XCBD 157" CrossCountry Design by Drela

[Spekd'out]

I am very interested in this design by Mark Drela which was introduced in the XC on the cheap thread. It seemed obvious that it needed its own thread so here it is. Take note that its intended to be REF (rudder, elevator,flap).I don't suppose anyone is that quick of a builder to have built one yet :eek: No specs on AUW which is an interesting topic. It has 1330" sq. area or 9.236' sq. which means at max FAI mass of 5 kg (176.37oz or 11 lbs) loading would be 19 oz/ft. Not sure of the lowest practicle flying weight but at 10 oz/ft it would weigh 92oz or 5lbs-12oz (2.608kg).

[ozmo01]

I was going to comment but I see this is the composite XC Bubble Dancer and am not familiar with it. Good Luck. :o :D Sounds like a Cross Country Monster!

[Anker]

I was literally minutes away from starting a build thread. I will be building a wing and set of tail feathers for John Elliott using stressed carbon skins and Hi Load 60 foam.

Since you started a thread I plan to just use this one.

Just to recap for those who didn't see the posts on the other thread, the XCBD is a REF Cross Country plane designed by Mark Drela.

Even though it looks like a scaled-up Bubble Dancer, it is a very different ship. The airfoils are similar in performance (lift and drag) to the AG4x airfoils used in the Supra, and this ship will be able to carry weight and move. The high aspect ratio wing will also put it in a very different performance class from the BD.

Both the REF design and the airfoils will make it easy to fly and very forgiving, again characteristics you will want in an XC plane. It will have a light wing loading, but be able to carry ballast for windy days.

If the prototype XCBD works as well as it promises, I will be selling wings and tail feathers for them. A fuse may be in the works at a later date.

Anker

[Anker]

I was going to comment but I see this is the composite Bubble Dancer and am not familiar with it. Good Luck. :o :D

You are mistaken, it is NOT a Bubble Dancer. The Bubble Dancer has different design parameters, uses different airfoils, has a different planform, and was designed for a different purpose. What they have in common is that they are REx ships designed by Mark Drela, that's it!

I am being pretty forceful because I can see that the looks and the name are going to invite assumptions like yours'.

Actually, I may start a new thread. The title "XC Bubble Dancer" is misleading. If the original author of the thread can rename it, please rename it to XCBD.

Anker

[soholingo]

Anker,

The plans say XC Bubble Dancer, what else would you call it?

Not trying to start anything, but if you use the Bubble Dancer initials you will get people calling it a Bubble Dancer.

Jay

[Anker]

Anker,

The plans say XC Bubble Dancer, what else would you call it?

Not trying to start anything, but if you use the Bubble Dancer initials you will get people calling it a Bubble Dancer.

Jay

We should have edited the plans. Mark and I agreed that it should be called the XCBD, just the initials.

:)

Anker

[Spekd'out]

Thanks Anker, I'm sure we will be fine from here on in. I was going to ask what kind of foam you used. Care to leak out how much is the foam core set goes for? I have been cutting templates on the CNC at work lately so I may cut them myself , but good to consider all the options. ;) I was not able to use the AG23 file posted, how thick is the section?

I may be thrifty on this build and use Dow blue or pink foam. Not sure if better foam is readily available in my area. I wish I had more 3-D programming experience, it would be neat to machine a fuselage plug for it on the CNC. I do have a more competent machinist friend that I may have to pay a visit. I may be able to get some "scrap" kevlar from a kayak manufacturer close by, for the fuse lay-up. I may also try salvaging spar caps from composite hockey sticks for the wings.

Jan B

[Anker]

Jan,

Thanks for changing the title.

I use Hi Load 60 exclusively. For stressed skin construction the compression strength of the foam is paramount.

The problem with the AG23 posted is that it uses exponential notation, I believe. I changed it to use regular decimal notation.

I'll put it in the Files section in a moment. Woops, this isn't Yahoo groups, so I attached it as a .txt file.

I'll charge $100 for a set of cores, plus shipping. I can cut the cores for the "tube spar", cut a spar slot for a capped spar, or leave it with no cutouts, your choice. These cores are close to perfect, thanks to my CNC setup and a lot of tweaking and programming.

If someone wants cores I'll be happy to take orders now.

Anker

Thanks Anker, I'm sure we will be fine from here on in. I was going to ask what kind of foam you used. Care to leak out how much is the foam core set goes for? I have been cutting templates on the CNC at work lately so I may cut them myself , but good to consider all the options. ;) I was not able to use the AG23 file posted, how thick is the section?

I may be thrifty on this build and use Dow blue or pink foam. Not sure if better foam is readily available in my area. I wish I had more 3-D programming experience, it would be neat to machine a fuselage plug for it on the CNC. I do have a more competent machinist friend that I may have to pay a visit. I may be able to get some "scrap" kevlar from a kayak manufacturer close by, for the fuse lay-up. I may also try salvaging spar caps from composite hockey sticks for the wings.

Jan B

[slozuke]

If the prototype XCBD works as well as it promises, I will be selling wings and tail feathers for them. A fuse may be in the works at a later date.

Anker

If/when you decide to sell bagged wings and tail feathers I would certainly be interested in set. I don't have the ability or desire to do bagging and this would be a perfect addition to my AVA and Supra!

Thanks,
Brian

[Axle209]

This design looks promising for a building project down the road.

[ChuckA]

Very interesting design for cross country. I see only one problem. Or rather don't see one problem. The stealth fuselage will do nothing for keeping the model in sight at 4000 feet. :)

[Spekd'out]

Jan,

I'll charge $100 for a set of cores, plus shipping. I can cut the cores for the "tube spar", cut a spar slot for a capped spar, or leave it with no cutouts, your choice. These cores are close to perfect, thanks to my CNC setup and a lot of tweaking and programming.

Anker

Does that include the tail feathers as well, and I guess they're Hi-Load 60 as well.?

I was finally able to get the root foil loaded into Compufoil. There was still one expo notatation in your file.
I get 8.9% thickness with 2.19% camber

[Spekd'out]

Very interesting design for cross country. I see only one problem. Or rather don't see one problem. The stealth fuselage will do nothing for keeping the model in sight at 4000 feet. :)

That doesn't matter, the wing will have disappeared at 2800 feet.:eek:
There was discussion of the chord size on the other thread. I will be using mine for sport flying so in my case I am not too concerned.

[nuevo]

Very interesting design for cross country. I see only one problem. Or rather don't see one problem. The stealth fuselage will do nothing for keeping the model in sight at 4000 feet. :)

Chuck, the only thing you can see at any real height is the wing. Even the stabs are gone... if you're daring enough. ;)

[Oian]

How about as an electric with Bubble Dancer type wing structure?

John

[jbeanelliott]

For comparison the RNR SBXC (not the newer MXC) has the following specs.

RNR SBXC Wing Span……...170 in
RNR SBXC Wing Area………1545 in²
RNR SBXC Weight…………. 10.5 lbs
RNR SBXC Wing Loading…. 15.6 oz/ft

So it appears that the XCBD is not that much smaller in squares than
the SBXC (1330" sq versus 1545" sq).

Above stats are from the webpage:
http://www.xcsoaring.com/techPicts/manualSuperXC.pdf

Does anyone know of the specs for the MXC?
(There is not any mention of the MXC on the RNR website).

-John Elliott

I am very interested in this design by Mark Drela which was introduced in the XC on the cheap thread. It seemed obvious that it needed its own thread so here it is. Take note that its intended to be REF (rudder, elevator,flap).I don't suppose anyone is that quick of a builder to have built one yet :eek: No specs on AUW which is an interesting topic. It has 1330" sq. area or 9.236' sq. which means at max FAI mass of 5 kg (176.37oz or 11 lbs) loading would be 19 oz/ft. Not sure of the lowest practicle flying weight but at 10 oz/ft it would weigh 92oz or 5lbs-12oz (2.608kg).

[ChuckA]

Chuck, the only thing you can see at any real height is the wing. Even the stabs are gone... if you're daring enough. ;)

Been there. Done that. Great race 1985.

