Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 Assuming my calc's are correct - if you put 10kg 20" back from the prop then your CofG should move to 64" back from the prop (which is reasonably close "ideal"). Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 battery is behind pilot. Link to comment Share on other sites More sharing options...
Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 So it's aft of the CofG point. Depending on it's size it could be a big benefit to move it to hang off the firewall - but the possibility of that would depend on the battery size, the room you have inside the cowl and whether or not there's anything that can support the weight of the battery. All alot of if's and maybe's really. I thought of the battery because the Karatoo I am building is typically built around a Subaru engine and W&B can be tricky because a Soob is heavy - it was suggested to me that sometimes the battery can be placed right down the rear of the aircraft to help the process. I am curious - is this the Supercat that was advertised on Ebay for a long, long time? Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 yes and the plane that had engine failure flying it home after 3 hours circut work.(earlier thread) Link to comment Share on other sites More sharing options...
Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 Ouch... Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 pics Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 any ideas? nothing stand out? Link to comment Share on other sites More sharing options...
facthunter Posted April 29, 2012 Share Posted April 29, 2012 They really are a short coupled aeroplane aren't they? In the absence of a longer moment arm, would VGs below the horizontal stabiliser do the job? ( or fins ahead of the horiz stabilisers like fitted to some DH-82's. (Tiger Moths). Also what is the movement of the Cof G from full fuel to fuel empty, at max AUW on take-off ( corresponding to full fuel ). Nev. Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 They really are a short coupled aeroplane aren't they? In the absence of a longer moment arm, would VGs below the horizontal stabiliser do the job? ( or fins ahead of the horiz stabilisers like fitted to some DH-82's. (Tiger Moths). Also what is the movement of the Cof G from full fuel to fuel empty, at max AUW on take-off ( corresponding to full fuel ). Nev. my calculations = with pilot and full fuel =13.11" 1/2 fuel=14.1 empty NO fuel =15.3 allowable c of g = 12" to 16" ,,14.3" optimum Link to comment Share on other sites More sharing options...
Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 What am I missing then...? Tail wheel : 162" x 21kg = 3402 RH Main : 47" x 93kg = 4371 LH Main : 47" x 88kg = 4136 Pilot : 85" x 70kg = 5950 Total Moment : 3402 + 4371 + 4136 + 5950 = 17859 Total Weight : 21 + 93 + 88 + 70 = 272 CofG : 17859 / 272 = ~65.7 Less Prop -> Leading Edge = (65.7 - 50) = 15.7" from leading edge with 1/2 fuel Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 just done a very rough balance on a beem with plane level and 70kgpilot.set beem(1 1/2" thick beam under fuselarge which is flat) at 14"(optimum cg) and it lifted the mains ? looking tail heavy? will move beem tomorrow to find at which point the tail lifts and find where current cg is sitting. Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 What am I missing then...?Tail wheel : 162" x 21kg = 3402 RH Main : 47" x 93kg = 4371 LH Main : 47" x 88kg = 4136 Pilot : 85" x 70kg = 5950 Total Moment : 3402 + 4371 + 4136 + 5950 = 17859 Total Weight : 21 + 93 + 88 + 70 = 272 CofG : 17859 / 272 = ~65.7 Less Prop -> Leading Edge = (65.7 - 50) = 15.7" from leading edge with 1/2 fuel by no means trust my calculations.. Link to comment Share on other sites More sharing options...
Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 Or mine. Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 What am I missing then...?Tail wheel : 162" x 21kg = 3402 RH Main : 47" x 93kg = 4371 LH Main : 47" x 88kg = 4136 Pilot : 85" x 70kg = 5950 Total Moment : 3402 + 4371 + 4136 + 5950 = 17859 Total Weight : 21 + 93 + 88 + 70 = 272 CofG : 17859 / 272 = ~65.7 Less Prop -> Leading Edge = (65.7 - 50) = 15.7" from leading edge with 1/2 fuel not sure how much it matters but a re measure and pilot cg is 82" NOT 85" Link to comment Share on other sites More sharing options...
Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 The nose drop seems odd though - my experience with RC aircraft and having their CofG too far back is that they tend to do the opposite. The porpoising seems right - but not the nose drop. That being said, the CofG being too far back may simply make the aircraft unstable, and the fact that I've seen a few RC aircraft land with their nose in the air because of this might just be because of total loss of control. The elevator's effect becomes really twitchy - and the first you know is when you pull back to climb from take off. The other thing that strikes me as odd is that this resulted from a change of engine (and from reading the Ebay ad - the old engine didn't sound like it was too "old"). Perhaps the new engine is lighter than the old (Assuming it is a new engine and not a rebuild)? Is there a way you could find out? Link to comment Share on other sites More sharing options...
Guest pookemon Posted April 29, 2012 Share Posted April 29, 2012 Moving the Pilot's Arm to 82" moves the CofG to 64.9" from the prop (14.9" from the leading edge). So that should be well within tolerance. Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 The original engine was not what advertised.it was worn inside lot of scores on both piston and bores,,replacement engine has genuine 47 hrs from new and still has factory machining criss cross marks on bore ,piston No scores and is slightly later made..that said it should be lighter in the front end but not by much as I have removed the add on exhaust(which some bright spark welded between the manifold and tuned exspansion chamber) also removed pullstart bell)thinking back to when I flew at purchace I really dont recall searching for more elevator. must say from a pilots point of view it feels front heavy.Take off feels normal and balanced,level flight at cruise seems normal but the intant you back off revs she nose over and want to contiue noseing without 3/4 stick,now with revs set 60 kt level I pushed her forward and set approach angle and she still wanted to pitch forward asi now 70...? if not front heavy I would say tail plane,I spent 15 years designing and biulding R.C scale models and if it where a large model doing this and the balance was right I would 1.adjust tail plane (NOT a thrust line issue as it does it throttled back)and check for adequet air flow over them,2 exsteand the tail to main gap and re balance.but she aint a model and I cant stretch her..lol Link to comment Share on other sites More sharing options...
djpacro Posted April 29, 2012 Share Posted April 29, 2012 OK flyerme, a few more questions: - pilot moment arm - what did you take as the location of the pilot cg? (I can suggest where it would be for a seated male pilot) - the fuel tank - what is the useable capacity, moment arm, and when you said 1/2 fuel - how many litres of that useable capacity? Link to comment Share on other sites More sharing options...
Guest Maj Millard Posted April 29, 2012 Share Posted April 29, 2012 Flyerme, that looks like a Powerfin prop you've got there...happy with it ?.....I've got 650 hours on mine now on the Lightwing and it's going fine....................................................Maj... Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 fuel moment from prop to centre of tank is 37" 1/2 full=22ltrs full=45 litrs pilot c.g was taken with me seated, measuered to my belly button from prop Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 Flyerme, that looks like a Powerfin prop you've got there...happy with it ?.....I've got 650 hours on mine now on the Lightwing and it's going fine....................................................Maj...[ATTACH=full]17360[/ATTACH] wow thats an awesome looking lighty,,,,I would def say the best i've seen, and yeh love the power fin,very smooth,easily ground adjust,very very forgiving on gravel and such ,love it.. mine has been cut down from 60" to 56",NOT sure why? but no complaints with da prop Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 sorry was a double post..appologies:augie: Link to comment Share on other sites More sharing options...
djpacro Posted April 29, 2012 Share Posted April 29, 2012 fuel moment from prop to centre of tank is 37" 1/2 full=22ltrs full=45 litrspilot c.g was taken with me seated, measuered to my belly button from prop I wonder how the forum censor will deal with this: horizontal cg of a seated male is at the tip of the erect penis - please redo the sums, flyerme, and don't be shy Link to comment Share on other sites More sharing options...
flyerme Posted April 29, 2012 Author Share Posted April 29, 2012 make it from prop to erect penis tip 81" Link to comment Share on other sites More sharing options...
djpacro Posted April 29, 2012 Share Posted April 29, 2012 CG moves aft as fuel is burnt - seems to me that it would be good to have the cg 10 mm further forward based on these figures - however as others are suggesting, CG doesn't seem to be the cause of your problems, flyerme. (added note - moving CG forward may aggravate your problem so I am not suggesting to change the CG at this time)- Link to comment Share on other sites More sharing options...
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