Dollar Tree Foam ... EZ-Jets - Page 4 - RC Groups
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Mar 01, 2015, 04:14 AM
Registered User
Th reason the canard CG calculator can be used in this case is illustrated in the attached image (even though the "canard" right up against the "main wing"). Yes, I was the one who recommended using it.
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Mar 01, 2015, 05:58 AM
Not always grumpy!!
insomaniac45's Avatar
i think the longer nose really makes it look mean.

how am i going to decide which to build. (pulling names from a hat springs to mind)

this is definatly on the list. the looks are great, clean, sharp and she looks like she is going mach 1 just sat there.

great work


tony.
Mar 01, 2015, 06:24 AM
Registered User
Quote:
Originally Posted by Lockey
I have just done some tests ringo42 and the results have astounded me

By adding 70 mm to the nose (lengthening the nose by just 70 mm) it has taken the C of G 105 mm further forward making it even worse to balance

I just can't understand it and not sure what to advise ... or what to do ??

I used the same canard CG calculator on all 3 examples ... the first one being the original one and that seems to be the best place for the C of G after flying both EDF and Pusher

Please can someone explain

Lockey

Lockey
70 mm? That's 7 cm. The drawing you showed does not look like you extended the nose by 7 cm!! It looks like a much smaller extension. Am I missing something?

One thing is certain: Extending the nose will move the CG forward to some degree.
Mar 01, 2015, 08:11 AM
Tony Audsley Retired Locksmith
Lockey's Avatar
Here is a better drawing showing the amount I added to the front

I did realize that the CG would move forward by doing this but not as much as the figures showed

I imagined if I added 70 mm (about 3 inches) that it would move forward slightly but not that much

Lawn Dart Flyer, thanks for the comments, I must admit, I hadn't given this aspect much thought at this stage ... I didn't think there was a problem with the pusher motor being set at zero on this one ... I have others where I did as you suggest and added some upthrust (motor pointing up at the back)

I don't altogether remember, but I don't seem to recall it pitching up when the throttle was applied (I was having too much fun before I decided to use it as a lawn dart

Lockey
Last edited by Lockey; Mar 01, 2015 at 08:30 AM.
Mar 01, 2015, 10:43 AM
skumgummi dave
I'm no help here Lockey. I'm the student and had to ask the question.

Dave-
Mar 01, 2015, 11:25 AM
Registered User
OK, I'm going back to the image below (you posted it earlier in the thread) and I think I see what's going on.
In the middle image (the one you say has the dramatically forward CG), you not only increased the "nose section" area (both its root chord and its tip chord), you also reduced the "rear wing" area in comparison the the right image (the root and and tip chord is smaller). Yes, those two changes together could very well lead to the drastic shift in CG.
Simple solution: Add plenty of chord to the "rear wing". And you could do that by increasing the elevons and making them travel less.
Last edited by nuteman; Mar 01, 2015 at 11:39 AM.
Mar 01, 2015, 06:54 PM
Tony Audsley Retired Locksmith
Lockey's Avatar
These are the 3 images again Nuteman

The first (Green) is the original ... The second (Pink) has just front section only pulled forward ... the third (Blue) has the front section and the wing pulled forward

I don't really want to make the elevons any bigger, 1. it will spoil the look, 2. it already has too much elevon and is very twitchy and 3. it would only add more weight to the back and make balance even more difficult

The balance is very easy with the EDF in fact I had to move the EDF unit back a little to make it easier to balance

The Pusher however was not quite as easy to balance and I had to use a 2200 / 3s to balance it and would like to have been able to balance with a smaller battery if I (or other guys) wanted to

The 3rd picture .... the "Stealth" I assume will have the same problem only worse ... EDF will be no problem as the unit can be moved to enable an easy balance, but the pusher is waaaay back and could be difficult to balance

I did think about a prop in slot pod to replace the EDF ... this might prove to be a winner on some ?? (see 4th pic) oooohhh my head hurts
a swappable EDF , Pusher or Prop in Slot ... now that would have to be a first

Lockey
Last edited by Lockey; Mar 01, 2015 at 07:07 PM.
Mar 01, 2015, 08:13 PM
Tony Audsley Retired Locksmith
Lockey's Avatar
Swappable EDF ... Pusher ... Prop in Slot

