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BVM thrust issues

Old 10-25-2017, 06:17 AM
  #26  
dbsonic
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Theuns, one thing about the BVM intakes is that they maintain an constant cross sectional area from inlet all the way to the Viofan(or if any reduction it is done very smoothly). I believe this was critical for the fan to operate optimally. The hard part is actually checking that due to the complexity of the inlet shapes being 2 approx. half circles being slightly oblong. You will probably want to smooth out any transitions to avoid any losses
Darren
Old 10-25-2017, 06:48 AM
  #27  
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I wonder if more nitro would help. I never ran a BVM engine, so I don't know what they like to drink but the OP flies at 5800'. I fly at 5000 and use more nitro up here than I would at sea level. Maybe someone more familiar with this engine can help? And probably a colder plug?

Scott
Old 10-25-2017, 08:51 AM
  #28  
Theuns
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I think what I will do is to just put the FOD screen on and run it down the raunway to see what happens, I think maybe I am close enough with the static thrust to attempt a flight after some high speed runns, they will indicate if the airframe /power setup is a viable option.

If it flies on this setup (making at best 12 Lbs) a 6-8kg and up turbine would fly it really well.
On a hot day we have a dencety alt of around 8000 ' pluss , so maybe the intakes as is can not keep up with fan demand, the fan itself is only making about 75-80% of what it can anyway due to engine suffering a little.

I have read that (at optimum) the setup I have coud deliver 12-13 Lbs , so taking into account our alt and the ducting not being optimized I think I am pretty much as the apex of what I can expect on the ground for now. To gain the extra lb I would have to totally rebuild the whole ducting system.....as long as it flies OK and not fall out of the sky I am happy, blistering speed is not essensial for me, besides my eyes and reactions are not what they were 15 years ago LOL

Theuns
Old 10-25-2017, 11:28 AM
  #29  
SECRET AGENT
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Be real careful in those turns, it's so easy to bleed off needed airspeed on a jet like yours. That first turn after takeoff needs to be babied or it's snap city.
Old 10-26-2017, 04:13 AM
  #30  
Art ARRO
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I concur with Secret Agent on bleeding off speed in sharp turns with a delta wing planform. Also note that deltas are very responsive in roll but generally sluggish in pitch. On my A-4,a delta with a horizontal tail, I would let the model takeoff by itself without forcing a rotation. Also, I set a fair amount of positive angle-of attack to allow the model to takeoff without elevator input. The first turn after takeoff was always exciting especially on a hot humid day. Note that this A-4 was initially powered by a K&B 7.5 driving a Turbax I fan producing only about 6 lbs of thrust on a 9 lb airframe. Good luck on that maiden flight and let us know how if goes.
Regards,
Art ARRO
Old 10-26-2017, 07:15 AM
  #31  
Theuns
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I had a few prop driven deltas i n the past an all of them had a tendancy to want to "skid' and mush out of a sharp high G turn, that do however have a nice quality of being able to fly at much higer angle of attack than other planes.
I hope to get most of the outfitting done this weekend and then find a suitable place to go try taxi trails.
I will also have a positive angle while on the ground to help with takeoff, sometimes these deltas battle to "unstick"

Theuns
Old 10-26-2017, 02:31 PM
  #32  
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Big big big difference between a prop delta and a "jet" delta. You can escape a lot of problems with propwash over a control surface that you just don't have in a jet. If you put your DF jet in a high alpha situation you are guaranteed to fall right out of the sky. Trust me, seen it dozens of times.
Old 10-26-2017, 08:22 PM
  #33  
Theuns
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Thanx for the advice, will keep it in mind :-)

Theuns
Old 01-11-2021, 06:01 PM
  #34  
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Hi, I know this is an old thread but i see here the right scenario in order to solve my question. I saw this BVM thrust tube in google and it looks like a coke bottle shaped, ¿is this design a better or more efficient one over the traditional straight?

I am in the process of molding my own thrust tube and i will appreciate your inputs about this TIA
Old 01-11-2021, 06:23 PM
  #35  
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With ducted fans, pinching down the exit increased the velocity. Even the Byron lexan thrust tubes were tapered......... Sorta like putting your finger on the end of a garden hose.
Old 01-12-2021, 05:07 AM
  #36  
patf
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Looks like an "area ruled" duct, where the cross section of the exhaust duct was maintained around the tuned pipe then tapered. by this time the tuned pipes were getting pretty large.

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