|May 13, 2012, 03:08 AM|
THE BIGn KFm4 project
The idea is for an update for the basic propjet design..
A big build by Otto D has a couple of good ideas that I want to use..
First is the capability of sliding grass take offs and second is a sub rudder that works well on his bigger version..
He achieves his grass take offs by using a higher motor location, and a fair lot of power
Ottos build.. http://www.rcgroups.com/forums/showthread.php?t=1504288
Ive been working on a generic propjet build to enable a change of shape to determine what you end up with..
As with my smaller models profile outlines are all that are needed... changing them to make what ever sorta jet you prefer..
So using a downloaded 3 view drawing I did a trace over for the basic shapes using sketchup and combine them with the drawings already made, using both my previous build and Ottos
I am going with a sorta YF22 It will be 48" long x 36" span
First powered by an emax 2215/20.. 3 cells.. 9x5 prop.....edit this flew but lacked power...
It was replaced by a 35-36 1800 kv motor.. the 9x5 prop was cut to 8x5.. It needs a 60A ESC and 2200 30C 3cell lipos... now its well overpowered and goes superbly..
Note.. this is a powerfull setup, it takes the jet up to 60 MPH , its not running to get speed but for thrust.. vertical is at half power, full power takes it up like a rocket.. Cruise is at half power or under...
The construction is basic...
EDIT now recommended and because of the massive power is that the main wing and controll surfaces and fins all be made as a doubled layers, so any ailerons and elevons need to be removed prior to laminating these together.. cloth hinges for all the surfaces must be placed between the two layers as you laminate
cut out 2x main layers and two front outer layers for the KFm4 section..laminate..dont forget the cloth hinges..then fix the ailerons and elevons in place...
Note the pictures only show my single layer build , so they dont show any cloth hinges being fitted for elevons....because they weren't...
The only cloth hinge I used in the prototype was in the sub rudder construction...see post 138 for the hinges construction..
Make up the sides and lower fuzz using two 6mm depron layers..
The sides and lower front are chamfered if you want angled sides or fixed in place vertically...
depending on what you are building..
Make a sub-rudder to the depth of the fuz sides..This is made using a double layer fixed fin a cloth hinge and a double layer rudder see pic 3 below
Later the bottom will be sheeted over the front fixed part of the rudder and forward of the prop slot, but first the servos have to be mounted
Make a top fuzz shape again two layers of 6mm and fix/slot in the motor mount ... I used one of the square tube mounts (10mm sq), and cut it into a slot with 3 degrees of down thrust, no side thrust
The mount was glued in place with cool melt glue and then some additional side cheeks , two on each side, were placed to further block in and reinforce the tube type mounting .. .
.Further reinforcing using tapes and cool melt glue were used to strengthen the motor block for the high powered motor..I used small side cheeks.. longer ones would be better.. pic4
All joints are butt glued only using UHU por.. they are then beaded with a cool melt adhesive on both sides of the joint.
Later tape covering will be added to the wing and fuz..this will be also lapped over the joints and the end result is well strong
|May 13, 2012, 03:33 AM|
I will post final drawings later, at the moment some parts are being modded as I progress..
note... when designing..The motor is mounted higher ... on the 48" version I decided to use a known setup, with a 9" prop.. so I put the motor center 1.5" above the main wing layer and then I needed the lower fuzz sides to go deep enough to give the prop clearance.. 3 1/4" deep below the wings layer gives 1/2" clear of the prop, and it doesnt look too deep when viewed side on..
For vertically fixed lower sides its not problem , but for angled sides it means that the sides need to be deeper by an (X) amount.. to allow for the foam removed to make the chamfer .... simplest is to stick with vertical sides, I went angled to better suit an F22, but the F14 and SU look fine vertical ..