Although I very much doubt that a pusher would be viable on this one as it would probably be too difficult to balance

Lockey
Mar 01, 2015, 08:53 PM
Tony Audsley Retired Locksmith
Lockey's Avatar
And the SR-72 as prop in slot

Only trouble I see with these prop in slot is that it makes the fuselage very weak at the slot

Lockey
Mar 01, 2015, 09:38 PM
Registered User
Quote:
Originally Posted by Lockey
3. it would only add more weight to the back and make balance even more difficult
Hi Lockey,

This is the part that I think you're not evaluating correctly. The weight penalty would be tiny in comparison the much more amicable CG (we're talking about a relatively small amount of weight in foam for a relatively large shift in area). The more rearward area you have, the more rearward the CG can be. Just like increasing the "chord" across the entire span of the nose section dramatically shifted the CG forward, so too would increasing the chord across the entire span of the rear wing dramatically move the CG rearward. Compare your blue layout to the pink one. The blue has a CG that is 19 mm more rearward than the pink - simply because the rear wing has more chord.
Now, go a step further and add chord the the trailing edge of the rear wing (to offset the added chord to the leading edge of the nose section) - not the leading edge of the rear wing. You could even add just enough such that it exactly offsets what you did to the leading edge of the nose, and get the CG back to it's original location. You just need to do the math - or better yet, let the CG calculator do the math for you.
Mar 01, 2015, 09:49 PM
Registered User
Quote:
Originally Posted by Lockey
And the SR-72 as prop in slot

Only trouble I see with these prop in slot is that it makes the fuselage very weak at the slot

Lockey
Yes, you would need bracing CF tubes (or rods) running parallel to the aircraft center-line on either side of the slot. Wooden dowels would do in a pinch, but are heavy, of course.
And yes, it would be extremely challenging to balance this one as a pusher prop.
Mar 01, 2015, 10:00 PM
Tony Audsley Retired Locksmith
Lockey's Avatar
Thanks again Nuteman for being so patient with me, I think I understand what you are saying mate ... but doing what you say only gets us back to where we were in the beginning

Its not really a big problem, and I don't want to change the shape too much and spoil it so we will just have to live with the fact if you are going to build as a pusher, make sure you keep the back and as light as you can

Ways to keep the back end light are:

1. Move the servos forward
2. Use a smaller / lighter motor
3. Just stick with the 2200 / 3s battery and it will balance

As I have already lengthened the nose on the repair, I will let you know if it works as soon as I get chance to fly it again

I think the slightly longer nose looks good and if nothing else, it will give a bit more room for the battery and gear up front

Lockey
Mar 01, 2015, 10:28 PM
Tony Audsley Retired Locksmith
Lockey's Avatar
Another solution to helping with the balance the a pusher configuration is to move the motor nearer to the front.. I have moved it 25 mm (one inch) without changing the look too much ... every little helps eh ... and it doesn't effect the EDF version, I am sure it will be quite happy with an inch off the length of the thrust tube

Lockey
Mar 02, 2015, 03:23 AM
Tony Audsley Retired Locksmith
Lockey's Avatar
OK, repair is finished (before paint touch up) and ready to fly

I am going to try a Floater-Jet Replacement Motor with a 6 x 4 prop ... its a bit lighter than the other motor and it seems to balance OK with a 1800 / 3s battery (slightly longer nose than the original)

http://www.hobbyking.com/hobbyking/s...dProduct=18198

Much too windy today ... over 33 km/h and gusting to 45km/h

Lockey
Mar 02, 2015, 06:04 AM
Tony Audsley Retired Locksmith
Lockey's Avatar
Well, the wind did drop a little and I did fly her (very short flight) I think she was a little tail heavy but it was really too windy to know for sure

It was VERY FAST with the little Floater Jet motor and a 6 x 4 prop ... BOY did it have some grunt ... AUW 620 grams

Lockey

EZ SR 72 with Floater Jet motor (2 min 28 sec)
Last edited by Lockey; Mar 02, 2015 at 06:21 AM.


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