Mig -25 EDF conversion

[Richard Sharman]

Tony Hijhuis has thrown the guantlet down to EDF fans(!) to convert his Magazine feature, pusher prop Mig-25 (RCME May/June "Dogfight Double") to ducted fan.  This forum thread is intended solely for those who are interested in this challenge (or, perhaps, have already done it and would like to share their insights?).

As a relatively recent convert to electric ducted fans, I would like to kick off this thread by listing a few of the concepts that seem to me to be important. Are these right? are there more important things to consider? where can improvements be made? etc.etc.   Basic ground rules seem to me to be: 

1. the EDF Mig-25 should have the same overall shape, size, weight and external appearance as the pusher prop model featured in the plan.  Minor modifications of a "cosmetic" nature are ok.  

2. The model can have 1 or 2 fans (or more?) but they should be installed in such a way that they can be removed and re-installed for maintenance/modification (ie not glued in until the model crashes).

3. At least Aileron, Elevator and Motor control must exist, but other functions (rudder, steering, retracts, flaps, brakes, etc.) are fine, provided rule (1) is not violated.

So ---- some questions for starters, are:

•  what changes to the plan for the pusher prop are needed? Or, to put it another way: what is the INTERNAL structure of the model?
• where is the CG? is it the same as the pusher prop, and if not, why not ?
• what is the relationship between intake, fan and exhaust sizes?
• what motor, fan, battery combination is needed to get performance similar to the pusher prop?

Conversely, if anyone has already done it (well done!) -- what is your experience?

best wishes, Richard

[Tim Mackey]

My .02P worth.

1. Forget retracts and rudder etc - the best advice for ANY EDF is to keep it as light as possible.
2. If possible stick to one fan as this reduces the need for 2 ESCs and again keeps the weight down as well as reducing complications and worries of one fan not being totally in synch with the other.
3. I Imagine the COG will be the same, after all its about balance, control response, fly-ability and stability.
4. The relationship and final results of intake / FSA / and exhaust or efflux is something which can often not be totally decided until flight tested. Generally and simplistically the intake must be large and smooth enouigh to provide sufficient air to the fan in order for it to be exhausted out of the efflux tube at the desired velocity - there is little point in it being any larger - obviously there is a difference between static thrust and airflow etc to that of in- flight. Think of the Impellor / efflux as a bit like prop as regards to thrust versus speed EG: larger diameter blades and larger efflux outlet diameter will mean more thrust ( like a big diameter prop ) but smaller faster impeller with a "choked" outlet diameter tube will give more speed. Obviously one needs to aim for a compromise between the two, or launching could be tricky and necessitate a bungee / ramp. Sometimes "cheat" holes are required to ensure sufficient airflow to feed the fan. More fan blades does not autiomatically mean more thrust. High RPM is normal for EDF units ( circa 40 - 50000 RPM is not uncommon.
5. The power-train as a whole needs to be as efficient as possible with the lowest weight possible to obtain the thrust / power needed. measurement of thrust is difficult ( I have done some basic static thrust testing the results of which are on the forum ) and this shows that well designed fan units can be much more efficient than some other cheaper ones, - producing far more thrust ( static )at LESS current !
6. Generally, I aim for at least 150 Watts per pound for EDFs - and consider a fight duration of around 5 - 6 minutes plenty - this should enable the use of smaller capacity batteries which again helps to keep the weight down - batteries also vary a lot in performance and weight - shop carefully !

[Richard Sharman]

Thank you, Timbo, for the thoughtful comments, which sound very sensible.  My responses are:

•  Rudders, etc. -- of course, we need to minimise weight at all costs!  My justification for putting rudders on the pusher prop model was to steer the model on the ground on take-off, and it worked fine.  I'm not into hand launching, and we take-off from a concrete runway, here. On a grass field I'd have to go for a bungee??
• Number of fans:  The reason for the 1 versus 2 fans was because a contributor to the Mig-25 Decals thread suggested 2 x 50mm fans (presumably running the ducts straight through from inlet to rear bulkhead, with minimum alteration to the internal structure).  I am favouring only 1 larger(70mm) fan on the grounds of mechanical simplictiy.  And if you do the math it turns out that the area of the one larger fan is nearly equal to two smaller ones anyway:  For the smaller fan, radius 25mm, pi*r*r = 1964sq mm, or a total of 3928 for two --- whereas for the larger fan, radisu 35mm, pi*r* =3850sq mm -- nearly the same,  pretty amazing, eh!!  So for the same foward velocity about the same volume of air passes through the two systems.  The simpler one fan solution must win!
• CG: yes, must be.
• Intakes, exhausts: I've seen various formulae for the ratio of intake/fan/exhaust and it's difficult to know what really works. It would seem common sense that a given volume of air is accelerated by the fan to produce thrust, so the frontal area should be bigger than the exhaust by the amount needed to produce the required difference in velocity. However, doing the sums requires you to know several unknown factors -- hmm, problem. Your comments are helpful, though.
• Power train: I am favouring using a 50,000rpm fan with 5 blades -- it worked well on the Tigershark
• Energy - I am planning 240 watts for a 25oz model -- should be enough, I think?

best wishes, Richard

[Tim Mackey]

Yep all sounds sensible to me, 160 wplb should be OK. As for bungee / hand launch etc, well the jury is still out on that one until you know the characteristics of it - I manage to easily hand launch my Alfa Sabre with little more than a gentle staright and level toss - I dont even run up, and she goes away with no problem at all, and that has just 120 wplb, so as we know a lot will depend on thrust V velocity.