And a Compulsion wing on an Ava fuselage is not as visible as a stock Compulsion. I found that out a few years ago when I bought Brian Smith's COP and couldn't fly it as far away as I could fly my stock Compulsion.

[jtlsf5]

So it appears that the XCBD is not that much smaller in squares than
the SBXC (1330" sq versus 1545" sq).
-John Elliott

John,
This isn't a reasonable statement. The SBXC has 16% more area, which is a pretty big difference. The main problem as I see this new design is the average and root chord (8.6" and 10.5" respectively). Great for a TD ship, but way too small for any serious XC flying. Remember, the root chord on the SBXC is 12", and the MXC is even bigger at 14".

Visibility is very important in XC. I can say from personal experience that the SBXC at 12" root gets pretty hard to see at 3900' agl. I wouldn't even consider thermalling my Supra (root chord around 10") to such an altitude. 3000' agl is about my limit for this size plane.

JT

[nuevo]

Jim,

what's the span, area, & airfoil of the MXC? I've never seen any specs published.

Jon

[jtlsf5]

Jim,

what's the span, area, & airfoil of the MXC? I've never seen any specs published.

Jon


JS,
I don't actually know. I've flown a couple of the ones that are around here, but haven't seen specs either. Wider chord than the SBXC, lower aspect ratio, thats about it.

Give RnR a call, I'm sure Rich Tiltman will be glad to share the info.

JT

[PizzaHunter]

Does anyone here try to design something new and unique before Dr. Drela will do this?

[jbeanelliott]

Is the RnR MXC really in production or are the ones that are flying now just pre-production models?

Is the older RnR SBXC still in production? I thought I read that the molds for the SBXC were getting warn so that they would only produce a few more.

The RnR website shows a lot of sailplane models that seem to be dated designs. There is no mention of the MXC on the RnR website. How active is RnR in the hobby sailplane business?

Many businesses simply do not update their websites very often so I assume they are simply not too web oriented.

-John E.

JS,
I don't actually know. I've flown a couple of the ones that are around here, but haven't seen specs either. Wider chord than the SBXC, lower aspect ratio, thats about it.

Give RnR a call, I'm sure Rich Tiltman will be glad to share the info.

JT

[jtlsf5]

Is the RnR MXC really in production or are the ones that rare flying now just pre-production models?

Is the older RnR SBXC still in production? I thought I read that the molds for the SBXC were getting warn so that they would only produce a few more.

The RnR website shows a lot of sailplane models that seem to be dated designs. There is no mention of the MXC on the RnR website. How active is RnR in the hobby sailplane business?

Many businesses simply do not update their websites very often so I assume they are simply not too web oriented.

-John E.


Good questions. I can't speak for RnR, but I know that model sailplanes have never been a big part of RnR's business. Model sailplanes don't have enough profit margin to sustain a business, especially in today's market where most consumers will save a few dollars and buy used or overseas at the cost of supporting current U.S. manufacturers. Rant over.

They can and will produce sailplanes as/when time allows. There's no value in keeping much of this type of product in stock, it doesn't move that fast. So its relegated to as/when.

If you are really interested, call them and find out what, when and how much.

JT

[jbeanelliott]

The terms "pretty big difference" and "not that much smaller" are somewhat subjective terms. I agree that the bigger cord is probably a big advantage in visibility at the limits of altitude. I was mainly trying to compare the wingloadings between the 2 gliders.

It appears that the RnR SBXC wings have a fair amount of taper towards the tips. If a person was designing a new XC glider, could the visibility improvement mostly be achieved by having a fat cord in the center 3 feet of the wingspan and having a lot of taper toward the tips (assuming this could be done without causing too much aerodynamic inefficiency)?

-John Elliott

John,
This isn't a reasonable statement. The SBXC has 16% more area, which is a pretty big difference. The main problem as I see this new design is the average and root chord (8.6" and 10.5" respectively). Great for a TD ship, but way too small for any serious XC flying. Remember, the root chord on the SBXC is 12", and the MXC is even bigger at 14".

Visibility is very important in XC. I can say from personal experience that the SBXC at 12" root gets pretty hard to see at 3900' agl. I wouldn't even consider thermalling my Supra (root chord around 10") to such an altitude. 3000' agl is about my limit for this size plane.

JT

[jtlsf5]

The terms "pretty big difference" and "not that much smaller" are somewhat subjective terms. I agree that the bigger cord is probably a big advantage in visibility at the limits of altitude. I was mainly trying to compare the wingloadings between the 2 gliders.
-John Elliott

You're right in that the smaller area plane will hit higher wing loadings at a given weight. Still doesn't solve the visibility problem, and loading one of these new designs up to 11# may take it out of its optimal performance envelope, thus self-defeating. I'm not saying this is the case, but should be considered.

JT

[Anker]

Lets get some real information into the equation.

If the XCBD can carry the full FAI max, which I believe it can, but I can verify, then the question becomes a comparison of the polars. If it can't, the XCBD could still perform better if it has better L/D running at full tilt, even at a lighter wing loading.

Finally, the XCBD might still do better even if you can't climb to as high altitudes. Clearly in light winds and lift, the XCBD can be un-ballasted. Also, if it is more efficient, you may not need to climb as high to jump from thermal to thermal.

If someone will get me the data for the RnR ship I'll do the calculations. I need airfoil, span, wing area and unballasted weight.

Thanks/Anker

[G Norsworthy]

I hand measured an MXC at about 1666 in2 wing area.

[jtlsf5]

Lets get some real information into the equation.

If the XCBD can carry the full FAI max, which I believe it can, but I can verify, then the question becomes a comparison of the polars. If it can't, the XCBD could still perform better if it has better L/D running at full tilt, even at a lighter wing loading.

Finally, the XCBD might still do better even if you can't climb to as high altitudes. Clearly in light winds and lift, the XCBD can be un-ballasted. Also, if it is more efficient, you may not need to climb as high to jump from thermal to thermal.

If someone will get me the data for the RnR ship I'll do the calculations. I need airfoil, span, wing area and unballasted weight.

Thanks/Anker

Interesting concept to not max a thermal to altitude in an XC event. Totally contrary to what preferred practice has been for the 25 years I have been associated with XC. Calculations are interesting and indicative of potential, but I will always put my faith in the potential energy that higher altitude brings with it.

When this plane is a reality I'd love to see it compete one on one against a traditional XC where the XCBD caps at 2000-3000 agl, and the traditional at 3000-4000 agl. I'll put my money on the traditional every time.

To paraphrase a very well known XC pilot and world record holder, "if you can still see it, it isn't high enough".

JT

[Anker]

Well, I went to RnR's web site and got some data on the XBSC. It uses the Selig 2048 airfoil.

I have attached polars for the S2048 and the AG24, and a plot of the two against each other.

I find it amazing how far airfoils have come in the 20-odd years since the S2048 was designed.

The S2048 must be flown at a specific speed for a given RE and wing loading. As soon as the speed falls outside the step, the efficiency of the airfoil falls of dramatically.

The problem is that the XBSC has a tapered airfoil, so only one part of the wing can be flown at its most efficient point. The rest of the wing is performing suboptimally.

Also, with the XBSC, you don't gain nearly as much as you will with a XCBD by spowing down in thermals and speeding up in sink. If you look at the comparison file you will see the dramatic difference. The L/D plot for the S2048 is pyramid shaped, while the AG24 has a nice, wide, rounded top. Also note how the S2048 pops over the AG48 at one Alpha value, but everywhere else the AG24 has better L/D.

The XCBD also has a blend of four different airfoils, each designed to optimize the performance of the wing.

Also, when I looked at the XBSC, it is HEAVY. There is no option to lighten the ship if the conditions favor that.

Its going to be interesting to see how this comes out.

As a final point, there's no reason not to build a bigger XCBD, I'm starting with one that has the size my customer asked for.

Anker

[Anker]

Given the need to fly the S2048 on the step, I wonder how you XC pilots control your airspeed to keep it right at the most efficient speed. You fly it too slow or too fast, and you come down in a hurry.

I would think you would want a XC ship to have a wide speed range and be very forgiving to changes in airspeed.

