Another Depron Delta but with 6 jets!
6 exhausts yes but only one fan.
Can you tell what it is yet?
A North American XB70.
A truly impressive beast that could cruised for 2000 miles at close to Mach 3 with its first flight 40 years ago!
The reason I this considering this was as a 'backup' to my four EDF Depron Concorde just in case using 40mm EDFs proved to be just too difficult.
Built to more or less the same size and in a similar way it looked like a 70mm fan could be used.
But what would be the effect of using 6 individual nozzles?
The only way to find out would be to build a stand alone 'test' duct. If it did not work out then not too much would be lost.
The duct is basically a tapered box built in 3mm Depron.
Each nozzle is made up of four pieces of Depron carefully formed to shape.
The duct is the correct scale shape.
The front of the duct has to change from a rectangle to a 70mm circle.
The completed duct with the basic 70mm AEO EDF glued on.
The thrust was tested by the simple method of taping the 3000mAh battery and several strips of steel to the duct until it was just 'neutral' in my hands. A rather alarming process but it weighed 27oz.
This figure was as much as my estimate for the all up weight of the Depron XB70.
To be continued!
Watching and learning.....
Watching with interest. Not sure how well the thrust will be because of the drastic change of airflow after the fan unit, but you never know until you try.
I hope it works!
Nothing like pioneering !!
Watching with interest ...
I have to say that my interest is more than just casual ... as I wondered before about split exhaust - wanting to EDF an F15 with single unit. I know there are factory models that do it .. but was thinking of a Schumate F15 scratch-job.
There's also the Concorde where twin 30's might work instead of the 50mm ? Given that I'm using a crap 50mm set-up ... a hyped up 30mm set-up might do the job ?
Although the airflow path may seem convoluted in fact the duct expands fairly smoothly to several times the FSA thus slowing the flow down and reducing duct losses. The air is only speeded up again right at the back within the nozzles themselves.
Hopefully this form of duct is not as inefficient as it might first appear.
As I seem to have a static thrust at least equal to the estimated weight of the plane I decided to make it a bit bigger, 40" span rather than 30".
By using the same Depron thickness the increase in the increase in airframe weight will match the increase in wing area yet as it will be using exactly the same electrics (the heaviest bit) it will result in a useful reduction in wing loading.
It will also make it easier to fit the fan within the scale duct outline.
A quick calculation showed the at its maximum the bigger duct would have an area of 6 times the FSA!
The first bit to build is the nose portion of the duct all in 3mm Depron.
With its characteristic central splitter.
To scale the inlet has nearly twice the FSA.
The rear portion that changes from a 70mm circle to a rectangle.
Joined together by the fan the side cheeks are added.
These add considerably to the rigidity of the assembly.
The completed nose portion of the duct.
A 2200mAh 3s (the intended battery) is shown to give an idea of the scale, however it is only a tiny proportion of the whole thing!
And its all going to made of just Depron!
I designed a pusher version with the motor in the middle of the wing to prevent torque roll. It flew well, but I still need to make a few modifications to beef up the forward fuse are (it's a profile, not full body).
Looking forward to more progress here!
Awesome! A single larger fan is going to be more efficient than several smaller fans. Do you have any idea what the thrust is in open air for that fan or what the duct losses are?
HobbyKing show the AEO 70mm delivering 24oz thrust on a 3s (12.6V) but with a 3300 kV motor drawing 28A. On a 4s they claim 32oz drawing 38A
My 6 nozzle 24" long duct delivered 27oz drawing 44A at 10V with a 3900kV motor.
Obviously any duct costs static thrust but I was pleasantly surprised with the figure I achieved as I had feared the losses would huge and thus make it an unrealistic project.
The duct on the 40" span version with its scale inlet will be quite a bit longer (33") so with its extra surface area I might loose a bit more static thrust.
Considering the amount of area inside the exhaust ducts, that's not that bad. I know you can build it light! It should be easier than the Concorde.
32oz = 907gr
27oz = 765gr
I am truly amazed at that ........... My HK 70mm Haoye on 5S, 2800kv only delivers about 950gr installed (F16).... ~34oz. Similar HK Haoye 70mm on 4S with 3400kv gives 870gr installed (Twister).
The Haoye unit is rated at 1.2kg thrust on 4S / 3200kv (uninstalled).
I've always noted the AEO are lower results than other budget units ...
You do have to be careful of static thrust figures for EDF units 'stand alone' in free air, particularly with a generous bell mouth in place. It alone can add more than 10% to the thrust.
It would appear that once in a duct of any length the loss in static thrust will be in the order of 25% or even more as your figures suggest.
The conventional wisdom is that the dynamic thrust of 'stand alone' EDF would fall off faster than one in a duct so the disadvantage of enclosing the EDF declines as the speed increases.
