David Ovenden Posted February 20, 2021 Share Posted February 20, 2021 I'm getting towards the end of my Curtiss Condor build now and so my thought have turned to "what next". A model that's been on my to do list for years is the Cessna T-50 Bobcat twin. Its got lots of wing area, fabric covered wings and fuselage so there is not acres of sheeting to do (balsa shortage / price increases) and the engines are well forward so CG shouldn't be a problem. I am planning on using a pair on Enya 53 four- strokes and will be attempting to have scale -operating retracts. My target weight will be 5kg (11lbs) and I'm thinking 1: 6.5 scale would be about right. That would give a wingspan of 77"; length of 60" and a wing area of 7 sq feet. At 11lbs that would give a wing loading of 25oz a sq foot which I consider reasonable. I have a French plan with a span of 73" which is pretty accurate in outline but has some quirky construction methods (like a 3mm square wing spar) that I will use as a basis and enlarge by 10% . I will redesign things a little and change the fuselage structure from 8mm square balsa framework to 1/4 spruce longerons with 1/4 balsa verticals and bracing. I would very much like to make this a "hybrid" model. That is, I would like to have both glow motor and an electric motor power modules that could be fitted, interchangeably so giving the option of twin four-stroke sound, or quiet electric flight. Mmm. I have managed to do something similar on a single engine model. But being a twin throws up a few more challenges. The engine nacelles are quite large on the Bobcat and the initial plan would be to design a power module that contained either the Enya 53 engine, micro throttle servo and fuel tank. OR, the electric motor. I imagine the front cowl being part of the module, but the top of the main nacelle unit being removable (moulded GF item) allowing the power module to be removed / exchanged) The cables from the ESC would stay in place as would the extension lead for the throttle servo on the glow module. The retract unit would stay in the lower nacelle and be the same for either power source. Retracts would need to be a custom build as on the Bobcat they pivot part-way up the oleo and not from the top of the leg. That in itself will be a challenge. But that's a lot of the fun of a new design - isn't it? 1 Quote Link to comment Share on other sites More sharing options...
Danny Fenton Posted February 20, 2021 Share Posted February 20, 2021 Should be a very interesting project to follow ? Cheers Danny Quote Link to comment Share on other sites More sharing options...
Dwain Dibley. Posted February 20, 2021 Share Posted February 20, 2021 Agreed, I'm tagging this one David, still have to maiden my Cessna 120. D.D. Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 21, 2021 Author Share Posted February 21, 2021 9 hours ago, Dwain Dibley. said: Agreed, I'm tagging this one David, still have to maiden my Cessna 120. D.D. I enjoyed your Cessna build DD. I'm sure the model will fly great, its was a BT design and expertly crafted by your good self. Odd times when we are building lovely models and can't get to fly them. Still summer and a vaccination jab are coming. 10 hours ago, Danny Fenton said: Should be a very interesting project to follow ? Cheers Danny This is not going to happen that quickly as I'm still at the thinking before drawing up the design stage. There are many questions to resolve. How to do the retracts being perhaps the biggest as that will affect the design of the nacelles and needs to right and designed into the wing structure. It also has to look like the full-size and be reliable! I wonder whether I could pivot a "standard" oleo leg half way up and then use a standard electric retract unit (with an output lever instead of a leg) to operate a pushrod connecting to the top of the oleo leg. That way the screw-jack mechanism and electronics are ready built and the unit has the up/down locking mechanism built-in. I may try a mock up with some HK units I have in the workshop to see if I can get the geometry right. Quote Link to comment Share on other sites More sharing options...
Danny Fenton Posted February 21, 2021 Share Posted February 21, 2021 Have you any pics of the full size UC so we can understand how it works? You like a challenge...... Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 21, 2021 Author Share Posted February 21, 2021 (edited) Found some pictures of full size. UC up and down. Quite complicated. Robart do some very expensive $600 units for the large scale Don Smith plan. Found drawings for those and a photo. Basically they pivot around a rear folding strut and a top mounted folding frame. I may have to rethink or settle for a less-scale solution? Edited February 21, 2021 by David Ovenden Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 21, 2021 Author Share Posted February 21, 2021 (edited) Also found a link to 2 (low quality) videos of a home -constructed Bobcat u/c made by a French guy (Henry Bruno)for a 100" model. Plans were published by Fly magazine, but they went bust and plans are no longer available (or so it seems)This system doesn't use a folding top frame but has a long travel screwthread drive to the top of the oleo leg. The geometry seems about right in terms of leg retraction as it still uses the rear strut as the pivot point. FLY-179-VID-Bobcat1.mov FLY-179-VID-Bobcat2.mov Edited February 21, 2021 by David Ovenden 1 Quote Link to comment Share on other sites More sharing options...
Danny Fenton Posted February 21, 2021 Share Posted February 21, 2021 Not as bad as I thought, but still a challenge ? Trailing link is the answer. Cheers Danny Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 21, 2021 Author Share Posted February 21, 2021 Cheers Danny. I will start looking for some suitable forked oleo legs to experiment with. They need to be able to take 3.5" wheels which seems to limit choice a lot (well at least in the affordable price range it does) Quote Link to comment Share on other sites More sharing options...