Anker

[Anker]

I decided to compare the polars of the S2048, AG24 and AG35. See the attached pdf file.

Even compared with that airfoils used in the standard Bubble Dancer the S2048 only has a narrow range that it performs better in.

Anker

[ozmo01]

Does anyone here try to design something new and unique before Dr. Drela will do this?

Well he beat us to a doctorate first. That is at least an eight to twelve year head start. And why reinvent the already improved and proven wheel? His Birds fly so well because he did a REALLY exceptional job on the research. So I for one will just fly his design(s) and smile while doing it... Bubble Dancer parts all collected, got plans from Mark today, will be my next build after the current RES Gentle Lady gets done( Drela Allegro Lite airfoils :p )

[Anker]

Interesting concept to not max a thermal to altitude in an XC event. Totally contrary to what preferred practice has been for the 25 years I have been associated with XC. Calculations are interesting and indicative of potential, but I will always put my faith in the potential energy that higher altitude brings with it.

When this plane is a reality I'd love to see it compete one on one against a traditional XC where the XCBD caps at 2000-3000 agl, and the traditional at 3000-4000 agl. I'll put my money on the traditional every time.

To paraphrase a very well known XC pilot and world record holder, "if you can still see it, it isn't high enough".

JT

What do birds of prey do when they want to travel from point A to point B? They fly rapidly through sink and slow down in lift. If the lift is really good they will make a few turns and then take off again in the direction they want to go.

Many, many times I have watched turkey buzzards and peregrine falcons employ this strategy.

They have a few million years on us, so I'd rather go with their strategy than one that has been deployed for a mere 25 years. :)

Anker

[DT56]

What do birds of prey do when they want to travel from point A to point B? They fly rapidly through sink and slow down in lift. If the lift is really good they will make a few turns and then take off again in the direction they want to go.

Many, many times I have watched turkey buzzards and peregrine falcons employ this strategy.

They have a few million years on us, so I'd rather go with their strategy than one that has been deployed for a mere 25 years. :)

Anker

Well, since birds of prey are actually motor gliders, with full span camber changing and variable wing planform geometry, they have far more options available than an average R/C X/C soarer. They also have much lower aspect ratio wings than even traditional X/C designs. They're quite the opposite of a high aspect ratio wing with fixed camber.

Your speed up in sink, slow down in lift strategy is correct though, it's the method employed by full scale soaring pilots. :)

[jtlsf5]

What do birds of prey do when they want to travel from point A to point B? They fly rapidly through sink and slow down in lift. If the lift is really good they will make a few turns and then take off again in the direction they want to go.

Many, many times I have watched turkey buzzards and peregrine falcons employ this strategy.

They have a few million years on us, so I'd rather go with their strategy than one that has been deployed for a mere 25 years. :)

Anker


You're still missing a key point. Slowing down in lift and speeding up in sink is a given. If you slow down in lift but top out lower due to diminished visibility, your potential range while speeding through sink to find the next thermal is drastically reduced. Altitude is king in XC, regardless of plane. You just don't do the bulk of your flying at 500-1500' agl, unless you like landing and relaunching alot.

Even if the plane is optimized for all situations you still have to fly it efficiently for the various stages of a flight. All the good design characteristics won't do you a bit of good at 3000' if you can't tell what the plane is doing because its nearly invisible.

Repeat the XC mantra one more time, "ALTITUDE IS KING".

And not to be critical, but the RnR plane is called an "SBXC", not XBSC. It has a meaning to those that know and care, much as the design you are touting does.

JT

[will_newton]

chill out....it's not like this first one is going to be the ONLY one ever built.

Like most designs it will hopefully get developed and improved over time. You don't like it, build your own...and hopefully, that's what will happen. ;)

Let's all say THANK YOU to Mark Drela for making an open source version available for us all to play with. This could be the Sailplane that launches XC soaring the same way the Supergee started a revolution (pun intended) in DLG.

Gentlemen, start your foam cutters!!!!!
:cool:

[markdrela]

The SBXC has 16% more area, which is a pretty big difference. The main problem as I see this new design is the average and root chord (8.6" and 10.5" respectively). Great for a TD ship, but way too small for any serious XC flying. Remember, the root chord on the SBXC is 12", and the MXC is even bigger at 14". There's nothing to prevent anyone from simply scaling up the XCBD to a 170" span. This will give a 11.37" center chord. Heck, go even bigger if you want.

But I think it's worth trying the 157" size first. One feature of this glider is that for a given cruise capability (L/D = X, at speed Y) it will be able to operate at lower wing loadings than the XBCD. This means it will climb faster, especially in weak thermals. This then translates to faster course speeds. Low-level saves get easier too. So perhaps there will be less need to start cruise at 3000'.

[Spekd'out]

I would like to order some composites for this project and would like to ask for some options on wing lay-up materials. It was stated that "Just using your stock Supra spar/layup should be OK for XC.
But drop off the carbon faster towards the tips if possible." This would be 2.4oz carbon/ 1.7oz kevlar / 1.0 oz kevlar according to the Supra plans. What would be suitable for an economy lay-up in glas? Perhaps 4.0oz unidirectional S-glas with 2.0 oz E-glas cloth on the bias. This is based on the soft foam spar recommendation with 1/8 balsa shear webs on the sides of it and soft foam for all cores.

The boom is another interesting topic. I will draw over the plans in Cad
but looks like it scales to 42" with 2" insertion to the fuse pod. I checked Hypers thread and it shows 40.75" lengths for his unlimited booms. Is there a source for longer tapered carbon tubes?

[will_newton]

Is there a source for longer tapered carbon tubes?just for fun i slid a polecatero.com supra boom over another supra boom and it gives you some length...with a bubble dancer pod on the end it's kinda cool looking... actually, it's very cool looking. wonder if it's strong enough once it's doubled up down the full length and epoxied together?

[Spekd'out]

[QUOTE=markdrela]There's nothing to prevent anyone from simply scaling up the XCBD to a 170" span. This will give a 11.37" center chord. Heck, go even bigger if you want.
QUOTE]

Funny, I have been thinking just that ! I will go with 4.5 meter version (177.164") which scales the root chord to 11.8125"

[Anker]

I would like to order some composites for this project and would like to ask for some options on wing lay-up materials. It was stated that "Just using your stock Supra spar/layup should be OK for XC.
But drop off the carbon faster towards the tips if possible." This would be 2.4oz carbon/ 1.7oz kevlar / 1.0 oz kevlar according to the Supra plans. What would be suitable for an economy lay-up in glas? Perhaps 4.0oz unidirectional S-glas with 2.0 oz E-glas cloth on the bias. This is based on the soft foam spar recommendation with 1/8 balsa shear webs on the sides of it and soft foam for all cores.

The boom is another interesting topic. I will draw over the plans in Cad
but looks like it scales to 42" with 2" insertion to the fuse pod. I checked Hypers thread and it shows 40.75" lengths for his unlimited booms. Is there a source for longer tapered carbon tubes?

Mark was referring to my layup, which is different from the ones outlined in the Supra plans Mark drew up.

Mark uses a spar structure where the spar is made from foam capped by carbon caps. The skin isn't structural. You will have to ask Mark for guidance if you plan to use that option.

My construction method is the one shown in the Phil Barnes/Bill Haymaker video "Vacuum Bagging Made Easy". This method uses a structural skin, called "stressed skin", where the area from just in front of the hinge line to the LE is layers of 3.7oz uni-carbon. 3 layers in the middle, going to 2 to 1 in the center panel. The reinforcement layers are diamond-shaped to prevent stress risers and the first one extends half-way across the panel and the second all the way to the joiners. Behind the carbon, on the control surfaces, I use 1.7 oz Kevlar on the bias. What I am describing here is the Supra. I was also describing the top skin. The bottom skin is simpler with just one reinforcement panel that goes half way to the joiners.

For the XCBD I plan to build the center panel exactly like the Supra center panel. The tips will have the carbon layer taper to a point at the tips instead of going 5/8 of the way from the LE. At the root of the tips I plan a single reinforcement carbon layer that is triangle shaped and goes half way across the first panel.

Stresses on a wing drop off very rapidly from the center towards the tips and you need very little structural strength at the tips. Most planes have the wing tips way overbuilt.

