BVM thrust issues
 

Hi All.
I have a BVM .91 ICDF with viofan I have in my Mirage 3.
I am strugling to get a good static thrust on this setup, the engine screams like a banshee so I think that part is OK but the thrust at the outlet of tube just "feels" weak.
I tried to measure it by having the nose of the jet pushing against my scale (scale standng vertical) and it only read about 2800 gr.

My thrust tube exit is 95mm (just over 3.75 in) diameter. It woeks out to be 81% of fan swept area.

Is this setup restricting the flow to much? I cant really alter the length of the thrust tube setup, only the outlet diameter id needed.

Would a 100% FSA exit size still give OK , not blistering fast, flight thrust?

Thanx

Theuns

You have to check the inlets. You need enough inlet area. And nice radius inlet lips and a full inlet to the front of the fan

Hi Matt

Just checked it now, radius on intake lip about 1/8" diameter and area is 100% that of FSA.

Theuns

Theuns,
The BVM Viojett/Viofan requires a full inlet to supply air to the fan for maximum efficiency. This inlet must go from the inlet lips to the face of the fan. Larry Wolfe, Jet Hangar Hobbies, used to sell a molded Hi-Tech Inlet for his Mirage semi kit. This inlet is not seamed and could be adjusted to provide a smooth flow of air from the inlet lip to the fan face. Once adjusted it can be seamed with a strip of fiberglass tape and epoxy. You might want to check with Chris Wolfe, Larry's son, on obtaining a Hi-Tech Inlet for the Mirage or directly with: www jethangarhobbies.com. This inlet should provide an increase in static as well as dynamic thrust. Let us know how you make out.
Regards,
Art ARRO

As you haven't included a picture of the install in the jet it's difficult to know but have you got full inlet ducting leading from the intakes to the front of the fan, and of course the shroud for over the fan and engine to enclose all of it? If you don't have full ducting from intake to exhaust you will have very little for thrust. That is why the byron fans were so inneficient. They were much larger diameter and required large cheater holes in the airplane. 1/8" rounded edge on the intake is very sharp, you are losing a lot of smooth air with that, 1/4" I believe is what was recommended. If the engine is running at proper RPM and your thrust is lacking then it is 99% likely a ducting issue, make sure your ducting is sealed and smooth from start to finish.

Guys sorry I should have posted pix of the actual duckting.
Yes it has full ducting from the intakes , arrounnd the engine and all the way to the exit. I tried to make this a smooth as I could.
It is the same tipe ducting as described that is made up of 3 sections and then assembled into place.

Tday I will make up 2 more thrust tubes from plastic just to test the difference. One with a 10mm larger and one with a 10mm smaller diameter than the one I have now.

How do you guys measure the actual thrust of a jet? I dont kow how acurate a digital scale would be if it stands up vertical.
I was thinking of maybe cutting a cheater hole into the centre section rear of the intake ( the rounded piece) , it should allow more air into the fan through the nose gear hole BUT i am not sure how this turbulant air will affect the fan. If there is enough space I could make a seperate little scoop that feeds into this centre section.
Could it be that the BVM needs more air than the 100% FSA the intakes provide, if the plane is moving forward, would it not ram more (smoother) air into the intakes?

Theuns

What static RPM are you getting?

Hi Matt, I have no idea, I cant get a taco in to see, I know however it must be very close to "max" by the sound of it , I understand it is not a acurate way but it is defenately revving way more than my OS 91 did.
I have a friend that flies a Mavric with the same setup, he told me his outlet is only 85mm diameter, 10mm less than mine so I removed the thrust tube and alttered it to be the same. I will run the engine today and see if tere is a difference.

Theuns

I ran a 85mm exit tube now, it only read 1900 gr on the scale but the thrust on my hand at the back felt quite a bit more than with the 95mm tube, the smaller tube also sucked my hand onto the intake stronger.......I am rather confused with this. I theroy the larger exit tube should give better thrust.
Maybe my way of measuring the static thrust is not all that acurate and this is giving the confusing readings.

Theuns

I'm wondering if your getting full rpm. Either too rich or your throttle isn't opening quite to full.

The smaller exit should give you a higher air velocity but often gives less static thrust. You're moving less air, but faster. Without actual data on things, it's hard to diagnose anything. A Viojett turning 20,000rpm will sound like it's screaming, but if I remember my DF days (a LONG time ago), a well tuned BVM .91/Viojett would turn upwards of 25,000rpm on the ground.