Danny Fenton Posted February 21, 2021 Share Posted February 21, 2021 If you really get stuck David, Brian Wood has helped a few of my friends with bespoke undercarriage machining, cannot promise he will help, but that looks like a real challenge he might enjoy! If you want to explore that let me know and I will put you in touch with Brian. Cheers Danny 1 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 21, 2021 Author Share Posted February 21, 2021 32 minutes ago, Danny Fenton said: If you really get stuck David, Brian Wood has helped a few of my friends with bespoke undercarriage machining, cannot promise he will help, but that looks like a real challenge he might enjoy! If you want to explore that let me know and I will put you in touch with Brian. Cheers Danny Thanks Danny. I've found some suitable?? forked oleos from HK. I've ordered those and will have a look at possible systems when I have them to hand. If that doesn't seem to work out I'll take up your offer to contact Brian. David Quote Link to comment Share on other sites More sharing options...
Max Z Posted February 21, 2021 Share Posted February 21, 2021 Nacelle structure of my Bobcat project (never completed, at least not in the last 25 years ?) Maybe it gives you an idea..... 1 1 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 21, 2021 Author Share Posted February 21, 2021 Thanks Max. Nice video and that gives me another avenue to pursue. I'm sure that I will need to make a nacelle "mock-up" (as you did) to test the retract operation before I start the build. Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 22, 2021 Author Share Posted February 22, 2021 (edited) Of course, I could always go for the fixed u/c option. I've found this photo! Would need small props though as there is little ground clearance. Edited February 22, 2021 by David Ovenden 1 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted February 27, 2021 Author Share Posted February 27, 2021 Not going very fast on this at the moment as I'm still finishing off the Condor. However, I did manage to use the U/C side view photos to plot the pivot points on the mechanism. I measured these and it comes out pretty close to the drawings I have of the Robart units, so it seems that its about correct. Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted March 30, 2021 Author Share Posted March 30, 2021 I now have some forked oleo legs from HK. They are nicely made, but unfortunately they won't really take the required 3 3/4" wheel without some modification. (In fact even a 3 1/2" wheel binds on the fork) Still they may do to create at a prototype set-up to prove the viability of a scale retract system for the Bobcat. 1 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted April 19, 2021 Author Share Posted April 19, 2021 (edited) I started drawing up the plan for the Bobcat wing yesterday. I have settled on 1:6.5 scale which will give 77" wingspan. I will be using my two Enya 53-4C engines but I am hoping to design the model to accept interchangeable electric motor power too. I have two SK3 4250 500KV motors and I am thinking that 13x6 3-blade props with 5S lipos will give equivalent power to the Enya using 12x6 2-blade props. More testing is needed. Once that is sorted I can look at designing the engine nacelles to accept interchangeable electric or glow power modules. In the meantime I better press on with drawing up the wing. I am currently planning on using NACA 2414 wing section at the root changing to NACA 2412 at the tip with 2 degrees of washout at the tip. I am thinking of modelling one of these aircraft as the colour schemes are not too complicated to do. Edited April 19, 2021 by David Ovenden photo alteration 2 Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted April 19, 2021 Share Posted April 19, 2021 Prefer the first colour scheme, not my project though. ? Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted April 19, 2021 Author Share Posted April 19, 2021 It's tempting John. Especially as I have 5 rolls of silver solartex in stock but no blue! 1 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted May 2, 2021 Author Share Posted May 2, 2021 I managed to do a bit more on the Bobcat wing design today. Designing the nacelles meant checking that fuel tanks would fit between the rib spacing/ nacelle side frames. I managed to find 2 unused 10oz Kavan fuel tanks and 2 Unused Dubro ones. How did I come to have 4 same-size tanks in stock? And why has the price of Kavan tanks gone down since I got them. Poor demand for I/C engines and therefore fuel tanks I guess. 1 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted May 18, 2021 Author Share Posted May 18, 2021 Been working on the retract system for the model. I want to have a scale (or close to) retraction system. I feel it will be easier to use a standard electric retract unit to power the system. Using the ratio measurements I got from the photos; and comparing them to the robart 1/5 scale units ; I came up with some dimensions. Today I mocked-up a system using a HK retract unit and some wooden links. Looks like the idea may work. I now need to fabricate some metal parts and test that the idea works in practice with real life heavy wheels and will be robust enough for a 5-6kg model. Here is a link to the Youtube video Retract geometry test 2 Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted August 19, 2021 Author Share Posted August 19, 2021 After bit of a break from the Bobcat project, I got some more done today. I have been doing some research into available materials and some experimenting with the design for the articulated oleo. I have finally decided on using some square aluminium tubing for the upper part of the strut. I fabricated some parts today to test the concept, which in general seems to work as planned. Video of the test on you tube here: https://youtu.be/YfYf_wcVnPUe Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted August 24, 2021 Author Share Posted August 24, 2021 (edited) I now have 2 folding leg struts mostly completed knowing that the geometry works. I need to make up frames now to hold the retract unit and the rear strut pivot point so it can easily be mounted into the nacelle. Edited August 24, 2021 by David Ovenden Quote Link to comment Share on other sites More sharing options...
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