In addition to the skin, the XCBD and Supras that I build have a 1/2" carbon tube running all the way through the center panel and half way into the first tip panel. This tube does little to the structural strength, bot its a very easy way to design joiners and it can be used as a ballast tube by sliding 1/2" metal and balsa slugs into the center tube. If you use a 1/2" metal tube in the center and its long enough it can add significant strength to the wing. You need to be careful of slugs the wrong length, you might create a stress riser that breaks the carbon tube. For ballast I would use short slugs, no more than a couple of inches long. For ballast/reinforcement I would use a rod that's a couple of feet long.

Anker

[ChuckA]

Wings are designed for two different loads. The center is designed for air loads while the tips are designed for door loads; car doors, screen doors, back doors. etc. :D

[mlachow]

One thing to keep in perspective is where you are flying. I can't say that I've flown in Cal Valley, but I have flown models in desert conditions. But if a guy is trying to do a level V distance on the east coast, and not racing, then it's likely that things will have to be done at a lower altitude.

If it's built light then ballasted up to 11 when needed, it might be fun to fly as a big RES some days. How many guys bring out XC ships to fun fly? I don't think there are any sport flyers who would buy an SBXC, but a light XCBD, that would be interesting to them.

[mlbco]

Guys,

I'm new to the XC soaring scene and was wondering what restrictions there are on electronic aids to stabilize the model at high altitude. Is there any rule against roll stabilization and airspeed holding autopilots? It seems these systems would dramatically improve performance and safety at high altitudes.


Steve

[G Norsworthy]

A few sample layups based on experience, and what happened to them

Plane 1 Sparless
Inner top, 7 oz uni sglass+3 oz skin coat
Inner bottom 4.1 oz uni sglass+3 oz skin coat
Tip top 4.1 oz uni+1.5 oz skin coat
Tip bottom 3 oz 0-90+1.5 oz skin coat
Additional 3 or 4.1 oz patch over the 1/4 ply half rib 4.5" from ply joint.
First bagged plane, heavy but almost indestructable. Built to 9.5 lbs. Could save quite a bit by painting the mylars first, bagging the center section in one piece to avoid the external patch and associated fill/paint work, and hard rolling or pre-bagging the skins wet with paper towels to pull out the excess resin. Met its end high and fast from too skinny tail boom and/or too thin pivot wire for full flying stab. Wing would take a full yank at high speed.

Plane 3 (from memory) Sparless
Inner top 2 layers 6oz 0-90
Inner bottom 6 oz 0-90+3 oz skin coat?
Two extra 6 oz over center joint, 12" and 6" wide
Tip top and bottom 2 layers 3 oz 0-90
Additional patch over 1/4 ply half ribs at poly joint
Center bagged in one piece on an angled table
Very cheap and easy to build. Built to 9.5 pounds. The 0-90 eglass makes me nervous but it lives to this day despite a collision with a fence post. Won Montague speed 2007.

Plane 4 (no records and not great memory)
Prepreg carbon spar caps top and bottom center section.
Minimal skin, something like 2x2.2 oz sglass +-45deg center section
Spruce spar caps in the tip, glass may have tapered to 1 layer in the last foot
Built to just under 11 lbs but the fuse was heavy
Center panel bagged in one pc on an angled table
Difficult to build but then I didn't know what I was doing
Blew the ailerons off at 1400m height, made new ones, used for experiments only, ended up overweight
Terrible finger dent resistance, easy to damage in transport.

Plane 7 Sparless
Inner top 2x6 oz uni+1.6 skin coat
Inner bottom 6 oz uni+6 oz uni half way out+1.6 skin coat
Tip top 2 each 1.6 0-90+6 oz uni 3/8 of the span
Tip bottom 2 each 1.6 uni+6 oz 1/8 of the span
Extra 6 oz uni over the center joint, 20" wide total
Center bagged in 1 pc on an angled table
Built to about 10 lbs
Tips are a bit fragile, have taken a lot of damage from bad landings, had to go back in the bag more than once. Lives to this day, won Montague distance 2007. Cheapest glass source at the time was Thayer, but you may have to order quantity or find someone to share the order.

2 was wood, 5 and 6 were carbon.

XCBD will be a delight to fly around and more than sufficient for level V tasks. The flat center panel with multiple poly breaks is particularly elegant for bagging the center on a flat board. If you want to race west coast, think about stretching the chords, not the span. Use Ellipse program and/or scale a planform from the multitude of F3_ molded planes. They are trending toward one solution. The polars indicate a solution which does not require camber, thank you Mark Drela. If your route/landing situation requires flaps, be sure to bridge adequately around the servo pocket, I lost one this way once.

A large enough white fuse will help with visibility on certain hazy days if you have to fly away from overhead. Likewise a white fin.

If you are only going to make one of these you can make a boom by wrapping around a rolled up cone of mylar, like the Frank Weston Magic. I never did one but I saw a nice example so I know it can be done. More than one it may be worth the effort to make a fuse mold.

[markdrela]

I'm new to the XC soaring scene and was wondering what restrictions there are on electronic aids to stabilize the model at high altitude. Is there any rule against roll stabilization and airspeed holding autopilots? It seems these systems would dramatically improve performance and safety at high altitudes. I've never heard of any XC restrictions on stuff like this. Varios are certainly allowed.

I don't think that flight-dynamic stability augmentation would help much, if at all. One possible exception might be a yaw gyro on the rudder to quash Dutch-roll wallowing at high speed if that's a problem.

Trim-hold augmentation is another matter. It does seem to me that heading-hold and airspeed-hold would reduce pilot workload, and also improve average speed by mostly eliminating excursions from a straight flight path.
It seems a simple way to implement heading-hold is with a heli heading-hold gyro on the rudder.
I don't know of a simple way to implement a stable airspeed-hold system with off-the-shelf components. My guess is you'd start with a pitch-hold gyro on the elevator (this is the easy part -- it's a heli gyro turned 90 degrees), and then bias that with an airspeed-error integrator signal (this is the harder part). This will also have the benefit of completely quashing any phugoid tendencies.

[markdrela]

A large enough white fuse will help with visibility on certain hazy days if you have to fly away from overhead. Likewise a white fin. Hmm. It seems to me that adding wetted area to gain visibility is self-defeating. Putting on a large fuse for visibility will allow you to go higher, which is fortunate, because you'll have to go higher because of the lower L/D :rolleyes:
On the XCBD, the sun-illuminated upper surface of the upturned tips is visible when viewed from the side, and perhaps can play the role of the large fuse.

[Anker]

I don't know of a simple way to implement a stable airspeed-hold system with off-the-shelf components. My guess is you'd start with a pitch-hold gyro on the elevator (this is the easy part -- it's a heli gyro turned 90 degrees), and then bias that with an airspeed-error integrator signal (this is the harder part). This will also have the benefit of completely quashing any phugoid tendencies.

Helmut Lelke has been working on such a system for years. The system I have seen uses a vane to measure pitch and a circuit to add a bias to the elevator servo input. The main problem with the circuit, which I have a copy of, is that it is not voltage-stabilized. I plan to switch to LiPos for my flight pack and use a BEC to stabilize the voltage. With such a setup it might just work.

I have toyed with the idea of using a small piece of wire and a strain gauge to measure airspeed directly. The wire would be at right angles to the airflow somewhere on the top of the front part of the fuse.

It should be very doable.

Anker

[markdrela]

Helmut Lelke has been working on such a system for years. The system I have seen uses a vane to measure pitch and a circuit to add a bias to the elevator servo input. That won't work here. Helmut's system is an Angle of Attack sensor, with some bias from pitch rate.
For airspeed hold and phugoid damping you need to sense Pitch angle relative to the earth, which is not the same thing as AoA.

AoA = angle between the chord line and the flight-path direction
Pitch = angle between the chord line and the earth surface

[Anker]

That won't work here. Helmut's system is an Angle of Attack sensor, with some bias from pitch rate.
For airspeed hold and phugoid damping you need to sense Pitch angle relative to the earth, which is not the same thing as AoA.