In the pics above you will notice the lower fuz goes all the way back to the front of the slot.. when the sides are in place the three should all be at the same height above the wing so the bottom decking fits across all nice and flat
|May 15, 2012, 04:42 AM|
note pic1... shows an uncovered rudder....It needs to be fully tape covered to make it strong and stiff
note pic3... make these side cheeks afew inches longer to suit the powerfull motor
Pic 4..shows the first motor which was too small
|May 15, 2012, 05:03 AM|
Order of construction
cut out the parts..x2 main layers, x2 KF outers, x4 elevons and aileron controll surfaces
laminate the wing.. making the KFm section....include the cloth hinges between the main layers..
Fit the ailerons and elevons
Fit and glue all the lower fuz parts... that is the two sides , the lower front fuz shape
Next the sub-rudder needs building and hinging.. I used two layers to build the assembly, with a fabric hinge between the layers.. when built, fit the servo, horns and pushrod to it.. then fix the sub rudder in place
That shown in pic1, post 124
The sides and the sub-rudder must all end up at the same depth because later the bottom sheeting will be fitted to make a nice strong box of it all..
The fuz sides should end at the sub rudder hinge line.. .. I initially made the mistake of taking the sides back further, as far as the elevon hinge line, this nullified the rudder, and I later had to cut them back... see the pic below for the correct end position
|May 15, 2012, 05:57 AM|
Before fixing the top structures, it is easier to fit all the servos etc. in place.
The elevons and ailerons, should be fully tape covered,
full tape covering makes them a lot stiffer for better results..
Fit horns and pushrods..pic 2 post 124, back linking the elevons to the ailerons
The first 3 pictures below were taken with the smaller power system componants,, however the "big stuff" all goes in the same location, and the bigger battery has plenty of back and fore adjustment to sort out the COG.. the velcro holds it nice and firmly
Velcro... on the fuz there are 4 strips ..two inners have hooks..two outers have loops...
On the battery I fit loop strips along all 4 long sides.
The battery then will grip the hooks on the fuz and wing..
I then use a loose hook strip tie down as security which will grip either side of the battery and onto the battery as well..
this is a good secure system that will only release in a hard crash, or when you pull the battery for a change over..
|May 15, 2012, 10:28 AM|
When the gear is all fitted, get the sub rudder working and adjusted..
Then the bottom gets sheeted... A hatch is built into the front area to make it easy to access the radio and place the battery ..
the hatch is tape hinged at the front and is held shut with strong button magnets at the rear..
The top fuz and fins are added..The fins should befully tape covered before gluing in place... butt join with UHU por and fillet the joints with cool melt glue....more tape is added at the base of the fins over the joints to make them secure to the wing
Tape it up... belly, leading edges etc...decor..
EDIT....Because I am fitting a bigger motor now, I will also be fitting additional blocking below the main wing layer to make a more solid structure for the more powerfull motor..this is the white depron seen in pic3
In the pics it is almost done , just needing some decals printed to finish it off
|May 17, 2012, 07:12 AM|
I maidened my 48" version but the power is only marginal.. on low power it is not so good so I need some sugestions as to a better motor..
At the moment I am using a 2215/20 on three cells with a 9x5 prop...
Idealy I want to keep the prop and three cells and have a much higher reving motor..
this is what I am on
what I need is twice the power...40A about 400W on 3 cells
Ive gone for this one....pic below...which is overkill but should do the job... + a couple of 2200 x30c 3 cells and a 60A esc
EDIT.. the big motor is well good see the video in post 132 and furhter on.....
Flying yesterday was only just ok, with the low powered motor......it really needs more go to power it round the turns...
problems at low power... trying to get it round it would go slow, nose going up ( thats the back end dropping when its on its side) and then get adverse yaw setting in, it would then turn tightly and quickly in the opposite direction, dropping the nose suddenly...
Luckily it is a real slow flier when needed so I could just correct it and fly away..ok
Banking...another pain, it was noticeably dropping its rear end in a bank, which to me means it has too much vertical area in a forward position, or the COG for the side area on is set too far back.. I was correcting for this with rudder, and I suspect with better speed it will not be a problem
roll rate...for me much too slow, but this will improve at speed..
heres the poor video
|May 19, 2012, 03:04 AM|
The big motor looks good hobyking do a review here
It looks like it will go well..
