Cliffwhacker

[Andy Green]

I think 81's should be fine. Just glad they're not 9g servos.

Looking for another cause.

Andy

[Martyn K]

It looks like there is quite a lot of dihedral on the design. Its natural stability suggests that there may be insufficient aileron authority to roll cleanly. Have you tried diving hard then rolling - get the airspeed right up?

M

[buster prop]

I've been plodding on slowly with my Cliffwhacker build and reading this thread. Wings and tail finished and just been fitting servos into the fuselage. I'm going to fix the tail rather than use bands because I'm concerned that it will move and affect the trim. Also I've put a triangular fillet between the fin and tailplane, without it the fin can easily get knocked off or in transit or just getting the model out of the car (been there done that). Someone suggested making the model electric powered and I had the same idea, according to the article Cliffwhacker weighs 47Oz just about 3Lb. So, it would need a 300-350W motor with a lowish KV so that it that could turn a 10" folding prop. A 2200 3S LiPo fits nicely into the nose compartment and I'd use a 40A ESC. In the end I made mine a glider, I have other electric models. Interesting discussion about roll response, I used HS85MG servos for the ailerons, they should be powerful enough. When I get the Cliffwhacker finished I'll try to post a picture.

[buster prop]

I meant to say HS82MG servos, not 85's.

Edited By buster prop on 28/07/2015 11:57:50

[Leslie Crane 1]

HI martyn, yes all my rolls are attempted after a dive but perhaps I wait too long after and am in a slight climb. I would be surprised if the dihedral was a problem as the original had some and others on this thread manage rolls ok. My dihedral is the same as shown on the plan. I think I need more speed in the dive and not to climb gently straight after. I will try different aileron movements as perhaps I am using too much and they are acting as brakes.

Hi Buster prop I made the same 2 amendments re tailplane and fin as you for the same reasons.

[Sean Murray-Smith]

Hi Leslie,

Perhaps using differential aileron with more up than down in order to remove the adverse yaw and drag effect. The weather in Adelaide is atrocious and I have been unable to throw mine off a cliff, but some weekend soon I shall have a go and let you know what happens

[Leslie Crane 1]

Hi Sean my flights were with 20% differential but was advised to remove it to increase roll rate!! I have yet to refly to see how that goes. Will let you know

Les

[PatMc]

Leslie, differential definitely slows the roll rate - don't forget that it's acting in the wrong direction during 50% of a roll. Another method of increasing the roll rate of a model with dihedral is to mix in some rudder with the ailerons if your Tx can do it - about 50% is usually OK. It may seem that rudder will be operating in the reverse direction during the inverted phase but so long as you don't push in too much down elevator during inverted the dihedral causes the rudder to keep working the desired direction. (I use coupled R/A in my Easy glider & a 100" ex thermal model, now used only for slope flying, when I'm throwing them around)

The ailerons on the CW seem to be a bit small but bearing in mind that several other folk apparently have no problem rolling the model I think perhaps your problem has to be a fundemantal operational one.
Have you done a dive test to establish the CG ?
Are you flying the model like a thermal glider ?
Most slope soarers are capable of being rolled from their normal flight path with little or no diving beforehand because they are trimmed to be flown at higher than "normal" glider speed.

 

Edited By PatMc on 30/07/2015 00:25:29

[Andy Green]

I don't get this:- 'don't forget that it's acting in the wrong direction during 50% of a roll.'

The direction of roll doesn't change to the differential is working with you all the way round isn't it?

Andy

[PatMc]

Andy, when flying right way up the downgoing wing's aileron is rigged to give more drag than the rising wing's. When inverted that the rising wing's aileron is creating more drag than the downging one.

So, no the differential isn't working with you all the way round.

[Leslie Crane 1]

Hi PatMc you ask questions to which I don't have an answer. How do you determine Cg using a dive - I am used to balancing to do this then if it flies level and climbs in a level attitude I assume it is reasonably balanced around the CG. Also as I dont' thermal soar I haven't a clue as to how they would be different. I am fairly new to slope soaring, about 12 months.

Les

[Sean Murray-Smith]

Hi PatMc,

The downgoing aileron causes increased drag and thus adverse yaw, but it would be on the upgoing wing.

Cheers

Sean

[Steve Houghton 1]

Let's not forget that, to my knowledge, this model was designed as a slope trainer, and so it isn't really designed for aerobatics, i.e. rolling. Loops, stall turns may be about as good as you'll get. The roll rate may improve by adding rudder as someone else stated but really this needs to be done manually and not through the computer on the tx. So for a right hand roll you would add hard left rudder as the model banks to about 45 degrees, easing off as before you're inverted, then hard right rudder as you begin to come out if the roll. I have to do this on a couple of my gliders to achieve a better roll rate.