AoA = angle between the chord line and the flight-path direction
Pitch = angle between the chord line and the earth surface

Thanks for the correction, Mark. :)

Anker

[jbeanelliott]

You might be interested in the article "Performance Testing of RNR’s SBXC Using a Piccolo Autopilot" By Dan Edwards at
http://www.xcsoaring.com/techPicts/Edwards%20performance%20test.pdf

I believe Dan Edwards and the Team Aloft team placed 3rd in the 2008 Montague Cross Country Challenge. They were flying an autonomous sailplane based on the SBXC airframe.

For more information see Dan Edward's website
http://soaring.goosetechnologies.com

-John Elliott

Guys,

I'm new to the XC soaring scene and was wondering what restrictions there are on electronic aids to stabilize the model at high altitude. Is there any rule against roll stabilization and airspeed holding autopilots? It seems these systems would dramatically improve performance and safety at high altitudes.

Steve

[Anker]

EagleTree Systems has a pitot-tube based airspeed sensor. They sell the stand-alone sensor with Pitot tube for just over $50. There are two ways to tap into the sensor: By reverse engineering the protocol between their sensor and data logger, or by tapping into the built-in LED display.

http://www.eagletreesystems.com/Standalone/standalone.htm

Anker

[DEAN GRADWELL]

Go bigger. 170" minimum or what can be built under 5k. You will have much more interest from XC addicts like me. Bigger is better no matter where you live.
Regards Dean

[Spekd'out]

I just finished my Cad overlay drawing and scaling. I went with 177" projected span. Even though the root cord was almost 12", I am making it 12" even and adjusted the other chords the same percentage. It works out to 1750" sq. or 12.15'sq. I recall how quickly planes like the Alpina 4001 get small in a hurry at the aero-tow events I attend!

[Anker]

I just finished my Cad overlay drawing and scaling. I went with 177" projected span. Even though the root cord was almost 12", I am making it 12" even and adjusted the other chords the same percentage. It works out to 1750" sq. or 12.15'sq. I recall how quickly planes like the Alpina 4001 get small in a hurry at the aero-tow events I attend!

Its going to be tough to find a fuse for this wing. The commercially available tail booms apparently barely go up to a size and strength adequate for the XCBD that I am building now. It will probably require a custom fuse and boom. Harder to do than the custom wings and tails.

Anker

[AMBeck]

Since you mentioned the S2048, I thought I'd put in some comments. It's been a favorite of mine since the Selig tests. The section has a low pitch moment, non-critical stall and a decent speed range. I built the wind tunnel model for the Selig tests, flew a model with it shortly afterward, and used that section on a couple models since then. They have all been 2 meters, so they wouldn't be exactly comparable to your XC project. All of the wings had D box structure with the spars at 45%, so there has been a a pretty good turbulator at that station.

Since then, I modified the S2048 to try to fix its problem areas. The mods consist of filling in the sag in the upper surface pressure distribution, moving the maximum thickness point forward a little and increasing camber, since the bottom of the low drag range is below the useful minimum Cl of a thermal ship. Thickness is 8.5% and camber 2.25%. In comparing the new section to the AG24 and AG35, drag is very similar to the AG24 from Cl=0.1 to 0.8, with the higher max lift coefficient of the AG35. In practice, it would probably be hard to tell the difference between the modified 2048 and the AG24. The attached plots are for flight Reynolds Numbers of 150K (an approximation of the root station) and 100K (tip).

[Anker]

Since then, I modified the S2048 to try to fix its problem areas.

Your changes did improve the performance significantly. Now it performs much closer to the AG24, and it also looks a lot like it. The weird rear ramp that I am sure created all the problems is gone. The attached polars contain the XFoil analysis. The AG24 is still abit better. Also remember that the AG24 is just the dominant airfoil on the XCBD, it uses other airfoils optimized for the position on the wing.

[Joe W]

Great to see some new enthusiasm for XC! And it is really nice to see MD jump into the XC fray with a new design. JT has echoed my thoughts though. XC contest performance has a bunch of different items involved, of which one is the planes performance. Another is the visibility at altitude, which can be a very large factor in the large scale desert conditions for most of the XC events on the left coast. The maximum altitude achievable is very important in a few aspects. One is that there is no explicit limit on the start altitude. This can be extremely important if the task is a short course. If I can enter at 6k ft agl, I will have a huge advantage over the guy that enters at 4k ft agl. The other aspect that rewards the big chord visibility is that it allows one to have more working altitude. Once you get below about 2.5k ft altitude, you have to start climbing in the weaker thermals. So the typical goal is to stay between 2.5k ft and your maximum altitude. If your max altitude is 5k ft instead of 6k ft, then you have lost about 30% of the desired altitude operating range.

One of the fun parts that goes along with the big desert lift, is the big sink that is also around. A mile or two of 1k ft/min sink can really eat up that altitude margin quickly. More altitude gives you more margin. If the conditions are lighter, or the inversion is lower, then the higher aspect ratio plane starts showing its pedigree.

As to the usage of electronic stabilization, autopilots, video downlinks, etc. in XC. There is currently no rules against using them. If you did use them, you would get around the visibility aspect (although you might be violating some FARs at that point). I think that using any video and/or autopilot control by a serious competitor should be avoided. It is pretty cool stuff to use, but I suspect that if a couple of competitors started using the fancy electronics in the XC competitions, they would find that the competition attendance would drop away, as the cost (and avionics knowledge) to be competitive would take a big jump.

Guess I should hit the drawing board someday and build a new XC. Still have my old Wile E wing, which is now almost a quarter century old. I'll put that in my In Box of things to do in the future... right next to about 30 other items!

Joe

PS One thing that scares me a little bit with the XCBD is the required structure. My rule of thumb for design integrity is that the plane should be able to take a full elevator pull after about a five second dive with maybe 100 mph initial speed. Sounds a bit severe, but this stuff happens. One scenario that comes to mind is a high energy final glide into the wind, with lift on course. You have lots of energy to burn, so you push the plane hard. Of course, the chase car is also going quickly... You hit a bump on the road, misplace the glider for a few seconds, and reacquire visual. As it is diving, the typical reflex is to pull UP. Seen more than a few planes go boom in similar situations... Montague in '07 on one final glide, we got to three digits on the speedo, going into the wind. It is nice to have confidence in the ability of the plane to survive these speeds!

[Anker]

Joe,

Great to hear from you. Are you having the same heat wave as the South Aussies?

It looks like a certainty that I will be kitting two models. The current one for East Coast and fun flying, and a bigger one for serious West Coast XC. The latter with a bigger chord and bigger wingspan.

Before I build the prototype of the bigger ship I want to finish the smaller one and know we got it right.

I would also love to build the first big one for someone who will be flying it in the conditions it is designed for.

Anker

[tonyestep]

Consider a pilot with 20/20 vision. He can discern the horizontal bars of a capital E in 20 point type at 20 feet. Each bar subtends about 16 seconds of arc. The limit of visual acuity is determined by the physical diffraction limit (the Airy disc), but more often by the construction of an individual's eye. For somebody with exceptional vision, it may be possible to differentiate the bars of an E that subtend as little as 12 seconds of arc.

Using this relationship, we can figure out that for a person with 20/20 vision to see a wing at 6000', its chord would have to be about 17 inches. If the pilot's vision was 20/15, a 12" chord would be visible, although approaching the limit. No matter what your vision, the maximum altitude at which you can fly is directly proportional to the wing chord.

[DT56]

Consider a pilot with 20/20 vision. He can discern the horizontal bars of a capital E in 20 point type at 20 feet. Each bar subtends about 16 seconds of arc. The limit of visual acuity is determined by the physical diffraction limit (the Airy disc), but more often by the construction of an individual's eye. For somebody with exceptional vision, it may be possible to differentiate the bars of an E that subtend as little as 12 seconds of arc.

Using this relationship, we can figure out that for a person with 20/20 vision to see a wing at 6000', its chord would have to be about 17 inches. If the pilot's vision was 20/15, a 12" chord would be visible, although approaching the limit. No matter what your vision, the maximum altitude at which you can fly is directly proportional to the wing chord.

