My Lidl RC conversion Biplane - 2nd test flights

My Lidl glider RC Biplane conversion had its second test flight. A bit more successful than the first, but still has a tendency to climb like crazy sometimes and was quite pitch unstable. At half throttle it flew quite well, so I don't think it's tail heavy and maybe not enough downthrust is the problem. I've reduced rates a bit and added more expo to improve the twitchy behaviour, plus added more downthrust. I did add a little more nose weight, though you can see that when it climbs and then stalls the nose drops after the stall so like I said, I'm doubtful that CoG is the problem. What are opinions please?

I admire your tenacity Bonzo, you will get it in the end mate.

Perhaps a change of prop pitch ?? Down thrust.

C of g shift again ?? are you measuring it on the top wing ?

Lower the throws on the surfaces ??

Anyway, keep up the effort, it will be worth it.

D.D.

I could be very wrong with any advice although from experience I would say:

• Not enough motor down thrust if it pitch changes due to motor power, not the same thing as pitch up with increased speed
• Looked to me quite rearward c of g, that tends to make things lively with high elevator rates, IMHO leave the rates where they are and move the C of G forwards a bit and see.

Well done and keep on with the model and posts as its always interesting seeing a different take on things.

Hey Mr Moon, I do enjoy your endeavours, keep going. My hunch is to play with down thrust, and top wing incidence. As she looked quite good at half power my hunch is not to keep adding nose weight. Try and measure the incidence of the top wing and aim to get it negative in comparison to the bottom wing, presume tail plane and bottom wing are equal.. good luck Sir Moon of Bonzo

Bonzo Moon

You need to consider where the centre of drag is, particularly with a biplane and a low set fuselage as you have, and how it equates to the thrust line.

With the top wing set high, and with extra incidence, there are two effects taking place over and above what the tail plane/elevator might be doing.

As the speed increases the centre of drag will move up as the top wing is set at a higher incidence and will create a greater share of the lift and drag. This will tend to make the plane pitch up.

With a thrust line below the centre of drag as the thrust is increased relative to the total drag it will tend to pitch up. Note as the plane slows down in a climb the drag decreases, the ratio to thrust and drag increases and so does the pitch up effect. Not really what you want.

All this is going on at the same time as the centre of pressure of the wing moves backwards or forwards relative to the CoG with changes in air speed.

With two wings biplanes have more variables than a monoplane (and there are plenty with them!) so in testing it pays to minimise as many of the biplane related effects as possible.

My suggestion would be:-

Set both wings to the same incidence.

Set the thrust line so it goes as close as possible to the centre of drag.

Adjust the tail plane incidence, and with the elevator at neutral to it, so at a particular cruise power setting it flies level. To achieve this may require the CofG position to be adjusted as well.

Then set the down thrust to achieve the required degree of climb, or not, when more power is applied.

Then you can explore the plane's stall characteristics which may require adjustment to the relative incidence of the wings.

Setting up a plane so it 'behaves' as you want is not quick and all planes have their own characteristics. Some you just have to live with but it is very satisfying if and when you do get it to want you want!

Have fun.

Edited By Simon Chaddock on 29/11/2020 10:32:50

Hi that top wing looks to have too much positive incidence .. That amount of incidence might be ok for a glider or a slow flying vintage model but your flying it quite fast in the video and the top wing will create a lot of lift (too much ) and will stall quicker when it goes pear shaped . Should be a great little flyer when sorted a sort of lidl panic.

Big thanks to everyone for their input and a lot of interesting suggestions and theories, many of which I had already considered to be honest.

My first attempt at biplane 'creation' and yes, as Simon said so many more variables. Angle of incidence of top wing and overlap for a start.

Regarding incidence of top wing, I'd modified the angle following Andrew Newton's tests where he found a positive decalage of 3 degs was best. Maybe that was good for the model he was testing, but not good for this. Decalage is much discussed online but no definitive answers!

Andrew Newton's test

Anyhow, I have already since that 2nd test flight....

Changed the angle of incidence of the tail plane to try and get it to fly level without elevator trimmed down. Down trim didn't seem to be very desirable as it seemed to me that with down trim on elevator, as soon as you power down it's going to dive.

I have given a bit more motor down thrust and added a bit more nose weight. I'm hoping that will stop the tendency for it to climb like crazy sometimes.

I increased the expo a bit and reduced the rates. That might make it easier for me to handle!