Steve

A470soaring.blogspot.co.uk

[PatMc]

Posted by Sean Murray-Smith on 31/07/2015 08:38:13:

Hi PatMc,

The downgoing aileron causes increased drag and thus adverse yaw, but it would be on the upgoing wing.

Cheers

Sean

Sean, we're considering a roll. When the model is inverted the previously upgoing wing becomes the descending one - thus any differential is acting to increase adverse yaw during 50% of the manoeuvre.
In any case the downgoing aileron is more effective in causing roll than the upgoing because it's acting in a higher pressure airstream (due to the wings angle of attack). Since Leslie's problem is lack of roll rate having differential ailerons would be an undesirable factor.

[PatMc]

Posted by Steve Houghton 1 on 31/07/2015 09:09:25:

Let's not forget that, to my knowledge, this model was designed as a slope trainer, and so it isn't really designed for aerobatics, i.e. rolling. Loops, stall turns may be about as good as you'll get. The roll rate may improve by adding rudder as someone else stated but really this needs to be done manually and not through the computer on the tx. So for a right hand roll you would add hard left rudder as the model banks to about 45 degrees, easing off as before you're inverted, then hard right rudder as you begin to come out if the roll. I have to do this on a couple of my gliders to achieve a better roll rate.

Steve

A470soaring.blogspot.co.uk

Steve, it may have been designed as a trainer but several other people have commented that the model rolls OK - e.g. 26th April in this thread Leccyflyer posted that "she was both very stable and fully aerobatic".

Re improving the roll rate using rudder - bear in mind that Leslie in a newcomer to slope soaring. Using a rudder mix in the Tx is a lot easier than trying to apply it manualy in varying degrees & direction during a manoeuver that he's not done before whilst slope soaring. I doubt that many newcomers to aerobatic flying could handle the manual rudder inputs you describe without a lot of practice first.
In any case with a model that has plenty of dihedral, like the Cliffwhacker, manually mixing is unecessary as the rudder will give effective roll in the same direction through 360 degrees so long as too much down elevator isn't applied during the inverted stage.

Edited By PatMc on 31/07/2015 19:53:11

[PatMc]

Posted by Leslie Crane 1 on 30/07/2015 23:30:03:

Hi PatMc you ask questions to which I don't have an answer. How do you determine Cg using a dive - I am used to balancing to do this then if it flies level and climbs in a level attitude I assume it is reasonably balanced around the CG. Also as I dont' thermal soar I haven't a clue as to how they would be different. I am fairly new to slope soaring, about 12 months.

Les

Hi Leslie, the dive test is a fairly well established method of "tweaking" the cg for aerobatic (& thermal) gliders. Most published sports & beginners model designs show a fairly far forward cg on the plan as this makes the model more stable longitudinally. Without going into lengthy explanations why a forward cg has this stabalising effect I think it's sufficient to say that if you want to extend your model's performance beyond the complete beginners' stage it would probably pay to see if the cg can be moved back a bit without leaving the model uncomfortable to fly.

Here's a diagram that needs very little extra explanation.

I've nicked it from another site where it was refering to thermal gliders which is why "slow trimmed glide" is mentioned. Flying from a slope the Cliffwhacker should be trimmed to maintain a cruise speed of around 15 - 20 mph without losing altitude before starting the test. If any cg changes are made the elevators will need to be re-trimmed before a re-test & when the cg position has been finalised.
It's likely that the model will behave as in A at first so the end result will be model capable of a greater speed range in all lift conditions. The ability to fly faster will improve the roll rate.

I asked if you were flying the model like a thermal soarer because a lot of people new to slope soaring (including very experienced power flyers) assume that because the model's a glider it needs to be flown at a fairly slow speed to stay up. To get the best out of an aerobatic slope model it should ideally be kept at around eye level by keeping the speed up with a little down elevator within the slope's lift band. That way aerobatics can be performed in a similar fashion to a power model but at a better viewing level without the model necessarily being dived before each loop or roll etc. But that's the ideal situation with a good breeze & well sorted model. Meantime you will need to dive the model before trying aeros but IMO you should consider whether you are indeed flying the Cliffwhacker too slowly in general & if so try to do something about that rather than any major alterations to the model.
It might be worthwhile to have a look on Youtube at any slope soarers doing aerobatics & try to analyse what the control inputs are.

Page 2 of this PDF gives a fuller explanation of the dive test albeit with a (low) powered model but the principle's exactly the same.

HTH, cheers

Pat.

