Curtis Condor biplane twin

[David Ovenden]

1 hour ago, Colin Leighfield said:

That would be my instinct, not that it is necessarily correct! If you moved it forward say 3 mm it would at least be an indicator of what to expect and is unlikely to cause a disaster, I would have thought. I suppose the most important thing if you do move the c of g forward is what the nose does after you reduce throttle.

Well that's my thinking Colin. A forward move of the CG is the "safer" direction. It is easily achieved with a slightly heavier LiPo pack and involves no permanent changes so easy to undo if it doesn't work.  

[David Ovenden]

I have tweaked the downthrust and adjusted the top wing incidence very slightly as there apeared to be a very small difference between left and right sides. I will test fly again on Tuesday all being well with the heavier lipos which moves the CG forward just a bit. 

 

I am very pleased with the speed and ease of fitting and removing the outboard wing panels. It is just as quick and easy as I had hoped. It's nice when things work as planned - which isn't always the case.

 

 

Edited May 1, 2021 by David Ovenden

[David Ovenden]

Well, I got the chance for some more flights today. The tweaks I made improved things a little but there are still some unresolved issues with the model. I have checked that the wing and tail incidences are  as per plan, and they are all set a zero degrees. The plan shows 2 degrees down thrust on the engines. I now have 5 degrees downthrust on the motors. In spite of the model still puts its nose right up with full power (even when throttle is increased slowly) necessitating quite a handful of down stick to compensate. Equally, with power off it puts its nose down markedly. Surely it can't need even more downthrust?

The model is now balanced forward of the plan marked ( and hand-calculated) CG position. It still feels vague in pitch and sometimes seems to start dropping the nose/ losing altitude when flying level with no power change. Other times it seems to want to put its nose up. It just doesn't seem very settled in level flight. 

I also feel it needs to fly much faster than I would have imagined a big biplane would need too. Landings are also faster than I anticipated. Maybe its just too heavy?

Any thoughts??

[Colin Leighfield]

It’s difficult isn’t it David? You have made the obvious corrections. Perhaps best to fly it a couple more times to acclimatise to it as it is and see if these are just characteristics that you can adjust to? I wonder if the full-size plane was naturally pitch sensitive. I suppose it could be possible to mix some elevator trim into the throttle control. A little down with rising throttle and a little up with reducing throttle?

[EarlyBird]

On 21/04/2021 at 08:04, David Ovenden said:

The elevator with reduced throw is still sensitive even with 30% expo added.

 

13 hours ago, David Ovenden said:

It still feels vague in pitch and sometimes seems to start dropping the nose/ losing altitude when flying level with no power change. Other times it seems to want to put its nose up. It just doesn't seem very settled in level flight. 

Something does not add up in my mind. ?

I am going through the same process with my Ballerina. Too many variables causes confusion I find.

 

Vague in pitch indicates nose heavy?

Not being settled in level flight indicates it is not nose heavy?

 

I am at the bottom of the learning curve I feel. 

[EarlyBird]

I do remember being told that on a Bi-Plane the top wing should have a small amount of positive incidence for stability. I wonder if this is true?

[Bob Cotsford]

In the past I've tended to set the top wing at -1 degrees relative to the lower and that's worked ok.  It sounds to me as if it's nose heavy (nose drops off power, sluggish handling) and a little overpowered (zoom climb on opening the taps).  Vague elevator control may just be a factor of the design and short of a tail end rebuild with a larger tail you may be stuck with that - you did say that the elevator connection is free moving, slop free and with no flex didn't you?

edit - looking back at the photos the tail area looks to be more than enough.  I'd go back over the control system for the elevators and ailerons checking for slop, flex or stickiness and then check that the rigging doesn't allow any flex in the wings.  Other than that could it just be a characteristic caused by the tubby fuselage?

Edited May 5, 2021 by Bob Cotsford

[Piers Bowlan]

1 hour ago, EarlyBird said:

I do remember being told that on a Bi-Plane the top wing should have a small amount of positive incidence for stability. I wonder if this is true?

