HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Feb 17, 2013, 06:12 PM
Registered User
Yak 52's Avatar
UK
Joined Jan 2009
1,248 Posts
Quote:
Originally Posted by ECBoehm View Post
As I understand it, the air molecules are not acting differently at all. It's just that our wings are so much smaller than a real airplane and the Reynolds Number is a ratio of the inertial forces and the air viscosity, so I guess it equates to big differences in performance.
Eric, you have it spot on there. It's not that the air is any more viscous (sticky) just the airflow on a model has a heck of a lot less inertia. It is the same air after all And it doesn't matter what fluid you use, if the Reynolds number is the same you'll get the same behaviour. Some wind tunnels often use high pressure gas in a closed loop to manipulate this too.


Anywho...

Quote:
Originally Posted by slebetman View Post
His airfoil has slightly less than 3% thickness. Which is plenty thin enough for this scale. The ideal thickness for 10 to 12 inch span seems to be around 5% to 6% so actually his airfoil is a bit too thin.
Yes, this is a different aerodynamic task to a free flight rubber model which operates only at low speed.

The outdoor catapult glider needs to fly both very fast but also have enough camber for a good glide. You need to minimize drag at 'zero lift' or CL=0. This means avoiding any undercamber generally because at zero lift you will get a lot of drag at high speed from flow separation on the underside of the wing. A thicker flat bottomed wing around 5-7% thick will give a good compromise between maximizing height off the launch but still give a good glide. I've built 10" span CLGs with 1/16 sheet that go like stink to a great height but come down quickly because they are draggy in the glide.

However.... indoor CLG needs to be optimized so that you launch to just below ceiling height (obviously!) so camber is tuned to the ceiling height. A lot depends on the ceiling height and on the potential energy in your catapult. If you are launching easily to full height then you will be able to use a more cambered or thinner airfoil. ie adding a bit of launch drag but reaping the benefit in a better sink rate in the glide. Your ceiling is pretty low but maybe not if you end up with a piddling little office rubber band to launch with

I haven't tried this type of competition but I think I would start with something with an aspect ratio of about 4 or 5, you won't need the full 10g! Try for about 4-5g. Then make a lighter one Then another, lighter than that Better to build light and increase the aspect ratio that just aim for wing area.


Quote:
Originally Posted by BMatthews View Post
The WINNER is going to be the one that has it trimmed spot on for the climb and transition. Trimming and practice to know just exactly how far back to pull the model against the rubber so it transitions just below the ceiling without actually hitting it is the route to success.
Can't improve on that advice


Quote:
Originally Posted by Jim Thomerson View Post
One thing about the long nose. It is good for reducing needed nose weight, but it contributes to the barbell effect. Moment of inertia is proportional to the square of the distance of the weight from the CG. May not be a problem.
The long nose is an issue. It's understandable that you would try it but the inertia you are creating is harming stability and will spoil transition. It's a diminishing return. Stick to about 1.5x the wing chord and add a small amount of nose weight.


A Static Margin of 2% is pretty tiny. There is good reasoning behind Bruce's approach, but for reliable transition using 10% and trimming for a spiral climb is more common in CLG these days. Especially where launch height is limited. It's decalage not static margin that causes the looping at zero_lift, so go for a big tail volume (like 0.8) and a reasonable static margin and you will only need smidgen of decalage. You may well need a lot more dihedral too, depending how you trim it for transition.

I would consider launching 'upside down', that is so that the model ends up flying on it's back at the top of the launch arc, giving a roll off the top rather than a flop-type transition. You lose less height that way, especially with a low ceiling/shallow climb approach.


Jon
Yak 52 is offline Find More Posts by Yak 52
Last edited by Yak 52; Feb 18, 2013 at 05:25 AM.
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 17, 2013, 08:14 PM
Registered User
slebetman's Avatar
Malaysia, Selangor, Kajang
Joined Jun 2009
1,405 Posts
Quote:
Originally Posted by Yak 52 View Post
This means avoiding any undercamber generally because at zero lift you will get a lot of drag at high speed from flow separation on the underside of the wing.
Mitsuru Ishii's previous F1N record holder uses a Jeldsky style undercambered wing. Not sure if it's by design or if it's accidental (since his wings were hand sanded) but the polars of the wing turns out to have very little lift and very low drag at high speeds but high lift + high drag at low speeds.

His airfoil has approximately 7% thickness.

