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Old Feb 01, 2016, 05:23 PM
sbrunsen is offline
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how slow can you go?

as i was looking around the WWW for a design to "copy"/make a version of. I stumbled upon the Thumper And when i saw the Thumper i knew for sure that i'd found the right design on where to base my own design on/ lent some inspiration from.

here is my version of it which turned out far from the original Thumper.
I made my fuselage in Styrofoam, tail section is made out of 5 mm foam board, A wooden boom is connecting the tail to the front end, and the push rods made from fencing wire(works terrible as they bend) and to guide the push rods iv'e used zip ties

i made the wing under-cambered with polyhedral wing tips, but i'm not sure exactly where to mount the wing?

further more i have absolutely no clue as whether to make some air brakes at various places as my goal with this build is to fly as slowly as possible indoor.

more to come...
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Old Feb 02, 2016, 04:29 AM
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The key to slow flight is low wing loading. That means either:

1. Light plane with very large wings.

2. Super light plane with regular wings.

The best way to achieve this with foamies is to do profile planes (like indoor 3D fliers).

Camber helps a tiny bit but reducing weight helps a lot more.

Increasing the size of the horizontal stabilizer helps in 2 ways - first, at very slow speeds a small tail will lose authority. The bigger the tail the more effective it is at slow speeds. Second, if the tail is around half the span of the wings then it starts to work as a tandem wing and the tail actually counts towards wing area which reduces wing loading.

Use as small a battery as you can manage - I'm thinking 160mAh or less if 2s, 200mAh or less if 1s.
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Old Feb 02, 2016, 08:46 AM
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You can go pretty slow :

F1D Mark Benns starting (1 min 57 sec)


As slebetman says a very low wing loading is the key but you also need a wing airfoil that is capable of a high Lift Coefficient.

Conveniently for us a 'curved plate section' is ideal. This is just a thin cambered sheet foam wing. Camber should be about 4-5% of the wing chord and as thin as possible.

The following is perhaps more techy than you are after but may be useful - speed can be calculated by the formula:

V = √(2*W*g)/(rho*S*CL)

where
W is weight in kilograms
g (gravity) is 9.81
rho (air density at sea level) is 1.225
S is wing area in metres square
CL is Lift Coefficient

If you reduce something in the first set of brackets (weight or gravity!) or increase something in the second set of brackets (air density, wing area and Lift Coefficient) you will be able to fly slower. Since you cant do much about gravity or air density (fly at the Dead Sea maybe?) you need to reduce weight, increase wing area and find an airfoil with a high CL.


The function of air brakes is to slow down a fast flying aircraft by adding drag but they don't improve it's inherent ability to fly slowly. Flaps which increase area or CL do help slow flight, but all you need is a simple 'high lift' airfoil in the first place.
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Old Feb 20, 2016, 03:34 PM
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http://www.rcgroups.com/forums/showthread.php?t=2055551

I built one of these at 35" wing span. It has a very under cambered wing, weighs 7.36 ounces and has a wing loading of 2.7 oz./sq. ft. I timed it over distance at 6 mph. If you get in your car and do that speed in a parking lot you'll get the idea of how slow that is. With a blue wonder 1400kv and a GWS 8043 slow fly prop it takes off and lands in about 10 ft. Mine has wheels. Get rid of them and you could lose another 1/2 oz.

A skeletonized fuse would get it maybe about 1/4 oz. lighter. I use a 2 cell 800 pack and it flies outside NO WIND for about 20 minutes.

If you use a smaller pack instead I would guess you could get the wing loading down to around 2 oz./sq. ft.

Notice everything mine has is what is recommended by previous posters.

Here is a real example of something that works and flies very slow.
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Old Feb 20, 2016, 04:34 PM
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how slow can you go ?

Stationary, if you fly 3D and hover.

We 'cheat'.

To fly really slowly, we compensate by flying at high alpha to increase drag, and rely on power to keep the model flying forward.

A glider has to 'fly on the wing', and has a fairly fixed stall speed, a power model can use the prop power to fly below the models power-off stall speed.

So in a sense, even heavy models can 'fly ?' slow by relying on the prop to over come gravity and drag.

If a model can still fly slow when the motor stops, then you have a real slow flier.

Ray.
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Old Feb 20, 2016, 06:41 PM
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Quote:
Originally Posted by eflightray View Post
Stationary, if you fly 3D and hover.

We 'cheat'.

To fly really slowly, we compensate by flying at high alpha to increase drag, and rely on power to keep the model flying forward.
...
So in a sense, even heavy models can 'fly ?' slow by relying on the prop to over come gravity and drag.
In between 'regular' flying and a hover is the high alpha flying mentioned above, which in 3D terms is a 'harrier'.

For slow flying, it helps to have the prop wash go over all the control surfaces (including any ailerons), as the airflow from the airplane motion itself is often too little for much control.
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Old Feb 21, 2016, 05:01 AM
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Ray makes a good point

The formula I mentioned is for straight and level flight in 'equilibrium' where thrust is horizontal equalling drag and lift comes from the wing.

In the case of high alpha (and steep climbs) then the thrust line has a vertical component (ie its angled down some) which has to be included as it reduces the lift required of the wing.

In the hover, the thrust equals the weight and the wing makes no lift.
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