Hi Steve, My understanding is that there are three schools of thought regarding the relative incident angles for the upper and lower wings on a biplane. One says that  with forward stagger the upper wing should have more incidence as that will be the one that stalls first, so causing an automatic pitch down as the aircraft nears the stall (healthy). The second school of thought says the lower wing is subject to the downwash from the upper wing, reducing its effective angle of incidence so consequently it should have greater A of I. The Curtis Condor doesn't have stagger so neither should apply! The third school of thought says that neither of these theories are relevant at model sized because of the low Reynolds numbers! Personally I would keep both wings at zero. 

 

The model certainly looks beautifully built so no doubt the tailplane incidence is correct according to the plan, but Is the plan drawn correctly? The model does not appear to have a short tail moment or small tailplane for that matter so I can't see why pitch authority or stability should be an issue unless the C of G is out. With five degrees motor down thrust surely that is enough but then, the centre of drag appears to be a long way above the centre of thrust so perhaps large pitch changes with power is to be expected? Personally I would try moving the C of G further forward incrementally to check of any improvement - or not as the case may be. Peter Miller often seems to start with the C of G at 25% MAC when maidening a prototype model. This is on the basis that if it is too far forward the model will fly badly but if too far back it will only fly once!   

 

I hope you get it sorted soon Steve so good luck.

Edited May 5, 2021 by Piers Bowlan

[David Ovenden]

Thank you guys for your thoughtful and helpful responses. I think to some extent it may be that I just need to get more flying time in with the model and get accustomed to its quirks. It is, after all, a scale model of a 1930's biplane twin-engined airliner. They had relatively low power and a pretty small speed range. The model flies and has had 6 flights without any accident, so it is a matter of tweaking and improving things rather than major redesign. The wing and tail incidences on the plan match those on the Paul Matt 3-views of the full size Condor, so I don't intend to alter those at the moment.

So my plan is to tackle the issues below. I'm not completely sure of the order, but certainly I need to address one at a time so as to be able to evaluate what difference, if any, the change makes.

• The max  power available from the 2 motors is around 1200 watts, which is more than 100w per lb. That will be a much higher power to weight ratio than the full-size had. So I probably have more power than is required and need to manage that more effectively with the throttle. Danny F suggested earlier that I could add in some throttle / elevator mixing to reduce the climbing tendency. So I need to pursue that avenue.
• Whilst the elevator linking is smooth with no binding (and has a top-end & powerful digital JR servo) I will look at fitting a ball link instead of a quick-link to eliminate all possible slop.
• I also have a gyro fitted and need to experiment with the settings on that to see if that smooths out the pitching tendency.
•  I will gradually move the CG to see if that makes a difference.
• More flying of the model to get familiar with its characteristics.

Time will tell. I will report back as and when I get to try out the changes.

[David Ovenden]

Well another outing with the Condor today. Some progress is being made, but not fully sorted yet. So going through the points in the post above:

• The model definitely has more power than needed and flies better when the throttle is not fully open. The Throttle / Elevator mix works but needs adjusting. I used a mix with a curve and I need to reduce the amount of down mixed at mid throttle  and maybe add a tad of up mix at low throttle (or is that tempting fate?)
• I managed to reduce the slop a tad by adding a ball link on the elevator servo output arm connection to the pushrod. Zero linkage slop now. The model felt just a little more settled in pitch I think.
• The gyro is still not set right and was overcompensating on ailerons making turning less easy. I am not sure it did much in smoothing out the  pitch problem. More gain needed on pitch?
• I tried adjusting the CG a bit and am now back at the point shown on the plan! I'm not minded to move it further back yet until all the other adjustments are sorted.
• Flying the model more is helping. It seems to fly best at mid-throttle settings and needs co-ordinated aileron and rudder to turn smoothly. High drag means quite a bit of power is need on landing approach to avoid the nose dropping too much, but the speed is slow to "bleed-off" when flaring to land. 

So there was some  improvement in flying characteristics  but there are still some more things yet to get sort out.  I am going to try fitting counter-rotating props to see if that improves the turning characteristics. It may also reduce the (considerable) amount of right rudder needed to hold the model straight on take off. 

