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Mar 29, 2020, 02:18 PM
gpw
gpw
“There’s no place like Foam”
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The hole in the center ( donut ) works a treat too , but always draws the ubiquitous “ toilet seat” comments … and actually we made the rear of the Donut slightly negative incidence to the wing , which gave us great pitch stability …forming a sorta' joined tandem wing , ‘ish …
Which led to other geometric openings …


Is a Biscuit (yank) like a scone … I’d think so ?
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Mar 29, 2020, 08:01 PM
Sokol
JureZ's Avatar
Quote:
Originally Posted by pardshaw

I remember seen this model of yours, very nice !!!
Latest blog entry: Weasel-EVO
Mar 30, 2020, 05:10 AM
Pass me that hammer...........
pardshaw's Avatar
It flew well enough, but I could never decide whether I liked it as much as a Nutball - I think probably not, on balance!
Mar 30, 2020, 08:55 AM
Registered User
Terry Rigden's Avatar
An interesting discussion, I've always thought that it was a good idea to minimise mass at the extremities. However I'm coming at this from a different direction in that I'm developing models for flying indoors in confined spaces ( you may have seen my video of the nutball flying in my house). So slow flight and agility are more important than punching through gusts. I do fly it through its own slipstream at times but I find powerful controls and an agile model are the answer. In my never ending quest to save weight I'm considering cutting a hole in the middle but and concerned that it would raise the stall speed. So when you say the hole made no difference did you investigate the stall?
Mar 30, 2020, 10:48 AM
Pass me that hammer...........
pardshaw's Avatar
Quote:
Originally Posted by Terry Rigden
An interesting discussion, I've always thought that it was a good idea to minimise mass at the extremities. However I'm coming at this from a different direction in that I'm developing models for flying indoors in confined spaces ( you may have seen my video of the nutball flying in my house). So slow flight and agility are more important than punching through gusts. I do fly it through its own slipstream at times but I find powerful controls and an agile model are the answer. In my never ending quest to save weight I'm considering cutting a hole in the middle but and concerned that it would raise the stall speed. So when you say the hole made no difference did you investigate the stall?

Hi Terry. Hope you are keeping busy and keeping well?


I think I have a bit of a problem with Nutballs and stalling, in so far as I have never experienced a stall. Which is perhaps interesting because I mainly spend my flying time flying as slowly as possible on as little power as possible. So my experience tells me that Nutballs don't stall - but that is just my personal view.


What I have experienced many times is something akin to a stall when gliding in (dead stick) for a landing on Nutballs with no landing gear. Nutballs glide relatively fast and steep, and when I try to bleed speed off by bringing the nose up a touch they will reach a point where the nose wants to bob down again and speed up. If that happens within 2 or 3 feet of the ground it is irrecoverable and she noses in. But if it is at, say, 8 to 10 feet then it will pick up enough speed to flare out nicely. I think it is a stall, but the wings remain level and it is very benign.


Thats not what happens with a Nutball with a hole in the middle. The lightly loaded ones behave much like a Nutball but with slightly degraded elevator authority and slightly degraded high alpha. They will, however, stall if more heavily loaded and drop a wing. I personally find that a bit of an alarming trait because regular Nutballs never do that (mine don't anyway).


But.... looking at the little discussion above, one explanation might be that I never moved my CG aft in my "holey" nutballs, but both Joel Rieman (with his O'Fungle) and Les Vinall (his Doughnut) both had significantly further aft CGs than 25% (30-31" and 33% respectively). I expect that would give better elevator authority when it matters. Not sure it improve stall much, though!


One note of caution I would sound is that my best performing "holey" Nutballs were purpose designed to have precisely the same wing loading as a good, lightly loaded Nutball. So a 30" holey Nutball has exactly the same wing loading as a 24" regular Nutball and has identical wing surface area. Its an important point because I cut holes in several regular Nutballs and all flew worse because the wing loading went up (obviously).

So.... the main advantage of a big hole in your Nutball seems to come down to this - you get a bigger diameter Nutball for exactly the same weight and wing loading as a regular Nutball of 80% small diameter than the one with the hole. The main disadvantage is bigger flat wings are harder to make stiff.

Dave

One thing I have wondered about is making multiple smaller holes rather than one big one....
Mar 30, 2020, 11:33 AM
Learning to make
A nose-down that you can't recover from until you get more speed is definitely a stall.
Mar 30, 2020, 12:14 PM
Pass me that hammer...........
pardshaw's Avatar
Quote:
Originally Posted by fossil1999
A nose-down that you can't recover from until you get more speed is definitely a stall.

Yep, I guess so. The ones I described are stalls during glide, so when I get one just before landing I can usually just give a small burst of throttle and "go around". So its just a bit of a minor irritation, really - largely because I allowed it to happen! But under power it almost never happens and with landing gear it almost never happens - its just that I'm a bit c**p at landing without landing gear
Mar 30, 2020, 01:59 PM
Registered User
Terry Rigden's Avatar
Thanks Dave.

