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May 13, 2011, 03:05 AM
Build straight - Fly twisty
Whiskers's Avatar
A friend once had me test fly a model he had made with very small tail surfaces. It flew OK while the power was on and the prop-wash was doing its thing, but when I shut off to test the glide it wanted to fall out of the sky.
Slammed on the power and recovered but I was wondering how to land the thing.
In the end I flew it in with power and tried to ease it in as best I could.
Got away with it OK, but the model was instantly retired and hung up in his workshop.
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May 13, 2011, 08:19 AM
What could possibly go wrong?
nickchud's Avatar

The Cutting Plan


Here's my cutting plan - so far. I will edit it as I go along!

Nick
May 13, 2011, 08:23 AM
Use your imagination....
czirh's Avatar
Very cool ekranoplan project, good luck.

Cem
May 13, 2011, 10:24 AM
Registered User
hoverman's Avatar
Here's my FS-8 model, I never finished it though:




Here are a couple of other Reversed delta WIG models I've built last year:







And there are a lot more, as you probably have seen in the
https://www.rcgroups.com/forums/show...2#post18223917

Anyway, it will probably be a simple build, I would not underestimate the aerodynamic part. It's not a plane, it's not stabilised like one either. The CG lies behind the CP of the main wing, the stabiliser is positively lifting. It can even fly stable with a higher Lift Coefficient on the tailplane than on the main wing, for a normal airplane this is impossible, because mathematically it means you would be statically longitudinal unstable.

It's possible to achieve stable flight anyway, because the main wing looses lift during an increase in altitude, allthough the tailplane is somewhat uneffected. As the CG lies inbetween the two CP's, it will result in a pitch down moment, counteracting the forward CP shift.

The reversed delta doesn't offer a magically reduced CP shift! One might suggest that the CP shift is reduced on this wing planform, because the chord is smaller at the tips? Well, if you look at it the other way around, the wing chord is larger at the wing center than on a square wing planform. In total, CP shift on a rectangular wing will be somewhat the same, during a transition from in to out of GE flight.

So why is it more stable? Because the wing is swept forward very strongly plus the wing chord is small at the tips, the tip vortices are relativelly small and narrow compared to a square wing. When the vehicle travels from in to out of GE the amount of upwash & downwash at the tips is increased, allthough not as extreme as on a square wing. Therefore the stabiliser experiences less of a increase in downwash, which means it won't loose as much lift due too downwash changes, as on the square wing. Blabla, the wing planform makes the stabiliser a bit more efficient.

The second stabilising effect of the reversed delta is that viewed from the side, the trailing edge of the main wing is aswell before the CG(at the wingtips) as it is behind the CG(at the wing root). This means that if minor pitch changes occur while flying in extreme ground effect, either the tips, or the wing center, will experience a slight increase in Ram air pressure at the trailing edge, causing a stabilising effect on the vehicle.

Can it fly out of GE? sure it can, it has got wings and a propellor, just don't forget that it's not a plane. You can build it like a plane, but then you'll just be 'flying low' pretending to be using ground effect

There's a vast difference between the changing aerodynamic effects which occur when a wing comes close to the ground. A Boeing experiences a reduce in overall drag, because it's producing quite a lot of down&upwash, espacially at low speeds, high angle of attack & deployed flaps. It does not however experience RAM air, which can produce an extreme increase of lift! For this to occur the wing trailing edge has to be really low, and you'll need a high angle of attack, just like on the Flightship.

Marijn
May 13, 2011, 03:29 PM
What could possibly go wrong?
nickchud's Avatar
Brilliant Marijn, thank you.

Don't you just love the internet! One minute a seed of a crazy idea. Just a few days later I've got information, plans, photos, advice, encouragement, experience, controversy.

So, in ground effect the AC is further back because of the ram air effect, yes? We need the CG to be further back than in normal flight. When we leave ground effect, ram air disappears suddenly*, AC moves forward and the plane will try to increase its angle of attack, stalling unless the pilot adjusts the elevator to keep the nose down. Lift from the rear stabiliser compensates for the rearward CG.

*but not so suddenly with a reverse delta.

Sounds like a canard flying backwards. Buried deep in the back of my mind is a project, called an Each Way Bet by my friend Trevor. A plane that can fly both ways without even landing in between. Maybe this is the germ of the solution!

Just a daydream.

Nick
May 13, 2011, 05:42 PM
Build straight - Fly twisty
Whiskers's Avatar
Great work, Marijn.
A Master Class on the subject.
I love this place...
May 14, 2011, 01:41 AM
What could possibly go wrong?
nickchud's Avatar
Quote:
Great work, Marijn.
A Master Class on the subject.
I love this place...
Yes!
Canada, USA, Australia, Turkey, Netherlands, UK. And we haven't even filled 2 pages yet.
May 14, 2011, 02:03 AM
Registered User
hoverman's Avatar
Quote:
Originally Posted by nickchud
So, in ground effect the AC is further back because of the ram air effect, yes? We need the CG to be further back than in normal flight. When we leave ground effect, ram air disappears suddenly*, AC moves forward and the plane will try to increase its angle of attack, stalling unless the pilot adjusts the elevator to keep the nose down. Lift from the rear stabiliser compensates for the rearward CG.