Of course there are grass fields, and then there are grass fields - depends on the state of it. I am always a little wary of small light models such as these with undercarriages of any type - to keep the weight down theytoo have to be small and light, and that usually means flimsy - and of course non retracting means drag....so unlike yourself I am actually fond of hand launching - I am lazy too, so avoid things like having to set up bungees and ramps etc.

I have used dolly systems in the past, and can just about live with them

You are obviously knowledgeable about FSA and so on - and I have not even looked at the actual structure or duct shape etc of the mig, but would just stress that from my testing on static thrust rigs - fan design and intake smoothing made a big difference, with the good 'ol midifan coming out on top.

I wish you the best of luck and will follow the thread with interest as I had already stated that when ( or should that be if ) my building board ever gets cleared, I would like to do the fan project myself, although I favour the lightning purely from a nostalgia and patriotic POV.

[Tim Mackey]

Blimeys Jet you hooligan - twin 68 mm fans

[Richard Sharman]

Well, obviously I don't want to have a model flying at Mach 2.83, or whatever the original did!  However, this is a 1/23 scale model, so it would only have to do about 100mph for scale speed which is not excessive for a model.  Actually, for me, stooging around  would be just fine posing for photos, but I wouldn't want it sagging about on the point of stalling.

The fullsize original has a (double) tubular back end not the simple cut-off transom as in the TN pusher prop model.  Perhaps this should be an essential modification to the plan to counter the issue raised by Jetsome?

My most important question at the moment is WHERE SHOULD THE FAN GO?  Should it be:

a) as far forward as possible on the grounds of getting good inflow to the fan, all the rest is just exhaust, or

b) in the middle of the fuselage where it is easiest to install, or

c) as far back as possible, on the grounds that most of the thrust can be lost in an inefficient tail pipe.

We're not worried about a rearward CG as the pusher prop has the motor right at the back, and that didn't cause a problem. The weight of a fan (compared to the motor) is next to nothing. Any ideas?

[Richard Sharman]

A problem!!!! Does any one out there have a solution for this??

The Tony Nijhuis pusher prop plan for the Mig-25 shows an all-moving tailplane - a nice feature and quite scale like. It's done by passing a torque rod through the rear fuselage just ahead of the last former.  The motor for the pusher prop is mounted on the back of the last former, so no difficulty passing the wires of the motor over the torque rod, and all is well.

BUT, for the ducted fan version, the torque rod would pass straight through the rear exhaust duct -- doesn't sound good??.

So, is an all moving tailplane a good idea anyway? Would a fixed tailplane+elevator do as well? or, where can you put the torque rod instead? 

[Richard Sharman]

this is where the tailplane has to go:

(see next but one post, please, sorry)

the picture shows the basic balsa box as specified in the Tony-N free plan (no fins, no nose cone, no canopy/pilot  -- just a plank wing, and a box fuselage.

[Richard Sharman]

readers of this forum might be interested in the previous discussions at  nig-25 decals forum

[Richard Sharman]

looks like I messed up the picture, try again:

[Tim Mackey]

I doubt the small diameter torque rod would bother the airlflow much - stick a vane on it and use it as vectored thrust pitch control ! I suppose otherwise you are stuck with conventional elevator then, ( unless you do two halves driven by 2 x mini servos and pushrods fitted in  /on fus sides just under the wing OOS ? ) Ugly though, and scale appearance sounds like its a priority for you.

[Richard Sharman]

ok, the torque rod just goes straight through the fuselage, rear exhaust duct -- the lot! Let's hope the drag on the exhaust flow is negligible.   I assme the comment about vectoring the thrust was proverbial tongue-in-cheek !? I hope so!

The problem with fixed tailplane + elevators on a model such as this is that it's never clear where to put the elevator dihedral brace (for the same reason as where to put the torque rod for the all moving tailplane) -- there just isn't a good place!

[Richard Sharman]

The Mig-25 EDF conversion specification:

As a  result of the discussions above, and the contributions of everybody gratefully accepted, here is my attempt at a "specification" of the Mig-25 EDF conversion project.  I am building to this specification and will post pictures as it develops.  Any obvious faults, no-no's, or whatever would be gratefully accepted by a critical audience!