[Leslie Crane 1]

Hi Pat thanks, interesting comments. I have been flying fairly high and attempting rolls out of a dive but just on the up. I haven't tried them lower down on a fast run parallel to the slope in case either I lost it or it lost speed as it seems to do when I attempt a roll higher up so will try rolling on a lower pass parallel to the slope. And, yes, I have been flying it at times like a power model, slow circuits!! Also I already thought that I needed to remove a little ballast from the nose. My efforts and the model's performance can be seen on the sussex slope soarers site, middle column, either the last video on that first page or on p2 (older posts). Strangely it is captioned something like Les maidens the Cliffwhacker!! The video is about 8 mins long and includes 2 launches 2 landings and both close up and long distance. Very boring really - except to me!!

Les

[PatMc]

Hi Leslie, I had a look at the video. IMO the model is being flown too slowly most of the time but the speed as you prepare to land is about right for normal fast cruise & when you want to do aerobatics.

I noticed in the video that the pre loop dives were too steep & brief which didn't allow the model to build up enough speed. That's why it was sometimes almost stalling out & dropping a wing as it went over the top. As the model completes the loop it should be going fast enough to go straight into a second loop. In good lift conditions consecutive loops can go on indefinitely but that does take some practice.

My advice is to leave the cg where it is for the time being but trim in some down whilst in flight. The amount of down needs to be sufficient to increase the speed without the model descending (or diving) but maintaining a decent altitude above the launch point. at the site in the video 50 - 60ft should be adquate but if that makes you feel uncomfortable make it a bit more. You'll find that there's a "ceiling" for the speed that the model's trimmed to. Forget the aerobatics & practice flying around at that altitude & speed for a while, building up confidence possibly over a few flight until it feels the natural way to handle the model. Once you're comfortable flying this way you'll find you only need to dip the nose down to build up the speed to begin the aeros & you'll wonder why you ever had a problem.

HTH

Pat.

[Leslie Crane 1]

Pat thanks for watching the video it was obviously a much better indicator of the situation than I was able to convey in writing. I will do exactly as you say. Your comments are very helpful and do reflect my existing flying and I can see that the faster speed will enable the ailerons to work more efficiently - power models tend not to slow down much through the roll so airflow over the wing is more consistent which is what I obviously need to aim for in the gliding. There is provision in the design and as constructed for an additional 8 oz of ballast at CofG which I can take with me and try should I still need even more speed. I really appreciate the time you have spent helping me and will let you know how I get on although it may be a couple of weeks before I can get to the site again - unless I get brave and try it at Mill Hill where the landing area is much tighter!!

Les

[Leslie Crane 1]

Hi Pat thought I would let you know how I got on. Altered flying style and got 2 rolls but very hesitant ones. Had to trim in some down but still needed a lot of down when flying (much more than I felt happy with) which suggested more nose weight needed. However, the dive test resulted in a positive climb which if I have interpreted your diagram correctly suggests that the CofG is already too far forward requiring weight off!! I still need more speed I think so extra weight at CofG may be helpful.

Les

[Peter Ager 1]

Good afternoon gents.

Just a quick query, does anyone know what comes in the additional woodpack kit?

Regards

Peter

[Leslie Crane 1]

Hi Peter I bought the woodpack and if I remember correctly, did not need to buy any more it had sheet, strip, leading and trailing edges etc.

Les

[buster prop]

My Cliffwhacker is now almost complete and this evening I put the wing on and connected up all the servos for a stick-waggle. In the end I decided to bottom hinge the flaps with tape. The balance point was checked and I had to add about 5oz of lead in the nose to get it onto the wing spar. With a battery fitted (and nose weight) the model weighs 2lb 5oz, 37oz which is 10 oz lighter than the article said. Now, I'm not a particularly light builder so have to assume that the 47oz figure was with optional weight fitted under the CG. Must take a photo and try to post it on here.

[Sean Murray-Smith]

I'm tired of waiting for the Adelaide weather to settle down and for the wind to blow the right way to fly from the cliffs at Sellicks so...

I had the EDF lying around in the shed so I put Glad Wrap on the centre section and laid down fibre glass and 2 pre-drilled "L" pieces of aluminium. With a 22oomAh battery up front she weighs in at 45 ounces.

I have named the unit "Otis" (Electric lift)

[buster prop]

Here it is at last. Unflown as yet but balances on the spar and the controls all go the right way so when I get time and the wind's in the right direction for the slope I'll give it a throw. As you can see its the flapped version and I have put the flaps on the throttle channel of the AR610 receiver. With the stick fully forward the flaps are up and go down to about 70 degrees with the stick back. I haven't mixed them with ailerons so don't have full camber change. I'm a bit cautious about this as I don't want it to tip stall! A flap to elevator mix might be needed as well. The only mix at present is slight aileron to rudder, will maiden it as-is and see how it goes..