Another issue is that the ESC's are getting hot and on landing are giving "over-temperature" warning beeps. They are 60amp YEP units but the max current being drawn is only 37 amps. So I need to get some air flowing over them to improve cooling. Not sure how to do that without cutting holes in the covering which I'm loath to do. Maybe remove one panel of the cockpit side glazing (as if the window was open)?? The ESCs may be getting hotter because the model is flying at only 1/2 throttle most of the time.

BTW the 3700mah lipo packs were showing 64% capacity remaining after 5 1/2 minute flight; so 9/10 min flights may be achievable.

 

 

[Andy Gates]

David,

 

Could some of the"feeling" issues you have come from the Gyro unit? I don't really see the need for a Gyro in a machine like this.

Are you able to switch the gyro off once the plane is a few mistakes high to see how it really feels?

I only have one of my machines with a gyro, to help it remain stable in stronger winds while training others.

 

The pitching up with power issue I would expect with the layout of the plane - no surprise really.

Likewise with the co-ordinated rudder / ailerons in the turns - typical of planes of the era.

Wings at zero / zero incidence is fine.

How quick are you throttling up during take off, maybe slower progression will help with the rudder correction?

 

Are you using the BEC's on both ESC's to give power to the radio, could this be part of the reason they are getting hot?

While you say the ESC's are only at 1/2 throttle most of the time, that is when the electronics work hardest but only pulling around 15A should not tax them. You could try fitting heat sinks to the ESC's to improve the cooling, I used an old CPU heat sink on one of mine in a Chipmunk which flies like your machine with limited cooling, and it made a world of difference - so much so I am still flying it 16 years later with no modifications.

 

Best of luck with your next series of flights - I hope you get the issues nailed.

[David Ovenden]

22 hours ago, Andy Gates said:

Could some of the"feeling" issues you have come from the Gyro unit? I don't really see the need for a Gyro in a machine like this.

Are you able to switch the gyro off once the plane is a few mistakes high to see how it really feels?

Andy,

Thanks for your thoughtful contribution. Actually the gyro is switchable on/off from te tx. In fact most of my flying has been with it switched off. My tests so far have shown it does not help or make things worse. I thought I might persevere a bit to see if I could set it up to assist with handling in windy conditions. Its a pain though as I can only get to adjust the settings by removing top and bottom wings and derigging the whole model.

 

Re the ESCs. Yes I have both BECs connected as the YEP manual states that you can do this and that it doubles the current available to rx and servos. However, the model doesn't need 10A for the rx/servos so I should try with just one BEC connected to see if that helps. The Heatsink idea may be worth trying too.

[Andy Gates]

Any updates David?

[David Ovenden]

Well  Andy... a long time to reply. The Condor is not a pretty sight I'm afraid. The model has never felt "settled" and in the end it bit hard. It went into a terminal spin whilst flying straight and level and at a reasonable pace. I really don't know quite what prompted it. But once started, even though I had height, nothing would stop it ... till the ground did. Luckily the front of the fuselage acted as a crumple zone! In fact the wings , tail and rear fuselage are all OK. 

I will rebuild the model over the winter and try adjusting the wing incidences whilst I'm at it. Then have another try next spring.

The model just never did feel right in the air from the beginning, so there must have been some underlying issue. Hopefully the rebuild will improve matters. Here's hoping anyway.

Edited August 25, 2021 by David Ovenden

[Andy Gates]

Hi David,

 

Thanks for the reply and so sorry to see your machine in that condition. What a shame, you have my sympathies, but good on you for rebuilding over winter.

 

Can you try building in less weight during the re-build?

 

Around 11lbs does sound a little portly to me.

1st thing I would do is remove the battery tray I can see in the rear of the photo. Fit lipos to a small sheet of very thin ply (0.8mm ish will be fine) stuck to the fuselage frame and hold the lipos in place with velcro.

Can you use just 1 lipo instead of 2 to reduce more weight? 4S 3700mAH packs are still going to carry a fair bit of weight.

 

Could one of your bracing wires have snapped during flight?

 

Good luck with the rebuild. Please keep in touch.