The engine leans out well on the mixture controll, I will start on a rich setting and then lean it out till just before it leans to much so as for fuel supply I am getting the max I can. The throttle does open all the way.
I will try some more thrust tubes to see what they deliver thrust wize, I am not after speed but power for takeoff so in theory a larger outlet is called for.

Some more measurements wrt 85, 95 and 105mm outlet should give me the data I need.
Just a PITA to have to make a new tube and remove the fan to install it every time....ah well the joys of DF LOL
The hassle is that i live in an estate and noise is not permitted so I have to time my tests carefully not to make hassles with the people arround, I must admit this BVM is extremely loud!

Theuns

I seem to recall 22K or so with the OS91. We used a fish scale, which was just a spring with weight markings on it, hooked to the back of the plane. 10 pounds was about the best I could get, but I fly at 5000' altitude. I usually went slightly on the larger side on the outlet for more static thrust for easier takeoff and climb out due to the altitude. I also seem to recall 110% to 115% inlet area, but maybe that is just my failing memory?

Scott

I've got a BVM Viper converted to EDF(Tams Dynamax) and have pretty much the same ducting as the ICDF version. There the outlet is 3.75in and the inlets appear to be at least 100% FSA. Probably right at 100% FSA and larger than today's state of the art performance EDFs like the Electra. Theuns sounds like you are getting 7lbs of thrust but if you want perf go EDF(I think there is a JetFan that will work very well). There is just no comparison to the old ICDF even if you expand the outlet. Of course the runtime of ICDF is still superior vs EDF...

I also fly at hot and high elevation, usually round 30 deg C at 5800'
3.75" otlet brings me back to the original 95mm outlet I first had, I will try get an acurate fish scale to compare the tubes.

I think that the more "power" I felt on the outle side was due to the higher air speed, not thrust.

Wish I had the $ for a nice EDF or even Turbine but allas it is not in the cards, will have to make this setup work as best I can, more tinkering needed I think.

Thanx for all the input sofar.

Theuns

Theuns,

May I ask how old your setup is? What fuel are you using? You seem to have a lot of carmelization on the header and pipe. Have you run it too lean? Just curious. BTW, I had a BVM .96/Viofan setup in a Y/A A-4. The inlets on that plane were, by all 'expert' opinion, all wrong. WAY less than 100% of FSA, curvy and LOUD. On the ground, at 23k, the plane didn't push on my hand much, but once in the air, she hauled butt. If you're satisfied that you're getting peak RPM, why not just fly it and see what you think of the actual in-flight performance.

Shaun, a mate gave me this setup that was in his old F-4 so I dont really know the history of it but the bearings feel glld, not tight and sandy, the piston has slight heat marks on it but no glazing I can see.
I spoke to an old school ICDF guy locally and he said somthing interesting. These intake may be causing turbulance because the wedge in the centre does not go aal the way back to the fan face, it could be causing the fan to cavitate- loads of noise with little thrust.
Now, I have a few options to test...
1. re-enstall the 95mm tube that gave about 6 Lbs on the scaleand hope that the forward motion will clean out the air into the intakes. I can only test this by actually doing hight speed taxi runs.
2. or make a splitter plate that seperates the 2 intake halves to just infront of the fan face.


On my old ICDF I do recall having intakes that went all the way to the fan with a splitter plate and a fairing for the spinner, even on my T-45 EDF with 90mm changesun it also has a splitter with the spinner fairing all the way to the fan face.....must be a reason for this?

I recall reading somwhere that these style of intakes I have now are similar to the "old style" That JHH used, I see they also now have the ones with the full splitter to the fan.

As for fuel. I had my local HS mix up a bru of 75 Methanol, 15% castor ,5% synthetic oil and 15% nitro. A for the plug I thing it isan OS 3 or 8
Theuns
PS if I take long to reply it is because we are about 8 hour ahead of you guys ;-)

Back in the days when ICDF was popular it was generally refered that BVM power set ups only worked fine in his own designed models. And it was generally acepted that it was very hard and difficult to adapt it to other model jets, so be prepared to test and test againg till you get your desired results.

Regarding your last comment I agree that all ICDF models I owned had the wedge going to tha fan´s face and having a hole for the spinner thus it may well be a problem with your model, but I also notice something that takes my attention: the fan is very far away from the model air intakes. The BVM models I had and others I know have the fan far more close to the air intakes and, in fact, I only remember a R/C model Grippen with a Ramtec unit and OS .91 from Mr. Paul Gray to have the fan nearly so back in the fuselage.

Hope experts put you in the correct way and get the performance to get that beauty Mirage 3 in the air.

Best Regards.

