of flying, this baby made an 1hr LSF-IV flight.
Mar 21, 2010, 06:09 AM
Who needs a pilot??
danstrider's Avatar
Boy that looks pretty! Nice job guys :-)
Mar 21, 2010, 11:20 PM
Master of the Wind
G Norsworthy's Avatar
The kevlar hinge shown on the battery tray failed today, and the battery slid back on launch. The plane was very tail heavy but I got it down in one piece. Lucky it did not unplug.

I replaced the hinge with 14 mil mylar and servo screws. Much more reliable.

Flew the planes at under 9 lbs for the last contest. A bit slow but nice handling. Need a bit more tuning before Montague.
Apr 10, 2010, 11:11 PM
Master of the Wind
G Norsworthy's Avatar

XFLR5 explorations


Rained in and had some time to kill today, nothing like XFLR5 to make a few hours disappear. No doubt flight experience is needed to confirm the findings, but this program is a most useful tool to point one in the right direction. Ultimately the goal today was to find out why the MXC on course at the last race outran us so quickly, and if our strategy needs to be changed.

I reentered the data for the wing only again using the RCSD article from 2/08 as an example. There is a lot of good info hidden in the program with regards to plotting variables and it is well explained in the article. Definitely worth your time to wade through it, even if it takes a few sittings and some discipline. This time I varied alpha from -2 to +9 for the airfoils and the Re from 50K to 1 mil in 50K steps. Alpha was varied in steps of 0.25 degree. I ran the analysis at 3.84 kilos which is where we flew in the last contest and 5 kilos fully loaded.

Looking at the graphs I was expecting to see the velocity go up but what surprised me was how much the performance improved overall at the higher weight. Velocity at max L/D goes from 24.5 to almost 29 mph. The penalty in sink rate for the higher weight was less than I expected. The penalty for flying the empty plane too fast was much higher than for the fully loaded one. This means the plane should never be flown empty and completely explains why the MXC blew by us in the race.

Based on the slope of the curve the top end for running should not exceed 34mph. This goes contrary to the strategy of climb and burn. Medium and steady wins the race. Next step will be to move over to the XC#9 thread and see what the program reveals about the new designs on paper compared to what we are flying now.

Disclaimer I have no idea what I am doing so don't believe it until you try it for yourself.
Apr 11, 2010, 10:25 AM
Who needs a pilot??
danstrider's Avatar
I came to a similar conclusion during the SBXC flight testing when looking at the L/D curve. Some folks were really surprised how much the high-speed performance really costs ... on the SBXC with no reflex, going 37kts airspeed is in the 10:1 range for L/D, but simply slowing up to 27kts is back up to 20:1. Now it turns into a balance of thermal spacing and time spent climbing.

We can explore the "fly slower" strategy with the MacCready speed ring. From the MacCready standpoint, if you know (or gamble) there is terrific lift ahead, flying 10:1 still could be optimal.

Vstf = sqrt( (c + current_climb_rate - expected_climb_rate)/a ) + headwind

where c and a are constants from a polynomial fit of the sink polar (for SBXC I measured c = -0.0095, a = -4.6702 for 5kg, 0deg flaps). The expected climb rate is in a sense the strength of the next thermal. If you have a thermal ahead, all things being equal, MacCready speed ring says to fly faster. If you're going through sink, MacCready speed ring says to fly faster. The only time you fly at max L/D or lower is when you're in neutral or rising air, there is no expectation of a thermal ahead, or you have a tailwind.

I'm not trying to refute your statement Greg. I'm trying to suggest there is definitely a "too fast" speed. We can probably back out the SBXC parasitic drag value and assume that for the XC#8/9 to get some number estimates for what "too fast" actually is.

Wait, it's pretty outside here, why am I doing math? I'm going out to toss hand-launch ;-)
Dan
Apr 11, 2010, 06:35 PM
Registered User
Dan, I would like to calculate the speed to fly for an SBXC assuming various thermal strengths. I assume the sink between thermals is about one third to one half the thermal strength. For the equation:

Vstf = sqrt( (c + current_climb_rate - expected_climb_rate)/a ) + headwind

What are Vstf and sqrt ?

For example, on a typical September day at Cal Valley, the lift might be 700 fpm and the sink between thermals might avg 250 fpm. If I am flying thru 250 fpm sink on my way to a 700fpm thermal, what is the MacCready speed to fly for an SBXC ?

This could all be very useful. The new Westech vario has an option for airspeed callouts.

John


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