And the amount of full scale traffic you have to avoid!

[OVSS Boss]

And remember, when you loose the stab from view and the wing is all you have, attitude becomes more difficult to assertain. Except unless you are Joe, and he always knows.

Full scale needs to be watched at all times anywhere, doing anything. Around here, you can get involved at about any TD contest much less flying from 2-4K AGL.

Marc

[Anker]

Full scale needs to be watched at all times anywhere, doing anything. Around here, you can get involved at about any TD contest much less flying from 2-4K AGL.

Marc

So I'll have to cover my wings with stickers warning against flying at high altitudes, diving at great speeds, walking through doorways, using as a bridge, climbing trees to retrieve, etc. etc.

I guess there will be no need to paint the tops.

[markdrela]

One is that there is no explicit limit on the start altitude. This can be extremely important if the task is a short course. If I can enter at 6k ft agl, I will have a huge advantage over the guy that enters at 4k ft agl. Wow. I wasn't aware of this rule. I thought the clock started when you come off the line. In full-size soaring contest there is a maximum starting altitude, typically 5k ft, nowadays enforced with an onboard GPS flight recorder, which is also used to verify the course turnpoints.

It seems to me that this no-limit XC start height is a "bad rule", as defined by Blaine Rawdon. He once pointed out that "good" RC contest rules are ones which favor an airplane which is also good for sport flying, since this breeds participation and doesn't result in hangar-queen airplanes. RES and F3K rules meet this criterion very well. F3B rules somewhat less so. XC rules probably not, since it favors huge airplanes which are not very practical, at least for most people.
One could enforce an XC start altitude with a Garmin-type GPS recorder onboard, although this adds a significant cost item which is not ideal.

[jbeanelliott]

By the way, John Ellias recently posted on the XCsoaringpilot mailing list that Chris Bovis of the Naval Research Lab would be giving away free GPS data loggers to each team at the next XC contest at California Valley.
-John Elliott

Below is the email from John Elias:
----------------------------------
We are planning a two day XC event May 23 and 24 at California Valley.

The autonomous soaring team "ALOFT" is planning on attending. As part of their research Chris Bovis of the Naval Research Lab has very generously offered to provide a Skytrace GPS data logger to each team that attends. The only requirement is that each team provide the gps logs of each flight to the ALOFT team for their use in the autonomous soaring research project. After the event the Skytrace GPS is yours to keep! If you are not familiar with the logger take a look at:
http://www.magtechinc.net/SkyTraceGPS.htm

Wow. I wasn't aware of this rule. I thought the clock started when you come off the line. In full-size soaring contest there is a maximum starting altitude, typically 5k ft, nowadays enforced with an onboard GPS flight recorder, which is also used to verify the course turnpoints.

It seems to me that this no-limit XC start height is a "bad rule", as defined by Blaine Rawdon. He once pointed out that "good" RC contest rules are ones which favor an airplane which is also good for sport flying, since this breeds participation and doesn't result in hangar-queen airplanes. RES and F3K rules meet this criterion very well. F3B rules somewhat less so. XC rules probably not, since it favors huge airplanes which are not very practical, at least for most people.
One could enforce an XC start altitude with a Garmin-type GPS recorder onboard, although this adds a significant cost item which is not ideal.

[jtlsf5]

Wow. I wasn't aware of this rule. I thought the clock started when you come off the line. In full-size soaring contest there is a maximum starting altitude, typically 5k ft, nowadays enforced with an onboard GPS flight recorder, which is also used to verify the course turnpoints.

It seems to me that this no-limit XC start height is a "bad rule", as defined by Blaine Rawdon. He once pointed out that "good" RC contest rules are ones which favor an airplane which is also good for sport flying, since this breeds participation and doesn't result in hangar-queen airplanes. RES and F3K rules meet this criterion very well. F3B rules somewhat less so. XC rules probably not, since it favors huge airplanes which are not very practical, at least for most people.
One could enforce an XC start altitude with a Garmin-type GPS recorder onboard, although this adds a significant cost item which is not ideal.

JW is right. No XC event as normally flown has the start at launch release, as TD events do. Launch is typically off course (much as F3B speed and distance), then you enter the course when you are ready. The clock or oddometer starts when the course is entered. To my mind this is perfectly normal.

You mention RES. Great example of the planes evolving to the task. RES started life as a home for prior generation (generally built up) planes. That lasted for about 1-2 years until the current breed of HL on steroids came along, which were purpose designed to the task. The net result was the planes that RES was intended to promote were instantly obsolete for serious competition, and they were relegated to the Woody class was which was born for these relics. So how does RES qualify as a "good rule" when it made the planes it was designed for obsolete. Bad rule in my view.

With the RES example of purposeful evolution in mind, why should XC be any different? The planes evolved to the task. Go higher, further, faster. That is what XC is about. If smaller or less visible planes were the technological answer to XC, thats the direction it would have gone. Limited span/weight XC has been tried on occasion. Never took because it wasn't as much fun as real XC planes.

The task drives the technology, not the other way around. This shows up over and over in soaring, no?

JT

[jbeanelliott]

If a competitor launches early and then sinks out, is he allowed to relaunch without penalty?

(This might be a handy rule for me).
-John Elliott

JW is right. No XC event as normally flown has the start at launch release, as TD events do. Launch is typically off course (much as F3B speed and distance), then you enter the course when you are ready. The clock or oddometer starts when the course is entered. <snip> JT

[tonyestep]

The course entry rule is relevant, BUT even if it were changed, the advantage goes to the plane that can fly higher. No matter what the entry altitude, you can climb while you're on the course, and generally speaking you'll want to follow the JW rule, "get high and stay high." A wide-chord wing will have a practical advantage over a high-AR wing no matter what course entry rule is used, way more than enough to offset the small increase in L/D that the high-AR wing will enjoy at a low CL. A plane with a 16 oz/sq ft wing loading has to fly at a CL of < 0.2 to go 60 mph, so a higher AR is of only modest benefit -- whereas an extra 1000 feet of altitude is of considerable benefit. Skip or some of the other guys who have flown high-AR scale ships in XC may have some good comments on this subject.

[DEAN GRADWELL]

You can launch as many times as you want. We want people on course. Sometimes to get to the next thermal you need as much altitude as you can get at the start gate.
Fullsize contests at Montague will have a ceiling in the start cylinder, usually 10,000' msl.
Regards Dean

[mlbco]

Mark,

The UAV autopilots I work with have no trouble with airspeed-hold and turn rate command, but you're right that this isn't COTS hobby stuff. My interest is more along the lines of an unlimited event but not necessarily autonomous soaring. I've seen the autonomous soaring work done by others and it is impressive at being able to work a thermal that happens to be on the course line. What it doesn't do well is gather visual cues of where the lift is (clouds, birds, other thermalling aircraft, etc.) which are vital for efficient long distance soaring. I'd be more interested in a design that can use pilot input for finding lift, but is able to work the lift to maximum altitude without visibility restriction limits. I can't imagine trying to control airspeed precisely on a model that is more than 1/2 mile above me and I don't think it would be much fun for me with my tired eyes to even attempt this. I wouldn't mind using a long range data link and live video feed so I could guide the model to areas of potential lift and then let the autopilot do the rest. This is my idea of what an unlimited class XC machine should be able to do.

Another thought that this brings up is the issue of optimal wing loading. I'm guessing most XC models have low wing loading because they fly at relatively low altitudes (for visibility) and don't always work thermals to their maximum height. This results in many low saves and the need to be able to climb in weak lift and therefore, low wing loading.

If maximum height weren't an issue it seems that XC models would be flown with much higher wing loadings to maximize average XC speed (i.e. MacCready speed to fly theory). I studied the XCBD 157's average XC speed using an estimated polar and found that the average XC speed is maximized at a wing loading of 6 lb/ft^2 for a soaring day with 400fpm thermals and 80 fpm sink. This produces an average XC speed of 29 mph at 78 mph inter-thermal glide speed. The same model with 1 lb/ft^2 wing loading is 25% slower (average XC speed) according to MacCready and cruises slower between thermals making it more susceptible to headwinds on course.