Yes we are fine hope you are too. At the moment all my flying is done either inside the house or from my front garden. So my preference for small models is paying off.

Another thing I forgot to mention was the improvement in handling when I took the undercarriage off both my Nutball and mini vapor. They both became more agile as well as lighter. This was most marked when I flew my mini vapor at the same time as a standard one , I could literally fly rings round the other one. Mine could loop the other couldn't. Yes it was lighter but having wieght at a distance certainly didn't improve things for the indoor flyer.
Looks like from what you say I best leave my NB alone, but I recall someone made one from CF rod and cling film. Now that could be seriously light.
Mar 30, 2020, 06:07 PM
flyin' fool
goldguy's Avatar
Thread OP
If your buying your TP by the case, keep the cardboard ......................
Mar 30, 2020, 06:10 PM
flyin' fool
goldguy's Avatar
Thread OP
That my 24" span Box Fox sloper with ONE pound of lead in the nose.
Mar 31, 2020, 05:42 AM
gpw
gpw
“There’s no place like Foam”
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Cardboard … COOL !!! And you can waterproof it easy enough ( see: Chuck Felton … )
Last edited by gpw; Mar 31, 2020 at 05:48 AM.
Mar 31, 2020, 09:26 AM
Pass me that hammer...........
pardshaw's Avatar

Fixing the 19" Nutball


So far I haven't enjoyed flying the 19" one much at all.

My main point of comparison is with the 21.5" one which is now set up exactly how I like it, with the CG at 26.5% and an AUW of 8.75 ounces (248 grams).

I have almost 9 hours stick time on the 21.5" one and this morning I decided to sort out the 19" one until it flew as good as the bigger one.


Step 1: Lose some weight - by using smaller batteries. This improved things a bit.

Step 2: Move the CG back - this improved things a lot.

Step 3: Lose more weight - by cutting lots of holes in the wing behind the CG - this helped even more! AUW now = 6.65 ounces, cubic loading = 2.2 OPCF.


It is now in the sweet spot and handles every bit as good as the 21.5" one (it does somehow feel smaller on the sticks and is a bit nimbler in turns/flips).
Mar 31, 2020, 02:10 PM
Pass me that hammer...........
pardshaw's Avatar

How Nutballs fly....


The thing about Nutballs is that regular aerodynamics are not very relevant as there is probably no laminar flow anywhere over any of the surfaces.


The typical Nutball propeller is very large compared to wingspan, and it seems that most of the airflow over the wing is prop wash. Highly turbulent, rotating air mass in other words. Half of which washes over the upper surface, and half of which washes over the underneath.


So the idea of laminar flow, 2 dimensional smooth streamlines and a low pressure regime above the wing and a higher pressure regime below the wing seems just plan wrong - to me.


Nutballs fly because a big propeller ejects a rotating mass of air behind it and the reaction to that mass keeps the Nutball up. This has been elegantly summarised by others as "Nutballs fly on thrust". And mainly they do - and even when they glide there is that big propeller mixing things up and a square cut leading edge tripping the flow into turbulence. Not to mention all the protruding hardware (servos, pushrods, horns, Rx, battery, ESC, spline, landing gear etc.


Therefore it follows that cutting large holes in the wings has no significant effect on the airflow across the wing. The fast, large, rotating cylinders of air moving above and below the wing have no inclination to "talk to each other" when passing across holes - they just keep moving as if the hole isn't there.


Well, thats how I see it, and its why holes in Nutball wings could be a useful thing.


But... I reckon many small holes is a better bet than one big one
Mar 31, 2020, 02:20 PM
Pass me that hammer...........
pardshaw's Avatar
How to take advantage of holes.........


I made a rough cardboard airframe for a 28.5" diameter Nutball.


Then I weighed the 3 main pieces:


1. In front of the 25% CG : this piece weighs just 2.1 ounces. This equals just 18% of the basic airframe weight.

2. Behind the 25% CG : these two pieces weigh 9.35 ounces. This equals a whopping 82% of the basic airframe weight.



Of course everyone already has their favourite way of positioning the heavy bits of hardware (motor/prop/ESC/battery/servos/RX) to achieve balance.


But holes might sometimes be a handy thing if you want to design an increased diameter of Nutball without the added weight......just add holes instead.


If my argument in the previous post is correct (it might not be) and if it is true that prop wash just zips right over holes in the wing without "seeing" them (it might not be)..... then it would follow that you can ignore the area of holes in calculating wing loading and cubic wing loading. In the limit, of course, that logic breaks down for extremely large single holes. But for several small holes I think it is a safe, workable approximation.
Last edited by pardshaw; Mar 31, 2020 at 02:43 PM.
Mar 31, 2020, 02:29 PM
flyin' fool
goldguy's Avatar
Thread OP
Although, my slope NutBalls have no prop wash. But, there's no high alfa in slope soaring. As a sloper they flew 'on rails' and were capable of all the basic aerobatics, plus backwards flight.


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