I will be using only 3 cells, and either a 9x5 or 8x4 prop, so my power and consumption will be well under the max 60A.. I should still get a 2:1 power weight but at a reduced top speed, better for my lighter weight model..
|May 20, 2012, 03:52 AM|
The big power system is all fitted.. the AUW has increased by 13oz to 35oz for a wing loading of 6.4ozsqft..
power is the 35-36 1800 kv motor on a 2200 3 cell 30C lipo, with a 60A ESC and a 9x5 prop...
now the worry is about the build strength and the servos used.. It should be ok..
EDIT ive just run up the system indoors and its perfect..I had the motor blowing aginst my bedroom wardrobes, rather than the wall where my models hang. full throttle was only briefly used and the meter went up to 55+A.. I had to shut it down as the circulating airflow was pulling wings off the wardrobe top where they rest... At max I think it was using just under 60A which is the max on the ESC so likely it will drop a bit when in flight..
When I go outside to fly I will run it again to get more accurate readings with a Watt meter and tacho.. then it wont wreck my bedroom with the airflow it makes..
Its a WOW! motor ..
|May 21, 2012, 10:35 AM|
Doing a full power static test the motor was spinning the 9x5 at 13300 revs and drawing 60A.. It made a lot of noise and vibrated the rear decking quite badly..
To reduce the noise ive cut the slot with a bow tie shape at the front and moved the rear line back 1/2"..reinforcing with more tape..Ive also added another layer below the rear top deck which goes back from the slot to the front of the sub rudder..
To reduce the draw Ive cut of 1/2" from each prop tip to make the prop an 8x5
|May 24, 2012, 01:34 AM|
More power... wow... understatement....
Before this flight, I had done a bit of beefing up and added some wheels... Although these wheels works, I repositioned them more internal so they didnt show as much...
Internal they work just as well, and its better than having no wheels at all..
|May 24, 2012, 01:55 AM|
Conclusions ... from the flight ... the power is great, the flight is good, but the roll rate needs upping,
The model is far too noisy, but impressive..
Static testing on the 9x5 prior to the flight was giving 60A ,13300 revs, on a 3 cell... right at my max for the 60Aesc, so I chopped the 9x5 down by 1" to reduce the draw..
In the flight the throttle was mostly at below 3/4 max, where it was screaming along.. 1/2power is more than enough for a fast cruise..
To increase the roll Ive cut in some ailerons and back linked them to the elevons..
To reduce, or try to reduce, the noise the slot is further opened up.
The wheels aided take of and landing, but they looked too sticky out, and were a touch too far back, so they have been moded..
More tape and depron has been added to stiffen the rear top deck between the fins in an attempt to stop noise vibrations..
At the elevon horns the depron was showing signs of deformation so ive added some CF strip, going from under the horns to 3/4 back on the elevons, and tape covering to fix the strip.
|May 24, 2012, 06:51 AM|
Another fight set up as above is better for sound levels and much better for roll rate...
My next problem to sort is the performance of the sub rudder, which has little or no effect at all.. ..
ideas for causes...so far, are, the flexibility of the rudder and the lack of rear box decking is letting the air escape .. so I will do both and hope its better... I then realised that the sides that extended along with the rudder were the problem,straightening out the airflow..Otto came up with the same answer..
flying.... on linked elevons and ailerons is super, I just want a bit of yaw for high alpha and knife edging... this is an impressive model, as usual the KFm4 is making it a point and go steady flyer.. there is no worries about low passes its all very smooth..
Varying the power with the high mounted motor make no difference to the flight direction, it keeps level, and simply goes that much faster.. smooth big aerobatics are impressive, but slow it a bit and bank and yank will bring it about fast...
do a loop at too low a power and it rights itself and flies off near the top... enough power and it cruises the loop like it cruises through it all
|May 24, 2012, 04:37 PM|
Now Ive removed the sides that likely were straightening the airflow, Ive taped the rudder to stop flex, and ive strengthened the main wing center area with another layer of depron
EDIT...this was the correct solution
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