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Old Feb 27, 2013, 04:08 PM
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First fan has been balanced dynamically with the motor, a 75% reduction in measured vibrations from the as delivered condition. See the post in the link below

Fan balacing results
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Old Feb 27, 2013, 04:30 PM
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The rotation does have a factor. High alpha can cause differential drag on the lower and upper drag rudders. This caused one wing to stall.
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Old Feb 27, 2013, 05:52 PM
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The thing is, if you read Chris Gold's article in QEFI, he talks about his first B2 which was about a 10ft wingspan powered by 4 glow powered fans and he did not have enough control throw built in, the plane took off, flew in a straight line, completely stable and without enough control authority it continued to fly in a straight line until it crashed, presumably out of sight or radio range or something. Robert May, his accomplice describes the 8ft span EDF model as quite stable and boring to fly elsewhere in RC Groups. So the plan itself appears to be OK.
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Old Feb 27, 2013, 10:12 PM
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But all the B-2s I seen do not go into high Alpha on take off. This is where the stalls and differential drag comes into play.
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Old Feb 28, 2013, 02:32 AM
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I made the port re stability not to counter your posy about high alpha, but to state that inherently the plan has good perdigree so should not go unstable unless either some form of build / thrust misalignment or being flown into stall. I agree, it most likely was pulled up too quickly and what is desired is a gentler ROG and shallow climb.
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Old Feb 28, 2013, 02:46 AM
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I built this plane http://www.rcgroups.com/forums/showthread.php?t=1064868. One bad thing about high intakes is that they tend to get starved at high take off angles. This plane needed to have the nose down on take off or it would lose thrust. I can see with the B-2 that it would also loose thrust at high takeoff angles and would also have asymmetric thrust if one wing was down.
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Old Feb 28, 2013, 04:02 AM
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That would make a lot of sense based on what I have picked up on fluid flow in the day job. High bangle of attack, so the air flow seperates at the leading edge resulting in a poor air flow into the fan rather than streamline flow from the leading edge back into the fan suction.
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Old Mar 01, 2013, 12:18 PM
emersok
Malmesbury UK
Joined Jun 2009
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5 foot B2

Hi Peter, I feel bad about hijacking your thread with my problems but you never know there may be something which is of use to you.
Measured the jet outlet speed with my pitot tube. All were surprisingly similar within a few mph of each other. Hung the model from a central balance point. Lateral balance good. C of G about 2 inches back from on plan but that was the best I could do. Balanced the model so that it hung with wings & fuse level. Powered up. Difficult to tell as I had to restrain the model from moving forward but there was a very considerable force wanting to lift the nose. So if the C of G was marginal (It was right on the planned point) as soon as the plane lifted, the nose would lift, the wing would stall, little effect of control surfaces, plane would continue to lift on thrust alone and any roll would just continue. I've had a few conversations with Chris Golds and get the impression that I have more input power (don't know about thrust) than his 8 footer. Maybe the small model accentuates the down thrust effect of the ramps? I'd be very interested in your expert opinion. Thanks Keith
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Old Mar 01, 2013, 12:55 PM
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Keith, no worries about adding to the thread, it's what all this is about, I am learing things off other people and more than happy to add my bit in to help out. I am not an expert, I am learning as I go along with some of this from other guys on here who know their stuff. I am a chemical engineer in the day job so picking up a lot of the stuff is not too much of a stretch and I know fluid flow and fan laws from work.

One thing I do know about this model is that the CofG has to be just right on this, too far back and you are going to bite the dust. Chris mentions in the original QEFI article about getting the CofG too far back on his maiden, originally he had 48% of the centre line chord, to quote the article "it lifted itself off, reared up and rolled to the right in a classic tail heavy tip stall". He then moved the CofG to the current plan location at 43% of the centre line chord and it flew well. The calcs I have picked up wih others in this thread confirm that the 48% was too far aft (behind the neutral point) and 43% is correctly positioned in front of the neutral point.