Thanx, the length of the intakes are set by the location of the engine, if I move away from this it will affect the CG and I really dont want to put unnessesary ballast on as I think this model will weigh about 14 lbs....already way heavier than I wanted it :-(

This week I will experiment with a splittle plate and report back, I really hope it will imporve the power of the fan, granted it will not fix it 100% but every but helps.
If I can get round 10Lbs static I am happy, I also think that this engine needs to move forward into clean air to make the correct thrust, just sucking it in while stationary I think makes the engine/ intake setup work to hard.
Now I just need to figure out a way to make said splitter plate. I was thinking of epoxyglass board or even 2024 alclad that I can screw to the intake. The very last thing I need is a gluejoint only that might come loose over time and going into a 20K rpm fan!

Theuns

Theuns,
I just measured the inlets and tailpipe nozzle on my BVM Maverick Pro and the inlet radius is 5 mm and the nozzle diameter is 95 mm. The inlets have a splitter plate right up to the fan face. This Maverick has been flown extensively with a BVM 81, K&B 82 and a Fan-Jett 95 engine. It has been radar clocked at 165+ MPH in a racetrack pattern-no diving. I don't recall the takeoff RPM but it was just below peak where a quick pinch of the fuel line would yield a rise to the peak. I did have a special tach with a LED and sensor to read the RPM from inside the fuselage. My fuel mixture was similar to yours except the oil content was increased to 23%.
I recommend trying the 95 mm tailpipe and fabricating a splitter plate inlet. Be sure to secure this plate well and apply several loops of CF tow on the inlet exterior to prevent collapse due to suction.
Note that I have about 20 years of ICDF experience on various airframes including the BVM A-4 Skyhawk, Sport Shark/ Aggressor and Maverick Pro plus the JMP Starfire and DL Cyclone. Good luck with your Mirage 3 and please post your findings.
Regards,
Art ARRO

hI Art, thanx for the info.
I concur wrt the outlet size.
I just ran the setup with the 85mm tube now, I bought a digital fish pull scale and it also showed 1,8 kg of static (3.9 lB)
I will now remake the tube to be a 95mm exit and test again without the splitter plate, I want somthing to compare with when I put the plate in.
If I can get 10 Lb static I will be happy to try fly it. maybe the forward motion will add a pound or so of thrust.

Stay tuned :-)

Theuns

I removed the setup again and made this splitter plate from .0025" Alclad.
It is not the nicest looking or best fitting unit but I am sure it should stop the turbulance and hopefully direct the air into the fan a little better.
The problem I think came in with the rounded edge of the "wedge"
I now need to make up the tabs that I will use to bolt the plate to the inside of the duckts.

Lets hope it will increase the thrust, after this I am out of ideas.....

Theuns

Hi,

I hope that improves things. I do have to say, though, that the stock setup isn't, in itself, bad. I offer the Starfire (Tom Cook and Y/A versions) as an example. That inlet is very similar to what you started with, and nobody could ever say that it didn't work. That plane could hit 200mph on an O.S. 91 and Dynamax fan and even the static thrust on the ground was considerable.

Well. here is the latest.
I made the tube uotlet to be 98mm in diameter amd I cut a slot in the wedge basically removing the rounded lip at the back.
I did this so I could install a sharpedged aluminium wedge from the fromt of the slot to smooth out the air and by being installed from the fromt it can not come loose and go into the fan......then the idea struck me - I will forst run the setup with that "cheater hole" and see what it does.
Maybe the air that can now enter from the slot will fill that low presure at the back of the round wedge , and as a bonus I now have about 25% more inlet area.
If it works I will omit the alluminium sharp wedge or splitter and just put a FOD cover over that hole.
Hope to run the engine after work tomorrow.

Theuns

The latest static test now shows an imporved thrust of 3,5 kg (7.7 lb) and the engine is not making the same super banshee scream, although it still sounds like it is running at full designed RPM.
The feel of the exhaust and hand sucked to intake is about the same as I can remember of my old OS/ramtex setup.
With a predicted MAUW of 14.3 lbs I will need 10 Lbs of thrust to fly, I am hoping that the ram air of the plane moving forward will give me the extra 2.3 Lbs I need.

I think, even with the full FSA intake this fan needed more air intake so I will leave the cheater hole as is and put a FOD screen on it.

Even with the same size tube (without the cheater hole) I only got 2.8 kg (6.8 Lbs) statinc, so on this model the intakes as standard for 91 size fan seems a tad small.
I am sure en EDF or turbine will be way more efficient and the intakes should be fime as is.

Now I need to complete the fitting out of servos ans undercart, then take it out for some high speed ground runs, then I will be able to tell if it would fly.

Thanx for every one who replied.

Theuns