The static thrust I achieved with my complex duct was certainly in the same order as for a conventional duct of the same length so it made sense to proceed.
For my very light planes I have to consider the thrust to weight ratio of the complete fan/ESC/battery unit rather than the maximum thrust possible from a particular size of fan.
In this respect the AEO units do quite well although you do have to use a bigger diameter fan to achieve the required thrust figure.
Cool! The Valkyrie was one of my favorite 'future birds'. You are going to make this a three dimensional model and not just a cutout profile plane, right?
addendum: Actually, your ducting is very reminiscent of a de Laval nozzle. An EDF fan is not a rocket engine by any stretch, however I see no reason why it wouldn't work. Subscribed.
Indeed everything to exact scale except the wing section. It will be the correct thickness ratio (3%!) but with a bi-convex section. The original's wing section was actually a flat hexagon as they could only make flat brazed titanium honey panels.
As the XB70 fuselage has amuch smaller section than Concorde it would be built from thicker 3mm Depron. Apart from any stiffness benefit the extra skin thickness allows a bit more of a 'sanding allowance' to achieve a smooth finish.
Like Concorde the removable nose section is built with formers and a half shell over the plan.
The cockpit area is a remarkably complex shape requiring many narrow planks.
The completed nose section. A solid Depron nose block still has to be added.
Light 1.2oz (33g) and amazingly rigid.
Next the remainder of duct. As the wing itself forms the top of the duct it all has to be built together.
I love the "planking" with depron. :D
Best of luck with this one Quorneng.
The wing is built in the same way as for my Concorde. 2mm Depron skins over closely spaced Depron shear webs with no other reinforcing.
The main part of the duct itself is just an open topped box in 3mm Depron glued to the underside of the lower wing skin.
The wing skin is cut out so the nose portion (with the fan) is flush with the wing skin.
At this stage the duct alone is providing all the rigidity.
When the wing shear webs are glued on things are not quite so flexible.
However the wing will only achieve any real stiffness when the top skin is added.
A trial run of the fan showed a significant fault which in hindsight I should have anticipated.
The underside of the duct has large flat surfaces in 3mm Depron. As the fan was run up the pressure it created caused the duct to blow up like a balloon!
After a bit of thought the simplest solution appeared to be to add a 'divider' down the centre of the main portion of the duct thus in effect creating two narrower ducts running side by side with each feeding 3 nozzles.
A view through one of the nozzles shows the divider in place.
It seemed to work with no noticeable duct distortion even at full power.
Of course the divider does add to the surface area of the duct and thus to the losses but as the duct has many times the FSA the air is moving relatively slowly so hopefully any loss in thrust will be small.
The top skin goes on making the wing a whole stiffer!
Each wing half required nearly 2m of POR glue line. This took some time to apply so the skin had to be placed very accurately as there was virtually no slide ability remaining.
The main fuselage formers were glued on top of the wing.
The fuselage contours where it joins the wing are quite complex.
The fuselage is actually positioned slightly above the wing so its leading edge acts as an intake 'splitter' to keep any turbulent flow from entering the engines.
Unlike Concorde the XB70 really has a 'minimum' fuselage (it only seated 2 pilots side by side at it widest point and with a constant taper to the rear) so the battery will be quite a tight squeeze at the 50% CofG point.
Does that stiffen up the EDF plenum too? Is this going to be a belly lander? Skid plates on the plenum could serve double duty to stiffen the plenum while providing some abrasion resistance.
Yeah, battery placement was a problem on my profile version as well. It had to go right in front of the intake splitter. I had to cover the lipo in white packing tape as the packaging of it is blue.
The lower surface of the plenum is indeed supported by the divider which is 'hung' directly off the wing underside. The wing itself is plenty stiff enough to resist the plenum pressure.
Yes it will be a belly lander. I have found abrasion is not too much of a problem as I fly over fairly long grass although it can leave a green stain!
It will have a very low wing loading (5oz/sqft?) so with the ground effect the landing speed should be pretty low, not much over walking pace.
I have found with my slightly smaller Depron Fairey Delta 2 if you get the flair right it will just 'flop' in with virtually no forward speed at all.
My intention is to delay planking the top of the fuselage so with all the electrics installed the battery can be positioned to give a 50% CofG which should be fairly safe starting point.
The ESC is installed close to the EDF.
The rx (an Orange 3 axis stabilised unit is mounted slightly further forward.
With the nose temporarily taped in place the battery position can be determined to give a 50% CofG which in turn allows the fuselage planking to proceed.
This is always the slowest part of the build as with the electrics all built in any errors in setting the CofG can be hard to do.
|All times are GMT +1. The time now is 09:45 AM.|
Powered by: vBulletin
Copyright ©2000 - 2016, Jelsoft Enterprises Ltd.
Copyright 2005 WattfFlyer.com
RCU Eflight HQ