EDF Thrust Tube - design & usefullness
05-05-2016, 07:57 AM
#1
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EDF Thrust Tube - design & usefullness
Guys,
Looking at ways to increase thrust I read an old article from the 1956 Air Trail Annual on Ducted Fan Design. In the article they went through a pretty detailed series of development steps going from an open fan to adding a thrust tube then adding flow straighteners to finally varying the outlet diameter to maximize thrust. The bottom line for this was they increase their thrust over 800% as they went through the various stages. The flow straighteners and exit diameter seemed to be key elements in achieving this impressive increase. Looking at today's off the shelf units we get a lot of the elements the describe as part of the basic unit (correct fan design and flow straighteners).
One thing that I have not seen much information on is the thrust tube component. For a modern design fan unit does this add significant thrust? What is the current design parameters (the Air Trail article indicated that the exit should be 87.5% of the sweep area)? I saw a YouTube video where a guy did a test on a 70 mm unit where he changed the exit diameter of the thrust tube from 65mm (that was full FSA) to 63, 60, 58. He measured the thrust and found that max thrust was from the 65mm and 63mm outlet diameter, it lost thrust for anything below that. He also did a test with no thrust tube and found it was equal to the 65 & 63mm thrust tube. What he didn't measure was the velocity. It seemed that the 63mm (which is very close to the 87.5% in the AT article) would give max thrust plus a little boost to efflux velocity. Also saw an article that indicated that a 3 fan diameter is a good length that is pretty close ideal.
Are there commercial tubes available?
Best, DennisT
[ATTACH]2161367[/IMG]
Looking at ways to increase thrust I read an old article from the 1956 Air Trail Annual on Ducted Fan Design. In the article they went through a pretty detailed series of development steps going from an open fan to adding a thrust tube then adding flow straighteners to finally varying the outlet diameter to maximize thrust. The bottom line for this was they increase their thrust over 800% as they went through the various stages. The flow straighteners and exit diameter seemed to be key elements in achieving this impressive increase. Looking at today's off the shelf units we get a lot of the elements the describe as part of the basic unit (correct fan design and flow straighteners).
One thing that I have not seen much information on is the thrust tube component. For a modern design fan unit does this add significant thrust? What is the current design parameters (the Air Trail article indicated that the exit should be 87.5% of the sweep area)? I saw a YouTube video where a guy did a test on a 70 mm unit where he changed the exit diameter of the thrust tube from 65mm (that was full FSA) to 63, 60, 58. He measured the thrust and found that max thrust was from the 65mm and 63mm outlet diameter, it lost thrust for anything below that. He also did a test with no thrust tube and found it was equal to the 65 & 63mm thrust tube. What he didn't measure was the velocity. It seemed that the 63mm (which is very close to the 87.5% in the AT article) would give max thrust plus a little boost to efflux velocity. Also saw an article that indicated that a 3 fan diameter is a good length that is pretty close ideal.
Are there commercial tubes available?
Best, DennisT
[ATTACH]2161367[/IMG]
Last edited by Stuntguy13; 05-11-2016 at 02:32 PM.
06-15-2016, 02:07 PM
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Hi Dennis.
Interesting that the author found the best position for running a fan outside the thrust tube without an intake ring. I'll theorize that the vortices coming off the blades probably acted to helping direct flow.
In some ways it reminds me of whats known about best practices when positioning a turbine and thrust tube in a model.
Anyway, after I read thru it all best I could determine based on my current model is that I might get a 5% increase in thrust if I added a full length exhaust tube cone (Fig. 8F). Fuzzy math suggest the increase would be about .25 lbs going off the research.
Seems today, that with all thats been tested over the years that most accept that an exhaust diameter of 100% FSA gives max thrust and anything else down to about 85% FSA decreases thrust but increases exhaust velocity. So folks make a tube that best matchs their wants when they can. Most ARF's dont make it easy to test thrust tube diameters much less adding a cone.
To the best of my knowledge there are no generic thrust tube and cones even close to what Figure 8F shows. However, BVM and TamJets are known to include thrust cones with their systems and tend to be regarded as efficient.
Cheers,
James
Interesting that the author found the best position for running a fan outside the thrust tube without an intake ring. I'll theorize that the vortices coming off the blades probably acted to helping direct flow.
In some ways it reminds me of whats known about best practices when positioning a turbine and thrust tube in a model.
Anyway, after I read thru it all best I could determine based on my current model is that I might get a 5% increase in thrust if I added a full length exhaust tube cone (Fig. 8F). Fuzzy math suggest the increase would be about .25 lbs going off the research.
Seems today, that with all thats been tested over the years that most accept that an exhaust diameter of 100% FSA gives max thrust and anything else down to about 85% FSA decreases thrust but increases exhaust velocity. So folks make a tube that best matchs their wants when they can. Most ARF's dont make it easy to test thrust tube diameters much less adding a cone.
To the best of my knowledge there are no generic thrust tube and cones even close to what Figure 8F shows. However, BVM and TamJets are known to include thrust cones with their systems and tend to be regarded as efficient.
Cheers,
James
Last edited by gsoav8r; 06-15-2016 at 02:27 PM.
06-15-2016, 05:04 PM
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I need to add also that BVM's ducted fan systems are typically "area ruled" designs, intake and exhaust, and have been for decades. Thats what Fig. 2 pretty much is.
Last edited by gsoav8r; 06-15-2016 at 05:11 PM.
06-19-2016, 12:17 AM
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I wouldn't take much notice what they came up with back in 1956, 800% increase thrust seem like bit of nonsense. You are asking just about impossible question to answer because different model with different power unit and different batteries will give you totally different figures. The best you can do is to measure your current draw and static thrust thrust with few different exit diameters and go on from there. There is also bit of difference between static and dynamic thrust which you will only find out by flying your model and comparing performance you are after. The figures on paper come always totally different than in practical reality. Wemotec in Germany makes some thrust tubes but there isn't much science to make your own by rolling out mylar sheets or I often use old X-ray sheets.
I have couple of 90mm models (Mig-29 and F-4) where I have squeezed the thrust tube exits to minimum at the expense of shorter flight time longer take offs but more speed while flying. So there is always some compromise to make. Another example is when I was few weeks ago playing with one of my 90mm EDF prototype and the thrust figures varied between 2.5 and 4 kg while using various inlet rings and thrust tubes. So we could go on for ever and ever on this subject and never agree on anything. (few pics of the models I mentioned)
I have couple of 90mm models (Mig-29 and F-4) where I have squeezed the thrust tube exits to minimum at the expense of shorter flight time longer take offs but more speed while flying. So there is always some compromise to make. Another example is when I was few weeks ago playing with one of my 90mm EDF prototype and the thrust figures varied between 2.5 and 4 kg while using various inlet rings and thrust tubes. So we could go on for ever and ever on this subject and never agree on anything. (few pics of the models I mentioned)