[David Ovenden]

Thanks Andy,

I count myself fortunate that it is only really the fuselage that needs attention (And one wing retaining plate where the screw pulled out. 

This model does probably count as both the most interesting scale design I've built and at the same time the least successful flyer I have had. So I need to sort it out one way or another. I have seen other models (different designers and scales) flying happily on videos. The full-size aircraft carried thousands of passengers and was  intrinsically sound. So my model's flying characteristics must be "fixable" surely?

 

I will strip all the covering from the fuselage to get it rebuilt straight. I can at that point double check all the rigging angles against the plan and the 3-view of the fullsize.

Whilst its not easy to reduce the weight of a finished model I could try a number of avenues. Firstly I could, as you say, use a single lipo pack. That however will mean drastic weight reduction at the rear to compensate. (A 4S 3700 pack is 380g) I used no ballast to get the correct CG so it can't be removed! I can remove the rudder and elevator servos from the tail end and move them right up to the nose and use closed loop controls for minimal weight at the back. Losing 60g at the rear is equal to 120g at the front as its a ratio of 2:1 in relation to the CG. Then add in the 60g of servos at the front we have a net improvement of 180g. It would still mean 200g of ballast weight up front If I drop to a single pack.

I could use foam wheels instead of the heavier Robart scale one that are fitted. I can remove the gyro and associated extra extension leads. The ESCs could possibly go right in the nose. Maybe I could replace the wiring to the  motors with a lighter guage to save a gram or 2. Other than that it would be hard to lose much more weight without redesigning the whole model. Still, we will have a go. If it is possible to shave off 400g (one LiPo pack) without having to add extra balast to compensate it might make some difference, albeit with reduced flight times. Still, better a nice flying model with shorter duration than one with the potential for longer flights that doesn't get flown because of its unpleasant flying characteristics.

 

Edited August 26, 2021 by David Ovenden

[David Ovenden]

I have finally got round to starting the repairs on the Condor model. I have sripped out the servos from the tail and removed any excess weight I could from the rear of the fuselage.  I have largely rebuilt the front fuselage structure and need to think about where to relocate the servos.  I want to squeeze the servos and ESC right at the front of the fuselage.  l wonder how much weight I will actually be able to lose? Will the model fly any better afterwards? Only one way to find out.

[David Ovenden]

It's not an elegant job but it's coming back together. Bit of a 3D jigsaw puzzle!

[Jon H]

I missed the post about the crash, most unfortunate but the symptoms you describe are of an un corrected engine out so i would look into the motors first off. I know engine out situations are rare with electric but the symptoms you describe are fairly typical and its much more difficult to detect on electric as they are very quiet. 

 

That or the gyro freaked out, which is not unheard of. 

 

On the weight, 11lbs for an 82 inch biplane is very light. My Stampe is 82 inch and it wafts along at 16lbs so i wouldnt bust a gut trying to slim it down. 

 

Good to see it going back together. 

Edited February 16, 2022 by Jon - Laser Engines

[David Ovenden]

Hi Jon,

Thanks for the input. Yes, one engine out would have given that effect, but both engines were working fine before, during and after the crash.

However,  thinking about it I wonder if the gyro was the cause of problems all along. Intermittent malfunctions could have given the weird and random unsettled moments I experienced before it finally went u/s.

I will take it out competely and then I will know any weird flying characteristics are caused by the model not the gyro.

Saving weight is one issue. Removing mass from the extremities(front and back of fuselage) is another. Should help the handling???

I will also introduce washout on the top wing to help tame the plane.

Edited February 16, 2022 by David Ovenden
Spelling

[Paul De Tourtoulon]

Seeing you fly I wonder why you would put a Gyro in it,,,

 

 I have an HK plane gyro and a robbe heli gyro if you want them,,,

[Andy Gates]

Glad you are making progress with the repair David.

 

I have been watching for a while hoping you were going to progress it again.

[Chris Walby]

Hi David, Apologies but I skimmed a few of the posts back, but picked this up from you:

The Condor test flight was pretty successful with no nasties but a few issues to resolve. 

