By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

Grave lack of lift

How much puff needed?

All Topics | Latest Posts

Search for:  in Thread Title in  
Terence Lynock18/09/2008 22:53:00
avatar
2453 forum posts
46 photos

I built a nice little glider for slope from a plan for 'Flying Fox' an old plan I had in my colleciton for a while, extended the wing a little by adding a centre section rather than have the dihedral start on the centre line. Incidence is about 3'.

It is a high wing 45'' span with fairly slender fuse and wing loading about 7.5 oz with COG at 30% cord, balance is nice in both horizontal axis but when I tried to see how it would glide today it showed much reluctance dropping the left wing and nosing in as soon as released.

How much wind do you need for a bird to stay up on the glide? it was very calm today with just the feintest of breezes from the south but I would have expected the bird to at least glide for a few yards just to show me if it was going to turn into a lawn dart or not which it did try to.

Perhaps it just needed more forward speed as chucking a 45'' span fast enough for it to obtain lift looks to be harder than I thought it would be,

regards.

John Bunting19/09/2008 00:43:00
105 forum posts

Hi Terence,

With that span and loading, I guess your model weighs a bit under a pound and has a wing area of about two square feet, and should have a fairly moderate flying speed, not difficult to hand-launch, even in calm conditions. The C of G position sounds about right, so if it persistently heads for the ground , with no tendency to stall, maybe it needs a bit more up elevator. I assume it's just two-channel, rudder and elevator, with no ailerons, and you say the balance is OK in both axes, so if it always drops the left wing, could there be a wing warp somewhere?

What was the original wingspan and area, before adding the centre-section? The larger wing area means that the tailplane is now a smaller percentage of the wing area, which may affect the longitudinal stability. Can you confirm the weight, and give us the wing and tailplane areas?

John.

Simon Chaddock19/09/2008 01:12:00
avatar
5733 forum posts
3034 photos

Yes Terence is should glide quite well. From say 6ft you should get at least 40ft if you launch it at flying speed with no wind. Your description certainly sounds like it did not have flying speed, remember in still air it is likely to need quite a firm throw. Try holding it level & running with it, you should be able to feel it want to lift from your hand when you are going fast enough. As a rough guide to stay up on a reasonable slope you will need a wind speed of about half its normal flying speed but it can get more complicated than that!

Any chance of a picture?

Terence Lynock19/09/2008 01:56:00
avatar
2453 forum posts
46 photos

Well, not sure if I have calculated the wing area correctly the more I think on it,  the main panel is 30'' x 5'' = 150 sq inches.

The combined outer panels = 8'' x 9'' = 72 sq inches so total area is 222 sq inches.

weight is 360g all up with battery etc = 360 / 222 = 1.62g sq in = 1.62 x 144 = 233.2 /28.4 = 8.21 oz sq ft which is slightly higher than I first thought.

Area of the complete stab and elevator is 23 sq in.

Area of the Fin and rudder is 14 sq in with rudder = about 50% of the total area.

Original span was 40'', with addition of centre section and semi-circular rounded tips now 45'', outer panels have leading edge sweep from 5'' to 3'' over a 9'' length. Aerofoil section is very simular to a Clark Y.

This is about all I can tell you folks, by my calculations it should fly but is making hard work of it,

 regards.         Terry 

Terence Lynock19/09/2008 02:03:00
avatar
2453 forum posts
46 photos

Thoughts on thoughts, may try it with my short Bungee if everything checks out, the field I am using is long grass gone to seed now so soft landings are no problem even at speed,

 regards,      Terry

Alistair Taylor19/09/2008 17:13:00
avatar
602 forum posts
18 photos

Hi Terry 

I think I remember this plan - and if it's the one I'm thinking of, it had quite a slender winf profile, so it should penetrate well, but it will need a bungee or reasonably powerful chuck to get it into the air - was it pictured in pink solarfilm in the build article?

Alistairt

Terence Lynock19/09/2008 18:38:00
avatar
2453 forum posts
46 photos

Thats the one Alistair, quite a nice slender fuse and squarish stab abd fin, I have sprayed my fuselage  with a Ford metalic paint called Ameranth Pearl which is a pearlised Purple metalic and covered the wing in a deepish pink, the  wingtips are a square patterned holographic purple on the underside and very reflective like a faceted purple mirror.

Presently looking at making a smaller and lighter launching ramp than the one I have that I can take apart and pack into a bag with my bungee, do you think 15 degree angle is enough for the job or should I make it variable?,

                       regards,           Terry

Eric Bray19/09/2008 22:15:00
avatar
6600 forum posts
2 photos

Not much help, but thin sharp wings want to fly fast, and fat blunt ones want to fly slowly.

Simon, I think you have pinched my line!

John Bunting19/09/2008 23:12:00
105 forum posts

Hi Terence,

The tailplane area of 23 sq ins is just over 10% of the wing, which might be a bit too small; but of course the moment arm, the distance between wing and tail, also affects the stability.  As Simon and Alistair said, it might need a good heave to get it leaving your hand at flying speed. Do you have access to a good slope-soaring site? If so, with a good breeze up the slope, I don't see how it can fail to fly, even if only for a short hop. Once it's in the air, you should find how controllable it is, and what needs changing, if anything. Keep us informed!

John.

Terence Lynock20/09/2008 00:13:00
avatar
2453 forum posts
46 photos

Will do John, the problem with this area is most of the decent sites have been built on or filled with bloody trees so finding one where you have a half decent chance of testing a new airframe without major obstructions is becoming harder.

I can enlarge the stab by slicing off the leading edge and grafting on an extra piece to give it more cord and span and likewise with the fin if need be, it isnt major surgery so will keep it as an option for the future,

      regards,            Terry

Simon Chaddock20/09/2008 00:36:00
avatar
5733 forum posts
3034 photos
Eric - not really pinched, limiting wind speed to 1/2 flying speed is sound advice even on full size gliders, however you did say it first, although it was in another thread!
Eric Bray20/09/2008 14:38:00
avatar
6600 forum posts
2 photos

All Topics | Latest Posts

Please login to post a reply.

Latest Forum Posts
Support Our Partners
Sussex Model Centre
CML
electricwingman 2017
Slec
Sarik
Advertise With Us
Latest "For Sale" Ads
Has home isolation prompted you to start trad' building?
Q: The effects of Coronavirus

 Yes - for the first time
 Yes - but Ive bashed balsa before
 No - Ive existing projects on the bench
 No - Im strictly an ARTF person

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E!