Feb 22, 2013, 05:42 PM
The Last Question
United States, UT, West Valley City
Joined Oct 2006
528 Posts
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Did I screw up?

I made a little jet. It weighs 61g and maintains 65g of thrust. Anyway I was doing test glides onto the bed and power off glides were ok...not the best but power on glides at half throttle shows a bad nose down situation. I didn't really think about the fan being above the wing until now. So what should I do? Any fixes you can think of? I don't want to start over.

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Feb 22, 2013, 07:26 PM
Registered User
United States, MD, Annapolis
Joined Jul 2012
94 Posts
I'm not too sure...but you may want to check your CG. It could be a little nose heavy, or the moment from the thrust above the centerline could be causing a nose down (negative) moment. You can counter it with a positive moment from shifting the CG aft.

You could give the exhaust duct some kind of down angle...so the front of the duct points down and the exhaust is pointing up a little bit.

I drew a diagram to show basic moments from different CG locations...and what it looks like when you point the exhaust in that upward direction. **Please note, the diagram is VERY basic. There are a lot of other factors that come into play in regards to the moments, but I would start by moving the CG slowly aft**

The neutral point is the aerodynamic center of the entire airplane.

If someone thinks that I am way off, let me know.

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 Feb 22, 2013, 07:36 PM The Last Question United States, UT, West Valley City Joined Oct 2006 528 Posts Wow, thats an awesome diagram! Thank you. So either move CG back making it tail heavy or adjust the thrust line giving it a positive moment when throttle is applied. So am I wrong in thinking the best solution would be the change in thrust line?
 Feb 22, 2013, 09:17 PM Supersonic hover Michigan, United States Joined Sep 2003 1,043 Posts Hi Entropy, It looks like you have the start of a nice little micro EDF there. Definitely make sure the thrust line is correct because even a little misalignment can make a big difference. My rule of thumb for new designs is to make sure the thrust line passes thought the middle of the battery since that will be your heaviest single component. Additional thoughts: 1 - Where is your CG now? My hunch is that it's too far forward with the long nose, battery, RX, etc. out in front of the wing and not much behind. 2 - You vertical stabilizer looks too small and/or too far forward. Side area of the forward fuselage is larger than the vertical stabilizer which will make it very unstable in yaw. 3 - No horizontal stabilizer? Any reflex in your elevons? Good luck! -Matt
 Feb 22, 2013, 09:19 PM Registered User United States, WA, Vancouver Joined Mar 2012 739 Posts I would move the thrust angle, just a touch higher at the tail end of the outlet cone, the thrust is most likely pushing the nose down. Joe
Feb 22, 2013, 09:35 PM
The Last Question
United States, UT, West Valley City
Joined Oct 2006
528 Posts
Quote:
 Originally Posted by Esprit440 Hi Entropy, It looks like you have the start of a nice little micro EDF there. Definitely make sure the thrust line is correct because even a little misalignment can make a big difference. My rule of thumb for new designs is to make sure the thrust line passes thought the middle of the battery since that will be your heaviest single component. Additional thoughts: 1 - Where is your CG now? My hunch is that it's too far forward with the long nose, battery, RX, etc. out in front of the wing and not much behind. 2 - You vertical stabilizer looks too small and/or too far forward. Side area of the forward fuselage is larger than the vertical stabilizer which will make it very unstable in yaw. 3 - No horizontal stabilizer? Any reflex in your elevons? Good luck! -Matt
Thank you, CG is at the recommended point based on an online calculator for delta wings.

(The nose is hollow)

So about the rudder, you're saying it is too far forward and not big enough. When I adjust thrust angle I'll try and improve that. Yeah, no horizonsntal stab, it's a delta.

No reflex in elevons. I'll give it a mm or two.

Thanks.

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Feb 22, 2013, 09:43 PM
The Last Question
United States, UT, West Valley City
Joined Oct 2006
528 Posts
Quote:
 Originally Posted by sloperjoe I would move the thrust angle, just a touch higher at the tail end of the outlet cone, the thrust is most likely pushing the nose down. Joe
Thanks, will do. And I'll try making the vert stab bigger/further back. It did wag a little even throwing it on the bed.
 Feb 22, 2013, 09:44 PM The Last Question United States, UT, West Valley City Joined Oct 2006 528 Posts CG is about 1 mm forward of the servos BTW. Thrust line is through the middle of the battery also...
 Mar 03, 2013, 02:25 AM The Last Question United States, UT, West Valley City Joined Oct 2006 528 Posts Well that didn't end well. First and last flight was a few days ago. I put landing gear on it. I got it up to speed and took off. The tail was wagging up and down pretty bad. I banked it left to turn and it rolled upside down and nose dived into the ground. So next one will have a bigger wing, bigger rudder and I think most importantly edf and wing will be mid-body.
 Mar 04, 2013, 11:52 PM Registered User USA, WA, Bremerton Joined Mar 2009 936 Posts Hey, that's the way it is with these things! Most of my planes don't fly, or are uncontrollable or nose dive, or crash to pieces, or loop and hit me in the back of the head and break in two, or, or, or! Just think about it....most of us are not aeronautical engineers (except Jetsett44 he works for Boeing so he does not count! )And we're building one off custom airframes and boy aren't we all glad there are not pilots lives at stake. Keep at it and you'll have a winner you can be proud of.
 Mar 05, 2013, 05:21 AM The Last Question United States, UT, West Valley City Joined Oct 2006 528 Posts Thanks for the support! Yeah, I love building so it was fun anyway. The whole time building it I was wondering if it would actually fly. I couldn't just let it sit there and not know. Anyway the motor/fan combo is what I really wanted to test and it definitely had good speed on takeoff. Hey, it was only foam, right?
 Mar 05, 2013, 10:38 AM Registered User USA, WA, Bremerton Joined Mar 2009 936 Posts I used to start a lot of builds and never finish them, saying to myself "aw that will never fly". Now I just chuck everything I got into the air. I don't care! Yeah its only foam and I have yet to break any gear. Well at least not at this weight.