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Aerodynamics Discuss the concepts of aerodynamics here

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Old 09-06-2011, 11:49 PM   #1
quorneng
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Default Simple flaps

I built this wing for my Wing Dragon specifically to explore the effect of flaps and the result on the handling as the basic 4ch Wing Dragon is considered pretty forgiving.
The 40" wing is tapered 2:1 has a constant Clark Y section and with no washout.
Although quite a good shape structurally such a configuration might be expected to demonstrate quite 'aggressive' characteristics with the tip stalling first.
The flaps are quite generous covering 2/3 of the trailing edge, are fully variable and can descend to a high drag 70 degrees.
Compared to current fashion the ailerons are quite small but do have a differential action (more up than down) and are completely independent of the flaps.
The basic questions were:
1. Do the flaps work and how slowly can it fly.
2. What happens as they are deployed.
3. What are the stall characteristics and is adequate aileron control maintained at the minimum speed.
This video shows their effect.
YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.

Although not intended to be aerobatic it nevertheless has quite a spirited performance and the flaps certainly work allowing it to fly well below the 'clean' stall speed.
It is interesting to note that with full flap the angle of attack is hardly altered. This means the airflow at the wing tip is not stalled and thus the ailerons remain effective, albeit with decreasing effectiveness as the speed reduces. Their differential action seems to keep the adverse yaw in check when full aileron is applied at slow speed.
The only drawback is a substantial nose up pitch when the flaps are deployed at normal flying speed, however if the speed is reduced in conjunction with the flaps the final trim change required is no more than a few 'clicks'.

So far so good!
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Old 09-12-2011, 01:29 AM   #2
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Default More complex flaps!

The simple flaps certainly seemed to work so I wondered if a bigger more complex flap arrangement would give a similar slow speed performance from even less wing area.
With the same span and aileron size as before the flaps were broader hinged at nearly 50% of the wing chord. To make the flaps aerodynamically more effective they have two sections with the first travelling only half that of the second.
Click image for larger version

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To achieve this action from a single servo requires a rather complex linkage geometry.
YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.

The slow speed performance is impressive and as the flaps are hinged quite close to the centre of pressure the trim change is not severe but the lack of rigidity of the complex linkage rather limits the maximum speed and precludes any sort of inverted flight.

Some improvement to the flap linkage is required to fully exploit the 'double flap' capabilities.


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Old 09-16-2011, 03:19 AM   #3
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Default

So basically what you are looking to achieve is similart to the full size Fowler Flap arrangement ... where the flaps are in sections and form a curve. Are you looking to also create a slot between as full size have ?

You mention speed restriction for deployment .... as far as I'm aware from being member of a family that half were in the flying game .. that is same situation as full size airliners etc. Deploy at anything above recc'd speed and you're liable to either rip them out or do severe damage to the mechanism.

I have had and have now flap equipped models. My first was a Royal Kits P51D Mustang ... beautiful model ... HB50 glow up front. Latest now is a Cessna 182 with flaps ... electric. Do they really make that much difference in model size .. not with mine to be honest. Yes the approach speed could be lowered due to extra lift of the wing, but power needed to be tweaked along with elevator to stop dutch rolling ... touch ailerons and you'd add to the problem.
I pretty well leave my flaps alone now .. unless I want a 'photo'as she low passes in front of flight line !

I'm interested in the advantages / findings of what you have with your flaps ...

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Old 09-17-2011, 01:47 AM   #4
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solentlife
Compared to scale plane flaps mine are enormous. It does not surprise me that those on your Cessna do not have a dramatic effect, after all, in the full size 172 stall speed is only reduced by 4 or 5 knots with full flap. You would be hard put to detect that difference unless you looking at an air speed indicator.

I was looking for a much greater stall speed reduction and perhaps even more important could aileron control be maintained.

The double flap has no slots as in model sizes (low Reynolds numbers) air is rather 'sticky' so the airflow tends to adhere to the wing surface more easily and this stickiness also means that airflow though a scale size slot is so poor that it has little effect.
Scale leading edge slats also have this same problem.

The videos were made to show the extremes of flying speed but perhaps of more use is the slow steep approach possible with full flap.
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Old 09-18-2011, 02:22 PM   #5
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Two examples of a steep landing approach with the Double Flap Wing Dragon.
The conditions were virtually calm and both landings started from what would normally be considered an impossibly high point.
YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
The first makes use of the flaps high drag for a really steep approach. Judging the flair point is not easy!
The second is a 'parachute' type approach at minimum speed. Not quite as steep and being close to the stall all the time requires careful speed control but the actual touch down is simple.
In such good conditions it was safe to take each of these landings to the extreme. Normally somewhere in between would give the greatest reserve of control.
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