If your 5ft model is a true scale down of the 8ft, then my model has the CofG 421mm behind the leading edge (43%) at the centre of the plane, yours should be 264mm behind the leading edge at the centre of the plane.

Re your power, if you have 1200W of power on tap, you may well have around 4.5lb of thrust, the original 8ft model probably had that level of thrust on the bench, a little less in situ, so you do need to be careful, but your weight is well up. I know on my build I will probbaly have 7lb of thrust, again a lot more than the original and mine will be lighter as well, so I should also be careful.

Have you mentioned to Chris that someone is building an update with smaller fans.
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Old Mar 01, 2013, 01:27 PM
emersok
Malmesbury UK
Joined Jun 2009
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5 foot B2

Hi Peter,
Thanks for that. Off the plan the C of G is 250mm behind the nose so a little more conservative than your calcs. Chris suggested moving it forward about half an inch (he tends to deal in imperial units!). I took the liberty of emailing him a link to your thread as he is always interested in people building from his plans. He is not that computer literate and his wife organises that sort of thing (I'm sure he won't mind me saying that) so if he has accessed it then he should know about your model's configuration. As a matter of interest I sheeted mine through-out, are you using film for some of the wing area ?
I'll do a few more tests but I'm tempted to remove the rear ramps and allow for a horizontal airflow from the EDFs. I didn't realise that your EDFs were only 55mm but from what you have said you are achieving a very light build. I really would like to see mine fly but I have seen more videos of RC B2s crashing than flying.
Regards, Keith
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Old Mar 01, 2013, 05:41 PM
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Keith,

Yes, I came down in fan size to get the lower nacelle tops to look more like the real beast rather than flat topped with the fuselage. I am happy with the 55mm as I can get more thrust out of these than the original 70mm fans - I spent a lot of time choosing the fans and motors to get a powerful and efficient combination, possibly the highest thrust (but not cheapest) 55mm set up.

I have not yet decided what to do about the sheeting of the rear of the fuselage / wings, the original model was left open structure and covered with tissue. I will make up my mind when I have the fans / ESCs and batteries installed and see what the CofG is coming out like. If I feel I am tail heavy then I will cover with film, if I am nose heavy I will sheet with light balsa.

When finished I was going to send some picturs into QEFI of the plane for the modellers planes page, but not sure when that will be.

Peter.
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Old Mar 08, 2013, 03:24 PM
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Completed balancing my fans, results in thread

Fan balancing results

using a small accelerometer and voltmeter to measure vibrations caused by imbalance.

Good results.

Followed up by putting all 4 fans onto the test stand one after the other to make sure they were all giving the same performance to see if I had to match pairs toegterh to minimise asymmetric thrust. The results show 4 matched fans within measurement error.

Fan Speed Power Thrust
- rpm W g
1 52000 400 700
2 52000 396 695
3 52100 400 710
4 52000 390 710
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Old Mar 08, 2013, 06:04 PM
deltas are cool
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if it were me i'd swap #2 and # 3 to balance it out ...just in case.
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Old Mar 08, 2013, 09:52 PM
check for reversed controls
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Wow 4 fans and motors and ESCs in it sure makes for many possible points of trouble.
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Old Mar 09, 2013, 04:46 AM
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Quote:
Originally Posted by AIR SALLY View Post
if it were me i'd swap #2 and # 3 to balance it out ...just in case.
Yes, I will do that, the fans are just numbered at the moment how I got round to them, they are not yet in the airframe.

Quote:
Originally Posted by alpea 41 View Post
Wow 4 fans and motors and ESCs in it sure makes for many possible points of trouble.
Yes, I accept the risk of an issue is higher and hopefully by taking time over everything I can reduce these to some extent, such as going for high quality fans and high quality motors, the only area I have probably skimped is using the HobbyWing ESCs. There is a Youtube video of someone who built a B2 and put a single large fan in the body section, an enlightened approach perhaps.
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