Here's the plan for one of his record setting planes:
slebetman is offline Find More Posts by slebetman
Reply With Quote
Old Feb 18, 2013, 12:01 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,364 Posts
What can I say... I'm hard core old school... For me working with near neutral stabilty works better and I've trained myself over many years of the "school of hard knocks" that working with lower stability margins works for me.

I see folks that have models with higher margins that spiral up using two to three turns in the climb. Some work but a lot fail to transition well. I guess it can be done but it seems like a tougher approach. I'd have to see someone doing this style reliably and learn from them to feel good about trying it. The low to marginal stability just seems so much easier to use for me. I always got good results in my low ceiling gliders using a half to 2/3's turn in the climb before the pause at the top and transition to the glide.
BMatthews is online now Find More Posts by BMatthews
Reply With Quote
Old Feb 18, 2013, 01:51 AM
Registered User
slebetman's Avatar
Malaysia, Selangor, Kajang
Joined Jun 2009
1,405 Posts
Quote:
Originally Posted by BMatthews View Post
I see folks that have models with higher margins that spiral up using two to three turns in the climb. Some work but a lot fail to transition well. I guess it can be done but it seems like a tougher approach.
Agreed. All the gliders that I've managed to make transition have had very tail heavy CG which I assume equates to low static margin. Especially considering how small the tails are on these free flight gliders.
slebetman is offline Find More Posts by slebetman
Reply With Quote
Old Feb 18, 2013, 06:01 AM
Registered User
Yak 52's Avatar
UK
Joined Jan 2009
1,248 Posts
Hi Slebetman,

Quote:
Originally Posted by slebetman View Post
Mitsuru Ishii's previous F1N record holder uses a Jeldsky style undercambered wing.
Yes, it's a compromise. Undercamber can be utilized where launch height is limited, ie indoors. I've attached a plan from Jim Buxton that shows the flat bottomed style airfoil. Undercamber is sometimes added (sanded in) to suit the venue height when trimming if the launch height is too high. It reduces weight and improves the glide.


Quote:
Originally Posted by slebetman View Post
the polars of the wing turns out to have very little lift and very low drag at high speeds but high lift + high drag at low speeds.
Actually that is a very refined shape, you can see that he's paid attention to avoiding lower surface separation (the forward position of the thickest part helps.)

The high lift, high drag quality is called 'section power factor'. This is maximised for glide (and for duration in rubber power) but it's hard to acheive both a high section power factor and low drag at zero_lift and high speed a.k.a. penetration.

I'd be interested to see the polars you refer to?


Here's a video of HLGs in a relatively low ceiling-ed venue:
F1N Krosno 29.1.2012 (3 min 10 sec)

Shows how transition is the key. The glider shown in the video has a very thin section and plenty of undercamber (I think it's a flapper actually) but it's perfect for the ceiling.
Yak 52 is offline Find More Posts by Yak 52
Last edited by Yak 52; Feb 18, 2013 at 06:25 AM.
Reply With Quote
Old Feb 18, 2013, 06:42 PM
made of foam
appa609's Avatar
Ontario, Canada.
Joined Aug 2008
578 Posts
Prototype 1
appa609 is offline Find More Posts by appa609
Reply With Quote
Old Feb 18, 2013, 08:30 PM
Registered User
slebetman's Avatar
Malaysia, Selangor, Kajang
Joined Jun 2009
1,405 Posts
Quote:
Originally Posted by Yak 52 View Post
The high lift, high drag quality is called 'section power factor'. This is maximised for glide (and for duration in rubber power) but it's hard to acheive both a high section power factor and low drag at zero_lift and high speed a.k.a. penetration.

I'd be interested to see the polars you refer to?
Took me a while to google it because I saw it some time ago and thought "hmm.. interesting..." and quickly forgot about it. Anyway here they are. Taken from this site: http://www.flyquiet.co.uk/smf/index.php?topic=1974.15
slebetman is offline Find More Posts by slebetman
Reply With Quote
Old Feb 18, 2013, 08:32 PM
Registered User
slebetman's Avatar
Malaysia, Selangor, Kajang
Joined Jun 2009
1,405 Posts
Quote:
Originally Posted by appa609 View Post
Prototype 1
Excellent progress! Now get a lump of clay and keep adding and removing bits from the nose until you get it to glide well.