As a last resort I might change the incidence of the top wing again.  I've done it once and it requires more serious surgery!  Having reinforced the struts with tape now too it's even more difficult.

Someone asked where I was testing CoG? At first I was balancing at the lower wing, like you would with a monoplane. Most Lidl CoG's are around 50/55 mm. Then I realised that was wrong and I read that the upper takes more load than the lower so I tried to calculate by interpolating between the too. ie that moves it further forward. Someone suggested on another group that almost at the leading edge of the lower wing. But this will all depend on the overlap!

Re the thrust line: in retrospect I wish I had mounted the lower wing a bit lower by putting a couple of small struts under the fusi and not insetting it. That would have put the motor more central to the wings. But as an experimental job modifying a £9 cheap foam glider it's always a gamble and at the time it seemed the best idea to mount it on the fusi.

As someone said, at half throttle it was flying quite well , even if a bit twitchy , so if I can improve the balance a bit I should be able to improve things. I have done lots of reading into the subject and they are a lot more complicated than monoplanes! A lot of fun messing about with it anyway!

I'll get there with it in the end and if not, as you saw in the last very short flight as a monoplane, it will fly solo wing, so I might do away with the top wing. At £9 it's not precious, just a bit of fun and a good learning experience.

This is my 6th Lidl conversion, some have been good, especially the twin-engined build, some have been bad. ie the twin boom pusher configuration, but they've all kept me busy through this weird Covid year!

Stay safe and happy flying!

Edited By Bonzo Moon on 29/11/2020 11:07:36

Looks like you’re having lots of fun with that!

Have you established whether it is is pitching up due to increased thrust or due to increased speed? If the latter, that is usually a sign that the cg is too far forwards. I’d suggest that, once it is trimmed and cruising steadily at part throttle, push the nose down into a gentle dive (to increase speed without changing thrust), then release the elevator stick. If it pulls out sharply, then you have a nose heavy model. Until that is sorted, you’re chasing your tail with thrust line adjustments.

Good luck!

Trevor

Yes indeed!  Just got back home after test 3 (video will follow tomorrow   )    and it flew really well since my last few mods! None of that crazy climbing! Quite easy and relaxing to fly, which what I was aiming for. Beautiful flying day here today as well! And flew I flew at a better, safer, location.

As it was behaving well I did get a chance for a few stall tests etc. and it was quite acceptable. Maybe a little nose heavy now. I would have liked to try it with a little less nose weight , but I'd run out of battery, and the 2s I had with me wasn't enough to get it off the ground.  ( It has a 3s 650 mA .)  The only thing it did need was a tiny bit more down trim as trimmed max on my tx it still wanted to climb a bit. I might make another small change to tail plane angle. I'd already moved it down a little.  Incidentally someone mentioned the top wing angle of incidence looked high?  I'd only given it about 3 degs positive relative to the lower.  

Stay tuned for todays test flight video!

Edited By Bonzo Moon on 29/11/2020 15:51:50

Edited By Bonzo Moon on 29/11/2020 15:52:34

Edited By Bonzo Moon on 29/11/2020 15:57:19

Edited By Bonzo Moon on 29/11/2020 16:02:47

Bonzo

3 degrees more than the lower wing is quite a bit. It is not uncommon to have that sort of angle between the wing and tail plane so in effect it means you are running the lower wing at a seriously low incidence when flying at speed. It will then contribute little lift. No problem in itself but with just a bit of up elevator the lower wing quickly starts to generate lift, a situation that can make the plane feel twitchy when strictly it is just the way the two wings are behaving.

I did a bit of research on the angles and found mostly conflicting information about decalage. I arbitrarily chose 3 degs positive after seeing Andrew Newtons video here? Maybe it is a bit too much for this home made conversion?

Andrew Newtons biplane experiment

Anyhow, it flew very well today!  I'm just working on the video 

Edited By Bonzo Moon on 29/11/2020 17:31:29

Edited By Bonzo Moon on 29/11/2020 17:31:53

Edited By Bonzo Moon on 29/11/2020 17:32:15

Finally got it pretty much sorted. 😀 Lidl glider RC conversion biplane, 3rd test flight was much more successful. Quite stable and relaxed easy flying. Just needs a little more elevator trimming as I was having to keep the nose up while flying. Needs a bit more trimming. I think main problem was CoG was way too far back. Shows the importance of getting CoG right!

Build blog is link on the video.

Sweet as a Nut Bonzo.

D.D.

Thanks mate!