Chris Freeman 3 Posted November 11, 2024 Author Share Posted November 11, 2024 On Saturday we took delivery of the counter rotating unit that was the reason for the build of this airframe. The unit looks great and now needs to be tested so it has been mounted in the old faithful Stick! We need to use larger wheels to get some more prop clearance. 4 Quote Link to comment Share on other sites More sharing options...
Roy Thompson Posted November 26, 2024 Share Posted November 26, 2024 Look Forward to seeing your contra in action. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted November 26, 2024 Share Posted November 26, 2024 Whilst I applaud the design and build of your contra unit, there are commercially available contras that use 3 blade props - check out this link if you are interested. They turn 22 in to 20 in props and the revs are about 4,000 flat out. Since these are designed for aerobatics and push out over 3,000 watts you might want to reduce the throttle movement to a lower figure to give a more scale like power output. The sound off the props is amazing. Not cheap though! Quote Link to comment Share on other sites More sharing options...
Chris Freeman 3 Posted November 27, 2024 Author Share Posted November 27, 2024 Still not flown the unit yet, hope to get it done this weekend if things go according to plan which they seldom do. The commercial units are nice but as you say very expensive especially when staying in Africa. The big problem that you have with the commercial units are the size of the spinners, the scale projects all have massive spinners. I am also not sure what a nose over will do to the unit. 2 Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted November 27, 2024 Share Posted November 27, 2024 Nose overs are usually expensive! I fly F3A aerobatics and the aircraft tend not to nose over but with 55 mm diameter wheels they can be tripped up by rough lsnding zones - of course pilot error landing in strong and gusting winds, a prerequisite for most competition days(!), also takes its toll. The props are all carbon fibre as they need to be stiff to avoid flexing and, worst case touching, as they pass each other. That means expdnsive! My experirnce has been that either one or both props need replacing should you get a nose over. The prop shafts are concentric and while they might tolerate a gentle nose over the inner shaft is likely to suffer a minor bend that means a replacement. You can 3D print appropriate size spinners if you have the skills and equipment to do that. The usual range of spinners is between 80 and 82 mm for the rear one. Contras are fragile in that respect as I'm sure you will find if your MB5 tips over. Hope it doesn't happen very often and preferrably never. 1 Quote Link to comment Share on other sites More sharing options...
Bruce Collinson Posted November 27, 2024 Share Posted November 27, 2024 With my standard of landings, I’d be tempted to rig a nose skid until ground handling qualities are determined, then remove it. B Quote Link to comment Share on other sites More sharing options...
Chris Freeman 3 Posted February 24 Author Share Posted February 24 We finally got a video of the flying of the unit, now it needs to be installed the the MB 5 airframe. https://www.youtube.com/watch?v=pArm2EIwxWU Quote Link to comment Share on other sites More sharing options...
Roy Thompson Posted February 24 Share Posted February 24 On 22/05/2024 at 12:17, Simon Chaddock said: I do wonder if some day someone finds a practical way to fly an RC MB5 with a scale diameter contra rotating prop. It would turn at remarkably low RPM compared to what is considered normal. Prop tip clearance on take & landing would of course be a serious issue. A tail low take off and a 3 pointer landing would be a must. Just me dreaming. Been there got the T shirt. 😀 The trouble with building and maintaining a contra system is, it becomes a project in its own right. 7 Quote Link to comment Share on other sites More sharing options...
Chris Freeman 3 Posted February 24 Author Share Posted February 24 Very nice Roy You are right, the Unit that Jon made does seem quite robust but we do not want to test its strength as just the props are an expensive exercise. Time will tell but we will keep the option to go back to the original set up. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted February 24 Share Posted February 24 As a mztter of interest, how much has it cost to make your contra excluding the cost of the props? Be good to knos the weight as well. Commercial units for F3A are between £1,000 to £1,300. Designed for 10S and produce up to 3,600 W at around 4,000 rpm and 22 to 23 inch props. Some have 3 blade props but thd majority run 2 bladers. Quote Link to comment Share on other sites More sharing options...