Why are most XC gliders flown with such low wing loadings?


Steve Morris

I've never heard of any XC restrictions on stuff like this. Varios are certainly allowed.

I don't think that flight-dynamic stability augmentation would help much, if at all. One possible exception might be a yaw gyro on the rudder to quash Dutch-roll wallowing at high speed if that's a problem.

Trim-hold augmentation is another matter. It does seem to me that heading-hold and airspeed-hold would reduce pilot workload, and also improve average speed by mostly eliminating excursions from a straight flight path.
It seems a simple way to implement heading-hold is with a heli heading-hold gyro on the rudder.
I don't know of a simple way to implement a stable airspeed-hold system with off-the-shelf components. My guess is you'd start with a pitch-hold gyro on the elevator (this is the easy part -- it's a heli gyro turned 90 degrees), and then bias that with an airspeed-error integrator signal (this is the harder part). This will also have the benefit of completely quashing any phugoid tendencies.

[jbeanelliott]

I believe that the present XC gliders are flown at the present wing loadings in order to stay under the 5 Kilogram limit. You could go with a higher aspect wing and have the advantage of a higher wing loading while staying under the 5 Kg limit but then you would have a small root cord and the visibility would limit your maximum altititude.

On the other hand, if you went for a very low aspect ratio wing (say 100" span with a 20" cord) you would have great high altitude visibility but your glide ratio and speed would be terrible.

-John Elliott

Mark,
<snip>
If maximum height weren't an issue it seems that XC models would be flown with much higher wing loadings to maximize average XC speed (i.e. MacCready speed to fly theory).
<snip>
Why are most XC gliders flown with such low wing loadings?

Steve Morris

[Joe W]

mlb,

The primary drivers for XC design are the 5 kg limit, typical arid thermal size/strength/spacing, sink between thermals, and the classic mark 1 eyeball limits. Would be great to run a 5 lb/ft2 plane in an XC event, as long as the chase vehicle was a convertible Lamborghini! The open altitude start window also adds some spice to the mix.

BTW, I developed a MacCready speed ring for my XC ship way back in the '80s (nerdish aero engr hopelessly enthralled with rc soaring...). Found out later that it had a description/name. Just wanted to define what I should be doing on the course. XC is one of the purest forms of soaring, no launch, no spot landing (unless you gotta land out), just thermal work/utilization.

For the airspeed hold thing with primitive aircraft, the simple solution is lots of static margin, and a calibrated elevator trim. This is where an analog elevator trim capability is a wonderful thing. My old XC plane would be easily flyable if I removed 8 oz out of the nose, but that nose weight spreads the trim out such that you can define the airspeed via trim. One little caution is that one has to provide some lead gain/damping when changing the trim by more than a little bit.

Actually enjoyed doing XC in the midwest the best. What a great concept, look down the course, and map out your flight path via the cu's! That is like cheating compared to the left coast XC events.

BTW, the best I've done on short courses are about 30 min for 20 miles, and 57 min for a 42 mile closed course. The latter was done maybe in the late 80s, and the limiting factor for the day was the contest requirement to obey road speed limits on course. Good thing they didn't enforce stopping at stop signs (we went through a few at the road speed limit of 50 mph). The 57 min could have been about 3 min faster, we got held up by a bicycle race peleton crossing the course at just the wrong time... other than that, we drove the legal speed limit for virtually the entire course. The last six miles were done while looping, inverted flight, etc. due to the downwind final and low speed limit on the last section of the course. Not sure if we could get the conditions to do that again, was a great day for XC!!!

MD, one concern with the datalogger method of limiting start altitude. How does one get feedback on the recorded start altitude to ensure compliance? If this rule is used, the optimal solution might be to climb to an extreme altitude, then trade potential for kinetic to get down to the limiting altitude when crossing the start, then reverse the trade until you get to the appropriate speed to fly. Another reason to build for ballistic performance!

(Must be getting old, but have had some fun reminiscing.. time to get back to work)

[jtlsf5]

BTW, the best I've done on short courses are about 30 min for 20 miles, and 57 min for a 42 mile closed course. The latter was done maybe in the late 80s, and the limiting factor for the day was the contest requirement to obey road speed limits on course. Good thing they didn't enforce stopping at stop signs (we went through a few at the road speed limit of 50 mph). The 57 min could have been about 3 min faster, we got held up by a bicycle race peleton crossing the course at just the wrong time... other than that, we drove the legal speed limit for virtually the entire course. The last six miles were done while looping, inverted flight, etc. due to the downwind final and low speed limit on the last section of the course. Not sure if we could get the conditions to do that again, was a great day for XC!!!


And all these years I thought we did that run in 54 minutes after the organizers drove the course at legal speed during the lunch break and came back with 49 min. As I recall they didn't believe the 1:04 we did in the morning was legal, so drove the course to attempt to prove otherwise. The peloton was fun to watch but did hold us up about 3 min. I guess you were flying and couldn't watch. ;) Two other minor highlights of that trip were one, someone packed into the back of Gil's truck along with the luggage for the trip from the airport, and two, the look on your face when you first flew my Module SP tranny (the "backup") with its metal faceplate and discovered you didn't have to do the Ace Sniffler dance to get reception. I recall that became the primary for the weekend. Gotta love grounding...
JT

[OVSS Boss]

I have flown probably as high an AR sailplane as anyone, an EMS Albatross (4m basically generic scaley). We had it over 3K, and it was hard in blue sky, with a cloud behind it, not quite as bad.

As far as speed, XC flights are limited by what goes on the ground, not in the air when things are hopping. I have never had a chance to fly out west, someday, but back east, crocked roads, more enforcement, etc are real limiters. my Albatros could out run us coming and going compared to our ground speed capabilities.

Anker, I am surprise you are not placing warnings all over them already. I work with electric utilities, and no kidding, the belts they wear to climb and work on poles actually have warnings on them that climibing poles is hazardess. Shakespear and Don Henley were right, "Kill all the lawyers, kill them tonight!".

Marc

[Robglover]

My eyes must not be as good as some folks. I fly in the east (mostly) and still need a bunch of wing chord to see the plane when it's high. Poly wings, large fuselages, and big tails are also beneficial. Cloud base is the best altitude.

Bubba

[markdrela]

MD, one concern with the datalogger method of limiting start altitude. How does one get feedback on the recorded start altitude to ensure compliance? I was thinking some appropriate time penalty per foot of exceeding the max start altitude. A penalty which is several times the theoretical time advantage should do it. This is severe enough that you're careful to estimate the start altitude visually (not too difficult), but not so severe that a penalty kills your chances of winning.

[Robglover]

I was thinking some appropriate time penalty per foot of exceeding the max start altitude. A penalty which is several times the theoretical time advantage should do it. This is severe enough that you're careful to estimate the start altitude visually (not too difficult), but not so severe that a penalty kills your chances of winning.

Sort of like restrictor plates for Nascar or taking the EPO out of cycling.

Meh.

[markdrela]

Sort of like restrictor plates for Nascar or taking the EPO out of cycling. Uh, yeah. Exactly like that. :rolleyes:

[Bernd Brunner]

What about a gps based tool with real-time downlink, where you can check height and speed when entering the course? Too high or too fast... try again or accept penalty.

Bernd

ps. http://www.rcgroups.com/forums/showpost.php?p=11492719&postcount=3

[OVSS Boss]

A picolario can do all you need for altitude readings.

Marc

[Ralph Weaver]

By the way, John Ellias recently posted on the XCsoaringpilot mailing list that Chris Bovis of the Naval Research Lab would be giving away free GPS data loggers to each team at the next XC contest at California Valley.

Speaking of...

I'm just finishing off the 20 GPS's for John. Here's a few pics of the world wide headquarters of Magellan Technologies (my basement).

[Anker]

If you are contemplating building an XCBD you need to know how to do the layup. The long, high aspect, wing needs to be protected against twist at extreme loads with the proper layup. Materials, position and direction all have to be right.

You also need to have a boom of sufficient stiffness.

Bottom line, if you are doing one of these, let me know and I'll share the information I have.

Anker

[jtlsf5]

A picolario can do all you need for altitude readings.