I don't know much about catapult launched gliders so others may have to chime in on where and how to install the hook for the rubber.
slebetman is offline Find More Posts by slebetman
Reply With Quote
Old Feb 18, 2013, 09:48 PM
made of foam
appa609's Avatar
Ontario, Canada.
Joined Aug 2008
578 Posts
Well, I didn't have clay, but electrical tape seemed to work alright. CG at 50% root chord. I got a stable, leftwards spiral glide, glide ratio about 20:3. I don't know what the auw is right now, but I'm tempted to move the wing back to get rid of that nose weight. Based on the calculations, my static margin is right about -15% right now...
appa609 is offline Find More Posts by appa609
Reply With Quote
Old Feb 18, 2013, 11:58 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,364 Posts
Have you tried the rubber band luanch and if so how well is it transitioning?
BMatthews is online now Find More Posts by BMatthews
Reply With Quote
Old Feb 19, 2013, 12:37 AM
Registered User
slebetman's Avatar
Malaysia, Selangor, Kajang
Joined Jun 2009
1,405 Posts
Post a video if you can. It helps us armchair fliers help you with the trimming
slebetman is offline Find More Posts by slebetman
Reply With Quote
Old Feb 19, 2013, 08:23 AM
made of foam
appa609's Avatar
Ontario, Canada.
Joined Aug 2008
578 Posts
Headed to the gym today. I recall someone said that office rubber would be a worst case scenario. Given that is all I have right now, what sort of rubber should I get?
I'll probably have to compress the video to post it: hope the plane's still visible, then.
appa609 is offline Find More Posts by appa609
Reply With Quote
Old Feb 19, 2013, 09:17 AM
Registered User
Yak 52's Avatar
UK
Joined Jan 2009
1,248 Posts
Quote:
Originally Posted by appa609 View Post
Headed to the gym today. I recall someone said that office rubber would be a worst case scenario. Given that is all I have right now, what sort of rubber should I get?
Think that was me, I just meant if you don't have much energy available for the launch. Do you have any restriction on how much rubber you can use? Most CLG comps use a standard catapult by rubber weight or thickness which means the quality of the rubber is a factor in the height of the launch. If you have no such restriction then quality is less of an issue, just so long as you can get it to ceiling height.

The glider looks good. How's the weight? You might want to cut down the fin area if it doesn't transition well. Adding dihedral would be the other fix but it's less practical unless you build a Mark II...


(Thanks for the link Slebetman)


Jon
Yak 52 is offline Find More Posts by Yak 52
Reply With Quote
Old Mar 04, 2013, 04:02 PM
made of foam
appa609's Avatar
Ontario, Canada.
Joined Aug 2008
578 Posts
Sorry for lack of updates, life has been hectic. I got two test flight sessions this week, and I was told that the ceiling height will be 10 m. I had a little trouble getting the rubber setup to work, so about half the launches were by hand, and I managed to get about 20 s a couple times. it doesn't transition very well if launch angle exceeds 40, but is alright below that. At high attitude, it just slows down, stops, and dives. I also noticed that it was porpoising ever so slightly on e way down, so I moved the cg up a little bit. That fixed the glide, but reduced the tansition angle further. Launching with the rubber, I found that my catapult, which is a shoebox with rubber on it, cannot launch it to the 10 m ceiling. I tried putting a stronger band on, but it ended up snapping back on the tail plane, and smashed my tail to bits. Now I am fixing it, and will have a mk ii by tomorrow evening based off roughly the same design, but better built and with a little more dihedral. Contest Friday/ Saturday.
appa609 is offline Find More Posts by appa609
Reply With Quote
Old Mar 04, 2013, 04:11 PM
made of foam
appa609's Avatar
Ontario, Canada.
Joined Aug 2008
578 Posts
I think that for my glider, so far, higher launch speed allowed for transition at a steeper angle. I'll probably also increase the size of my tail to reduce nose weight, and leave a little bit of fuse at the rear end to pull the glider by. I am using really thin rubber right ow, and instead of going thicker, I'd like to just make it longer, to avoid over stressing the airframe. For trim, I've just been sort of bending the back side of my tail, so far, but is there a better way?
appa609 is offline Find More Posts by appa609
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Help! My Tail on my Glider keeps breaking off...Help!! rhampton Sailplane Talk 26 Feb 06, 2013 08:15 PM
For Sale Pods and booms for gliders and electro gliders ClmPro Aircraft - Sailplanes (FS/W) 3 Feb 06, 2013 10:34 AM
Discussion Help, I need more thrust to keep my rc chuck glider in the air aa35199 Power Systems 13 Feb 03, 2013 12:51 PM
For Sale Pods and booms for gliders and electro gliders ClmPro Aircraft - Sailplanes (FS/W) 5 Feb 03, 2013 09:04 AM
Help! Need help with polyhedral EPP glider wing assembly Alard Slope 8 Jan 15, 2013 03:08 PM