Roy Thompson Posted February 25 Share Posted February 25 On 24/02/2025 at 14:47, Peter Jenkins said: As a mztter of interest, how much has it cost to make your contra excluding the cost of the props? Be good to knos the weight as well. Commercial units for F3A are between £1,000 to £1,300. Designed for 10S and produce up to 3,600 W at around 4,000 rpm and 22 to 23 inch props. Some have 3 blade props but thd majority run 2 bladers. Thats a good question that I can’t answer, the two motors that form the base were less than £100 each and all the other materials could have only been another £200 tops. The main cost would have been my time, machining up all the parts, assembling and testing, excluding the cost of the tools in my workshop. On the other hand the power output of mine are only half that and running 4S. If I’m remembering correctly the RPM is in the realm’s of 8,000 spinning Ramosa 3 blade Vario Props 13 1/2 inch dia. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted February 25 Share Posted February 25 Hi Roy Thanks for the information. I've attached a photo of the Debowski Twin Motor Contra Rotating (TMCR) powerplant. The motors have a kv of 120 so with a 10S pack had a max rpm of around 4,800 swinging a 23x20 props front and rear. The great attraction of this setup was with 2 outrunners cooling was good and there was no maintenance of belts or gears. You did need 2 ESCs though but I found that controlling them using a Y lead to connect them to the Rx was perfectly acceptable. If my memory serves me correctly they weighed in at around 600 g. Sadly, following a failure of the JR X-bus converter board that froze the elevator and rudder control on the aircraft, terra firma put an end to its flying days. I was going to point you at his website but when I looked he seems to have taken the TMCR off it. Whether that means he's stopped making it or is about to launch a new version I shall have to wait and see. I have no idea how much the TMCR costs to make but they are selling, or were, for around £1,600 including the 2 ESCs, spinner and a toolkit. A good deal more than you spent if you exclude your time. I suspect if you include your time, depending on how you value it(!), you might be in excess of the TMCR acquisition cost! These days, there seems to be a move away from epicyclic gearboxes towards a belt and direct drive gear drive and replacing in-runners with outrunners. Most continue to cost in the £1,000 to £1,500 range though. They do not take kindly to nose overs though! I'm currently using an Adverrun V1 that is a belt and pinion drive. The early Adverruns had straight cut gears but they were soon replaced by bevel teeth that made far less noise. In fact, most of the noise is generated by the props and they do sound almost like a well silenced four stroke. 2 Quote Link to comment Share on other sites More sharing options...
Chris Freeman 3 Posted February 26 Author Share Posted February 26 Interesting information. I finally got some detail of our unit which is very heavy and also the front propeller is not powered so it is just for looks! we did manage to get a second hand unit at a very good price but we still need to find a home for it as it looks quite fragile for a warbird. Paid around GBP 200.00 for it! Not sure what our unit cost to make as a friend made it for us at no cost as Byron made all the spray masks for his FW 190 and Pitts. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted February 26 Share Posted February 26 Thanks for the info Chris. That is heavy! Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 26 Share Posted February 26 Given that a contra rotating prop system eliminates torque on the airframe a geared reduction drive does seem ideal in that it makes a scale diameter prop a possibility. A tooth belt will take up more space but is quieter in operation. After all even the full size had a gear reduction from the engine. Gears worked for my MB5. It turned scale props but was light, low powered and not RC but it did fly. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted February 26 Share Posted February 26 Simon Unfortunately, just having a single motor with a contra does not eliminate the torque effect. Yes, the propeller effects are cancelled out but the single motor still has a load to turn. The Adverrun I currently use suffers from torque effect but the TRMC, with 2 motors independently driving the props does completely eliminate the torque effect. What I have observed is that the prop effects are of a higher order of magnitude than the torque effect. Quote Link to comment Share on other sites More sharing options...
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