*but not so suddenly with a reverse delta.
Sounds about right, there are some smaller contributing factors, but yes, the main CP shift is caused by the occurence / dissapearance of the Ram air effect.

I'll have a look into my files, I think I have better/ more accurate plans of the flightship FS-8

& Yes, this really is a great forum haha

Marijn
May 18, 2011, 05:56 PM
Registered User
marijn, i have read much on ground effect and this is about the best i read!
i'm amazed that the tail being so high is still affected by the downwash of the main wing!
Q in the bixel/aircat what happens to cg shift that makes them stable although opposite to the reverse delta?
although a 'normal' wing does not capitalize on ram air pressure the closeness of the ground reduces down wash, reducing induced drag, so it may still be experiencing 'ground effect'(?)
the flaircraft video i look at often, showing the aircat by the dock, seems to show the stab having less incidence than the main wing, am i right?
May 19, 2011, 02:50 AM
Registered User
hoverman's Avatar
Well, if you take a look into airflow around wings, you'll find that the airmass that is actually influenced by the passing of the wing, is huge! Ofcourse, the closer you get to the wing, the stronger the air is deflected, but still even a T-tail on a WIG will experience another airflow direction than the free stream airflow, because of the presence of the main wing. Don't forget that WIG's have véry low aspect ratio wings, meaning they create a hell of a lot of downwash&upwash/induced drag when flying Out of GE

About the Bixel, damn I just don't know. I've had many discussions about the Bixel principles on this forum, via emails etc. but I still haven't quite figured it out. One thing I can say is that the Double wing Bixel layout is not what is creating the stability. The Longitudinal stability is hardly affected by the presence or dissapearance of the small outboard wings.

I believe the Bixel stability is caused by something like this:

By using a flat wing, or a relatively thin symmetrical wing profile, you now have a wing which has a natural tendency to go to 0 degrees angle of attack, and stay there. It's naturally self stabilising in pitch. Ofcourse as we have learned, using the ram air effect can create CP shifts, which ofcourse also occur on the Bixel. Why is it less influenced by this shift than other layouts(or does it only seem to be that way?)

Well, you'll notice that all bixels are built to have low angle of attacks, and also they naturally tend to fly at a very low angle of attack. You're using a flat wing, so a high alpha won't even be possible without stalling. However, when placing a wing only at a very minor angle of attack close to the ground, the amount of Ram air created below it, will also be very minor(and the amount of CP shift will also be ver...) If you can see where I'm going, then suddenly all the flight behaviours of the Bixel make sense!

- It flies faster than other WIG's
- As soon as it Takes off, the initial Ground clearance will start out higher
- Take off speed is also higher
- It flies at a low angle of attack
- It needs more power to fly in GE
- The amount of power needed to fly out of GE is not much higher
- The transition from in to out of GE flight seems to create no pitching moments

Alltogether, I believe that the main advantage of the Bixel is that it's just more of a low flying airplane and less of a WIG It's not very efficient anyway, so what's the advantage of a WIG if it's not efficient

About the stabiliser having less incidence, this is true, my Lippisch models also have 10 degrees alpha on the main wing, and 8-9 degrees on the stab. Still, it's positivly lifting, but 8 degrees ob the stab was sufficient for longitudinal stability in and out of GE.

Marijn
May 19, 2011, 08:28 AM
Registered User
slebetman's Avatar
The airfish even works great as a glider. Here's my build from last year:

https://www.rcgroups.com/forums/show....php?t=1071250



Parking test. (0 min 10 sec)
May 19, 2011, 01:27 PM
Registered FFF Addict
larryross's Avatar
Nice vid... she really did use the ground effect nicely.
May 19, 2011, 01:41 PM
What could possibly go wrong?
nickchud's Avatar
Quote:
Nice vid... she really did use the ground effect nicely.
Yes, thank you, and very re-assuring. Tonight's the night I start cutting out my FS-8. In principal, pretty much the same arrangement as your glider.

Thanks for posting the video. I'll try andpost some build photos soon.

Nick
May 19, 2011, 06:10 PM
Crazy, not stupid
Putting some extra encouragement in your bucket and and extra Dutchie in the international count
May 19, 2011, 08:39 PM
Registered FFF Addict
larryross's Avatar
Your going to do a little more scale than that glider I hope. Full fuse and all.



Quote:
Originally Posted by nickchud
Yes, thank you, and very re-assuring. Tonight's the night I start cutting out my FS-8. In principal, pretty much the same arrangement as your glider.

Thanks for posting the video. I'll try andpost some build photos soon.

Nick


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