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Sep 30, 2010, 12:04 AM
Caution:Makes sharp left turns
Troy's Avatar
Thread OP
Question

Up thrust for above-wing motor pod?


Trying to calculate needed up-thrust for a pod mounted EDF. I am guessing that I will need at least 1-2 degrees of up thrust (spinner upward, tail cone downward) based on the thrust line passing over the CG. I am aware that designs similar to this can have some pitching issues throughout the flight envelope if the thrust angle is not correct; especially on launch. I'm looking for any recommendations.

These pics are just mock ups. I am moving the EDF forward and down about 3/8" closer to the wing. There is going to be a fairing around the EDF so I need some extra clearance. Same fairing but not nearly as high up:
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Sep 30, 2010, 01:32 AM
B for Bruce
BMatthews's Avatar
The amount of up thrust is guided by the location of the pull. Props that sit out just in front of the leading edge seem to require a lot of upthrust. Often in the 5 to 6 degree range. But as the drive point moves back, like with your EDF, the angle needed seems to become less very quickly. And indeed in your case and depending on where the model senses the drive location it may even need to be angled with the thrust pointed slightly up.

This is because the idea is to counter the high thrust line's tendency to drive the nose down. For a prop mounted in front of the wing's leading edge a lot of up thrust counters this tendency. For your situation depending on if the model "feels" the push coming from the impeller or if it "feels" it coming from the point of the exhaust flow you may need no up thrust or even downthrust (output cone high). The problem would be to find some information on how to model this situation and it will also depend on the thrust to weight ratio and how high the thrust is located above the CG.
Sep 30, 2010, 08:55 AM
Caution:Makes sharp left turns
Troy's Avatar
Thread OP
I am confident that there will be a tendency to pitch the nose over given the power input (1500watts or so). I also have a narrowing thrust tube to increase efflux velocity. This force is behind and above the CG. Hmmm....
Sep 30, 2010, 12:03 PM
B for Bruce
BMatthews's Avatar
When I wrote my reply I had to really stop and think about just where does the force from a jet style EDF take place. Is it at the fan where the air is first accellerated or at the end of the tail cone where it's directed.

Which raises a good point. If we assume that the drive is felt at the impeller it doesn't mean that you could not use the efflux tube to slightly vector the flow and achieve a trimmable function just like the big stuff does. Whether you can achieve that by adjusting two cones that overlap and have slotted holes for adjustment or by just removing the one cone and reshaping it to angle the tube slightly. Either should work. And the redirection point will be further back than the impeller blade so angling the efflux up will lift the nose to compensate for the high mounted thrust.

Howzzat concept work for ya?
Sep 30, 2010, 01:20 PM
Caution:Makes sharp left turns
Troy's Avatar
Thread OP
The Salamander has these issues, too. It's almost as if it has re-directed thrust tube.
https://www.rcgroups.com/forums/atta...hmentid=108927
Sep 30, 2010, 03:24 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by BMatthews
If we assume that the drive is felt at the impeller it doesn't mean that you could not use the efflux tube to slightly vector the flow and achieve a trimmable function just like the big stuff does.
With an impeller in a tube that is not parallel to the free-stream the side force is produced at the inlet lip (really). However tilting a pylon mounted engine that is approximately above the CG is not going to fix the trim changes due to thrust. This is a common problem for single engine flying boats. The common solution is to move the horizontal up the fin so that the prop blast hits it. The H-stab's span should be at least 2* the impeller diameter so that some of it can react to the free stream and thus earn it's name

--Norm
Last edited by nmasters; Sep 30, 2010 at 10:28 PM.
Sep 30, 2010, 03:30 PM
internet gadfly
nmasters's Avatar
BTW notice the "similar threads" window at the bottom of the page. Check out those links, especially "Help me correct my thrust line on a pod motor above a wing"
Sep 30, 2010, 03:43 PM
internet gadfly
nmasters's Avatar
This one goes into more detail of the physics If you want that sort of thing.
Sep 30, 2010, 04:43 PM
Caution:Makes sharp left turns
Troy's Avatar
Thread OP
Good stuff! I do have an option of a T-tail fuselage where the horizontal will be in the air flow. So really the up-thrust compensates more for low speeds or launch because the lower part of the aircraft is trying to "catch up" with the high-mounted pod. I would imagine that in this current configuration the lower I can keep the pod the better off I would be and adding some upward angle to the thrust tube may aid as well. All of the build threads I have read on Pod-mounted EDF's talk about how much fiddling it takes to get the thrust angles correct for a given airframe. I'm okay with that but I would hate to add the angle in the incorrect direction on a maiden flight. I can make the exhaust tube with a slight upward angle and keep the inlet fairly level to the wing incidence.... does this sound reasonable?
Sep 30, 2010, 04:53 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by Troy
I can make the exhaust tube with a slight upward angle and keep the inlet fairly level to the wing incidence.... does this sound reasonable?
That would just add another vector to an already complicated picture. KISS
Sep 30, 2010, 10:19 PM
B for Bruce
BMatthews's Avatar
That's why I thought that if you could make the outlet tube adjustable to a small degree that you could vector the thrust to provide an adjustment to zero out the effects.

Let me see if I can describe this fairly simply.

You would have a fixed outlet tube that has a number of V shaped cutouts for the last 1/2 to 3/4 inch so that the end of the tube is somewhat flexible. LIkely something like 8 to 10 "petals" created by the V cuts. Over this petal area you'd have another shorter cone that covers up the V cuts by just a little at the front and be longer at the rear by about 3/4 of an inch to an inch. The outer cone would have slots that line up with the middle of the petals. Screws would pass through these slots to the petals inside. When loosened you would be able to flex the outer tail cone around by perhaps up to 5 or 7 degrees and then tighten the screws to hold it in place and pull the petals into close contact with the outer cone. This would allow for some thrust vectoring. And because the vectoring would be occuring near the end of the duct it would be a sizeable distance behind the CG and therefore have a decent amount of leverage.

At least that's my story and I'm sticking to it....
Oct 01, 2010, 12:39 AM
Caution:Makes sharp left turns
Troy's Avatar
Thread OP
That's an interesting idea. I have seen some variations of vectored tail cones for situations like this.

I did some more research into the EDF Salamanders and what type of experiences they had with this configuration. Essentially they set the thrust line as close to 0 as possible and the launch phase was done at partial throttle with a slow ramp up. Go-arounds were done carefully.

I may actually try and mount the motor close to the original pics above. I have come to the conclusion that pod mounts too far forward and too high up are a real problem. If I can get the CG correct with the pod back some, then the pitching moment is less than if it is forward like the Salamander (and some other designs I've seen). The Kyosho FV-1000 is built like mine will be and it does not have nasty tendencies... everyone raves about how well it flew.
Oct 01, 2010, 09:23 AM
Registered User
Yak 52's Avatar
The problem is directly related to the couple set up between the cg and the thrustline.

I think thrust vectoring at the pod will be less effective than the 'tail in slipstream' solution - with the tail you have a much greater moment arm to play with.

There's a good article here: http://www.djaerotech.com/dj_askjd/d...upthrust2.html
Oct 01, 2010, 02:21 PM
Caution:Makes sharp left turns
Troy's Avatar
Thread OP
Thanks, it confirms my other findings. I will set things up the best I can and be careful on launch. Considering I know this airframe quite well, it will be interesting to see how the pod affects flight characteristics.
Oct 02, 2010, 05:20 AM
German Engineering.......
HugePanic's Avatar
just fly it!!

i build my V1 with as 0deg. as possible, and it's no problem to handle.

just be aware of a possible nose down moment at handlaunching, or use a bungee if possible.


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