External dimensions, shape, size, general structure _________ as in Tony Hijhuis plan

Internal structure, ducts _______________________________ redesigned bulkheads, Y-shape duct

Fan postion ________________________________________ 3/4 of way down fuselage

 Weight, CG, control movements ________________________ as close to TN plan as possible

Fan ______________________________________________ 1 x 70mm powerfan, 6 blade

Motor _____________________________________________ Brushless, 3600/v, 22Amp

ESC ______________________________________________ BLESC-25A

Battery ____________________________________________ LiPo 3s  2000mA

Servos ____________________________________________ Aileron(2), Tailplane(1), steering(1)

Rx _______________________________________________  4 channels required

Wing loading _______________________________________ (tbd)

Power/weight ratio ___________________________________ (tbd)

 Obviously, a lot depends on the all-weight, so the building needs to be as light as possible.  TN said his prototype without gear weighed 12 oz, which is a good target.  My version of the pusher prop was 12.5 oz without gear, but including rudders and wheels.

So - let building commence!   

[Tim Mackey]

Yep tongue in cheek

So fixed undercarriage I assume, rather than none at all ?

If you still plan on circa 250 watts, then the 22 A motor and 25A ESC are getting a little close to their limits on a 3s

250w / 10.5v = 24A  ( static of course )....and the 2000 ma pack might not last long...perhaps 2300 or so better.

[Richard Sharman]

Why "tongue in cheek", Timbo?  You obviously don't think it's going to work??!!  The spec meets the requirements on power though?    I agree that the ESC and motor are a bit near the limits, but that is governed somewhat by what's lying around on my bench, rather than the ideal.  I'm trying to re-use some equipment to keep the cost of this project down  (some of my other projects are rather more expensive!)

[Richard Sharman]

well, here is a construction picture of progress so far -- front and rear ducts in, fan+motor mounted, etc,
--- comments?

[Tim Mackey]

You said this, " I assume the comment about vectoring the thrust was proverbial tongue-in-cheek !? I hope so! "

so I replied....

Your picture is too small for me / us to comment on really

[Richard Sharman]

Sorry, looks like I messed up with Photobucket again! I just can't get the hang of posting these pictures!

[Tim Mackey]

Looks very good Richard....I take it there will be a final bridging duct for the inlet to the fan. Excellent work .

[Richard Sharman]

Just an explanatory note on the construction picture in the last message:

tubes are made from "plasticard" cut to size from a template and rolled into a tube before being inserted into the space and expanded.

The two tubes from the front inlet arrive at fan inlet guide which is 120% of fan size.  Yes, there is a smooth guide (not shown in picture to bridge the gap from inlet to fan.

Outlet tube goes from fan size to 90% of fan size to produce exhaust acceleration to a small degree.

The front Fuselage is tapered to nothing around the F4 position.  Fan is mounted in soft balsa buffer. All other bulkheads are liteply. Other wood is lighter than used for pusher prop model.

(more photos of finished model later, when I get photos sorted out!)

[Richard Sharman]

another picture of the duct layout (minus the detachable throat). The removable throat tube sits between the inlet ducts and the fan and slightly compresses the inflow. It's removable (by simply lifting it up), so that the fan unit can be pulled foward and out for servicing. The pockets to the side of the throat are big enough to house the servos and Rx, and of course the battery is in the nose, so that takes care of the installation - more later.

[Richard Sharman]

here is the throat duct:

[Richard Sharman]

Balsa construction complete!  This is how the model is looking now....!!!!

if you look closely, you can just see the intakes are open, so it really is the EDF version!  The canopy is still a bit unfinished (and stuck on with bits of masking tape - ugh!).  The model needs a final sanding before covering, and installation of servos, etc.  weight is 14oz -- just a little up on the 12.5oz of the pusher prop, but then it does have 4 extra bulkheads and 4 ducts.

[Tim Mackey]

You may not like this idea at all, but consider a test flight before you cover it....after all why add more weight until you know how it performs ? Besides, you may end up having to increase intake sizes etc, possibly even adding cheat holes etc, and then recovering / alteration will be necessary anyway.  Looking very nice there Richard.

[Richard Sharman]

Yes, it's a very good idea, and you are absolutely right, Timbo, that changes may have to be made, so why not test it first?  The counter argument is that (a) it goes against the grain (sorry!) to fly an unfinished model, and (b) our flying site is rather harsh (concrete runway, oil, dirt, etc) so that any unfinished model will really suffer. So I will probably cover it first, and take any other hits later, but thanks for the suggestion.  The solarfilm will weigh next to nothing, too.

 There is one other factor: on small models like this I use Cyano stuck fibreglass hinges, and it's best put these in AFTER covering. If you do it before covering, it's virtually impossible to cover it neatly.  Just a thought!

PS. delays in posting due to actual flying of pusher Mig - more later!