• Or is there just too much power from the 5s lipo pack? I didn't need full throttle to take off and almost all the flight was 1/2 throttle or less. So maybe 4s would be enough? The model weighs 4.7kg but there is 1200W from the 2 motors at full throttle. After a 4 minute flight the 7000mah lipo was still showing 74% so it is not power hungry.
• The weird thing is that on landing both YEP 60A ESCs were "double beeping" which notifies Low voltage.  However, measuring the LiPo pack showed all 5 cells at 4.0v. The ESC is set for "slow down" at 3.2v per cell. So something is very off there. Because of this issue I stuck with just the one flight. Any thoughts.
• I have  a dedicated separate 10a UBEC so maybe I ought to fit that rather than rely on the ESC BECs?

Did you fully resolve these issues? The reason I ask is IMO they are contradictory to some extent ?

If it leaps off the ground and flies at 1/2 throttle why would it trigger the ESC low voltage? Either one of the cells is knackered or 1200W @ 5S equates to 67A and 5S7000 should be good for 200A at approx 30C so you should be nowhere close to invoking the ESC Low voltage so why was it?

 

Questions

• Is a lipo knackered?
• Is the battery to ESC cables too small causing volt drop to the ESC under full load, only apparent under prolonged full power
• Sure you have both ESC's set up for "slow down" mode, if one is set for lock out it will just go into reduced power mode (only resettable with power cycling) and all will appear normal until you throttle up (over half approx). you won't notice this by sound, just by more throttle producing more and more asymmetric thrust until it overcomes the gyro/rudder - spin. The problem is the more power you add the worse it gets, but you may not realize that in the heat of spin recovery as its not doing what you expect! 
• Where you using props in same direction or counter rotating props? just interested in swing and prop wash
• When it crashed was it flying slowly or still around 1/2 throttle or more?

I needed a long chat with George to get my head around this so bear with me...as I thought ESC heat generation was proportional to motor power e.g the faster the motor is being driven the more power going through the ESC thus the more heat it generates. Assuming the ESC efficiency is constant - WRONG ! ESC's are most efficient at full power and become very inefficient at low power setting (generating most heat). Flying slowly may have another effect where cooling air flow could be reduced. A point here is that the ESC might back off or shutdown with excessive temperature and on a single you would just notice a loss of power, but with a twin, one ESC ok the other shutting down and we are back to asymmetrical thrust!   

 

Lastly, long shot but do both motor pick up everytime  exactly every time without fail....you never get one that back turns a bit or is hesitant in any way? Just I have a mossie that did it sometimes....found the manufacturers motor bullet connector poorly soldered under the heat shrink sleeving....but not before it spun on me (never had a chance of working out which one had quit to get the rudder in.  

 

Hope that helps and keep asking questions 

 

PS - I have experienced more electric twin dead sticks than IC over my limited years, but then I am far more attentive with the IC stuff!

PPS - I did have a twin powered mono plane that was 80 inch and weighed over 7 Kg, it flew, but not well so I think you are ok with wing loading.

 

 

 

 

[Martin Harris - Moderator]

Nobody's mentioned the possibility of a dodgy elevator servo as far as I can see. IIRC you had complained of occasional random trim changes in pitch which could be signs of an ageing feedback pot in the servo preventing accurate re-centring - maybe it finally bit big time...

[David Ovenden]

Hi Chris

Thanks for taking the time to read the thread and to respond. You ask some good questions. To answer briefly. 

The 5s Lipos, though new, were just very poor. Though labelled  as 35c they certaint aren't and voltage dropped drastically very quickly. Hence the ESC warnings.

I swapped out the 5s lipo for 2 x 4s lipos (40c) which provided plenty of power for the plane and didn't sag under load.  So no more ESC warnings.

The motors ran and responded perfectly at all times - never any lag or hesitaion.

I was/ am using counter rotating 10x8 3 blade props which provided good power and with which there was negligible torque swing on takeoff.

 

I think the problems with the model are mainly rigging/ incidence issues or a faulty gyro unit. 

I am checking the incidences and adding washout. The gyiro is coming out.

I will try it on one lipo pack to reduce AUW. We will have to see whether that improves things