Flap geometry

[Graeme Poke]

I have a YT 72" Spitfire. It's the first time I have had flaps. I can get approx. 60 degrees but the instructions say I need at least 75deg, peferrably80deg.

My book, Radio Control Guide by Norman Butcher, states "the horn should rake back if below the surface some 30deg" and "rake effect achieved by moving horn back" from the hinge line on the flap.

I tried this and I do get more movement but the servo "buzzes" just about all the time. Do I get a longer servo arm, (more work for the servo)?

What is the accepted way to set up flap geometry and how do I get more movement without possibly drawing on the battery to excess? Any help would be appreciated please.

Cheers Graeme

[Frank Skilbeck]

The servo buzzing is usually a sign that the surface or linkage aren't moving freely so the servo is having to work hard. Without the servo connected to the flaps can the flaps easily move down to 80 degs? If they can you need to look at the linkage. If you need to fit a longer servo arm to get the full movement and your servo isn't man enough then you may need to get a higher torque servo.

There's a servo torque calculator here

[cymaz]

Can you put the push rod on the furthest hole on the servo arm and the closest hole to the hinge on the flap horn ? This will give you the maximum mechanical throw .

[Graeme Poke]

Not really well Frank. They are the fuzzy cyno type hinges and I reckon you are right. Maybe I should replace them with plastic mechanical ones. It's a pretty big angle for them and not ideal. Yes I think I will get bigger servos as well. I am using JR 539's. Thanks for the calculator.

Cymaz, I have put the longest servo arm I have and the last hole just pokes through the hatch cover. So the pushrod is in that hole. The closest hole on the horn doesn't work because the pushrod scraped across the wing. So I am three holes up from the hinge.

I will try bigger servos, replace the hinges, (yuck!) and get a bit longer servo arm. Hopefully that will fix it. I was being lazy and trying to avoid all that. Thanks for your suggestions.

Graeme

[IanR]

Graeme, if you are going to fit new hinges, would this type of flap-hinge be any good for you? **LINK**

Ian

[Graeme Poke]

Thanks Ian, I haven't seen those before. I need (?) some stuff from hk so will order these as well. Doesn't hurt to have a look. Graeme

[Don Fry]

I have used these, do a nice job, but be sure to secure the ends, I used a bit of fuse wire through a drilled hole. Without securing, the two haves of the hinge can come apart under flight loads.

Edited By Donald Fry on 15/12/2015 09:42:29

[Phil Green]

IIRC the Spitfire had 90 degree flaps, all or nothing, take it or leave it!

[Piers Bowlan]

The early Spitfires (Mk 1A I think) had 60 degrees of flap but this was soon increased to 89 degrees as this produced a more nose down attitude on the approach to land, improving the pilots view. It also enabled a steeper approach, good for getting into small airfields. Be aware there there will probably be a large pitch change with flap so you will need to re-trim, best done in the circuit rather than on the approach. Likewise if you go-around (overshoot) be careful of retracting the flaps too near the ground because of the trim change. Also check to see that you have enough elevator authority in the flair with the flaps down as you may run out of up elevator because of the trim change (you may need to increase the amount of travel). As previously stated in an earlier post, ensure the flap travel is full and free, don't just fit a more powerful servo. If the flap is binding it will drain your Rx battery quickly. Eighty or ninety degrees of flap produce a lot of drag (that is what they are for essentially) so use power on the approach as you can lose speed very quickly if you try a glide approach;- with potentially disastrous consequences! Good luck.

Just my 2p worth

[Graeme Poke]

Donald thanks for the tip. I will order some in the new year and then decide whether to try them.

Piers, that's really helpful information. I was concerned that I might drain the battery so as Frank suggested I will change the hinges and hopefully have them work freely. The power on with flaps whilst landing has also been recommended by others with experience of such things. So thanks for reinforcing that procedure. I will give it a try up high to start with so I can get a feel for the reaction, pitching, trim changes etc.

I won't be ready till mid January, busy time, so I will report then on the result.Thank you all for your help.

Merry Christmas all, Graeme

[stu knowles]

I see to remember that when I put mine together I just chamfered the edges of the flap to get full and free movement and then by fitting the flap horn on backwards I was able to get nearly the full 90 degree movement without binding or any other alteration.

This model does come out pretty heavy, I went through three engines, RCV90 (gutless and underpowered) OS 91FX, not too bad but it needed lots of ballast and then MVVS 26 petrol which stayed with it until death.

They fly very well, I did rebuild the U/C mounts, which reminds me that this model should not be 'three pointed' when landing, It will fly in quite slowly but fly in down onto the main wheels,, Much less stressful on the undercarriage.

Nice model, I was sorry to loose mine - killed by changing my phone to a Blackberry which caused my (35) gear to failsafe.

[Graeme Poke]

Hi Stu, I have seen your model elsewhere. It looks so good, so different. It's a shame you lost it. I suggest you buy another.

I have chamfered the flap edges, as you suggest, well you have to in order to get the required movement but I never thought to put flap horn on backwards. I will investigate.

I have fitted a Laser 155, lovely motor, but I was not happy with the landing and it was suggested that I needed more flap movement. So I am in the process of modification but it will probably be another month now (busy) before I get it ready. Then I will see if it helps at about 80degrees.

Thanks for your suggestions. Cheers Graeme

[Cuban8]

My YT Spit came out at 14lbs and was a delight to fly.

In anything but dead calm conditions, you really didn't need the flaps, providing you could make a reasonably long and flat curving approach. Nice thick (but non-scale wing section) made the handling very predictable with no signs of tip stalling no matter how ham-fisted the pilot. Model broke its back after a dead-stick landing off the strip (crank-pin failure on a new ASP120FS, but that's another story) Rear of fuz was devoid of glass so as thin and as strong as an egg shell really.

Edited By Cuban8 on 19/12/2015 09:03:51