Marc

Nice thought, but not practical. Who's going to pay to have them calibrated to respond accurately enough for a competition situation? Then who will safeguard them until used? Will the contest organizer purchase/have them available for all contestants at an event?

If left to themselves, you gotta know that someone with a little knowledge and skill could rig one these things to be off enough to allow a significant launch advantage. And the "big boys playing by the rules" has been proven wrong many times where ego/competition is concerned.

Personally, I think if it ain't broken, leave it alone. Anyone skilled enough to fly competitive XC should be fine with a simple vario. All the rest of the information a Picolario supplies is fluff. You should already have enough battery and maintain your equipment. And if you aren't going up, your going down is pretty clear; you don't need altitude reports to confirm what your eyes and a vario tell you.

Highstarter and Big Jer have already tried to promote their crippled XC concepts. The call for <127" RES planes for XC got no interest. Highstarter's low altitude/little plane/everyone wins hokey ham thing is a joke. Kind of tells you that XC'ers aren't interested in these kinds of limitations.

JT

[big jer]

Hey Anker,
I've got a nice 36" pod mould for X/C or record models. It is designed for a 1.75" boom, but, I never got the mandrell made. How big is your Boom at the big end?
I/ve got some Topaz booms that I can make work but I'd rather have a bigger boom.
JDK

[Anker]

Hey Anker,
I've got a nice 36" pod mould for X/C or record models. It is designed for a 1.75" boom, but, I never got the mandrell made. How big is your Boom at the big end?
I/ve got some Topaz booms that I can make work but I'd rather have a bigger boom.
JDK

Right now we are looking at using Troy Lawicki's booms. They have the right length and stiffness.

I have a mandrel, but its only .80" in diameter at the fat end.

My suggestion is that you use Troy's boom and if its narrower than the back of the pod use pieces of carbon tube from CST to adapt them.

That might not work if there is an extreme difference in diameter. I don't have a Try boom yet, but I suspect its no more than 1" at the fat end.

Anker

[jbeanelliott]

Below is an excerpt from an older post by Troy Lawicki with the specs of his unlimited boom.

Unlimited booms 40.75” long and a weight of 55 grams;
I.D. 1.000” Tapering to I.D. .625”
O.D. 1.050” Tapering to O.D. .670”

1-4 $45.00 ea.
5 pks $200.00

-John Elliott

Right now we are looking at using Troy Lawicki's booms. They have the right length and stiffness.

I have a mandrel, but its only .80" in diameter at the fat end.

My suggestion is that you use Troy's boom and if its narrower than the back of the pod use pieces of carbon tube from CST to adapt them.

That might not work if there is an extreme difference in diameter. I don't have a Try boom yet, but I suspect its no more than 1" at the fat end.

Anker

[Anker]

Hey Anker,
I've got a nice 36" pod mould for X/C or record models. It is designed for a 1.75" boom, but, I never got the mandrell made. How big is your Boom at the big end?
I/ve got some Topaz booms that I can make work but I'd rather have a bigger boom.
JDK

JDK,

I'd be interested in learning more about your mold. Do you mind contacting me offline?

Thanks/Anker

[big jer]

Anker

Love to talk to ya offline. Don't exactly know how to do it.

Are you located in the US?

JDK

[Anker]

Anker

Love to talk to ya offline. Don't exactly know how to do it.

Are you located in the US?

JDK

Email is Anker AT AnkerSoft.com, telephone is 978-897-1750. Yup, I am in Massachusetts.

Anker

[will_newton]

anyone got ideas for the pod and boom layup for this plane? I have sent a PM to Mark Drela, but haven't heard back from him. I have printed the fuselage out full size to get the dimensions for the plug, but my layup knowledge is limited to DLG pods.

I do have one of Vladimir's supra booms which is fairly close in diameter at the big end, but needs to be longer. I found that by sliding the fat end of a DLG tailboom onto it, I can get something that looks like the boom drawing, but of course, is not the proper layup.

I have a layup schedule for a supra style pod and boom, but don't know if it will work for the XCBD or not, due to the size differences.

Big Jer, do you have any layup info for your XC mold? Maybe it can be adapted to this pod.

[Spekd'out]

I was wondering about different ways to go about the fuse pod. I like the thought of a slip on nose cone and saw this idea at F3X http://www.rcgroups.com/forums/showpost.php?p=11388363&postcount=1

It looks like it would be possible to cut the cone from the pod if it was molded in one piece and use a separate mold for the inner hardware tray. I also thought of a pylon mount like the Supra but I will keep it simple for now. I am practicing 3-D machining on AlphaCam and I should be able to produce some code before long. I am having just as much fun doing that as anything else :D

[big jer]

Will Newton

Bame and I made our X/C pod mould in 85 or 86 and there has never been a pod pulled from it. I don't know what the lay-up will be. I'll have to ask Bame or someone. The tail boom from a Topaz is 1.1" at the big end and I think it is 34" long. I'd love to find a larger longer boom. I think the Topaz boom will be fine as long as I don't turn the model upside down or cartwheel it. unfortunately, those things happen

JDK

[jgs99v]

Your changes did improve the performance significantly. Now it performs much closer to the AG24, and it also looks a lot like it. The weird rear ramp that I am sure created all the problems is gone. The attached polars contain the XFoil analysis. The AG24 is still abit better. Also remember that the AG24 is just the dominant airfoil on the XCBD, it uses other airfoils optimized for the position on the wing.


Anker - How does the SD6060 compare to the improved 2048 and AG24 given a 12" chord same span?

Just interested if there is any gain over what I have.

jack ~

[Anker]

Anker - How does the SD6060 compare to the improved 2048 and AG24 given a 12" chord same span?

Just interested if there is any gain over what I have.

jack ~

Jack,

The SD6060 is draggy compared with the new, thinner airfoils. Especially at lower REs. It has a better Cl/Cd at high Alphas (angles of attack) as a result of being thicker, but in the area where the other two excel, it clearly inferior. See the attached pdf.

Anker

[jtlsf5]

Jack,

The SD6060 is draggy compared with the new, thinner airfoils. Especially at lower REs. It has a better Cl/Cd at high Alphas (angles of attack) as a result of being thicker, but in the area where the other two excel, it clearly inferior. See the attached pdf.

Anker


Interesting that the SD6060's even less efficient, crappier predescessor, the E374, holds the world straight line distance sailplane record of 140.7 miles, set in 1988. Be interesting to see if the highly optimized package you are putting together here is capable of anywhere near such a performance. Not talking theory here, but results.

JT

[Desert flier]

might be a bit off topic but heres a trace I did with my bubble dancer at 4000ft it was so close to being out of sight that i thought that it was lost untill i saw a glint definately to high to see a bubble dancer

[will_newton]

....and you got the data to prove it. that is sweet!

[Kiesling]

Interesting that the SD6060's even less efficient, crappier predescessor, the E374, holds the world straight line distance sailplane record of 140.7 miles, set in 1988. Be interesting to see if the highly optimized package you are putting together here is capable of anywhere near such a performance. Not talking theory here, but results.

JT

I'm sure it can - especially with Joe Wurts as the pilot ;)

[nuevo]

Aren't type-2 plots more useful for comparing airfoils across the range of Re's they might see?

[jbeanelliott]

>Originally Posted by jtlsf5
>Interesting that the SD6060's even less efficient, crappier predescessor, the >E374, holds the world straight line distance sailplane record of 140.7 miles, >set in 1988. Be interesting to see if the highly optimized package you are >putting together here is capable of anywhere near such a performance. Not >talking theory here, but results.

I'm sure it can - especially with Joe Wurts as the pilot ;)

But without any pilot at the controls a free flight glider flew 192.83 miles?

See http://records.fai.org/models/current.asp?id=308
Category F1: Free flight
(Class F1 Free Flight = No physical connection what so ever between the model and the competitor. )
Sub-class F1 Open (G) (Glider)
N°102: Distance in a straight line : 310.33 km (=192.83 miles)
Date of flight: 31/03/1962
Record holder: Zdenek TAUS (Czechoslovakia)
Course/place: Plzen - Bory - Holesov (Czechoslovakia)