Blended bodies

[PyroJoe]

Have been working with multiple intakes lately. This somewhat led to the idea of combining multiple engines from the head plate to tail. A "Blended Body" in effect.

Basically what this means is that as the aerodynamic valving of the intake increases, the body of the duct can have a larger cross section to length ratio.

This is nothing new, but was new to me.

Utilized this to review some valved engines as they typically have larger cross sections to length ratios. At first I considered this was an attribute of mechanical valves, instead of aerodynamic valving. Now I'm not so sure.

Solid modeled a Dyna-Jet and split it into 3 blended bodies. I substituted the 10 intake ports for 9 and corrected the intake cross section. This allowed 3 valveless intakes per blended body.

Also I took one of the bodies and applied the caliber technique to get a feel for how it stacks up.

[PyroJoe]

I tested the previous caliber method (Recipe for Beginners topic) on a small 2 blended body, and it worked well.

[Jutte]

Hey that's pretty cool.
Did you notice any starting/running issues?
Are the combustion chambers linked or totally seperate?

[PyroJoe]

Thanks,
The combustion chamber has no seperation. About the only running issue was the high frequency typical of smaller engines. Also it is painfully loud, most of the duct glows orange.

[larry cottrill]

Yes, Joe, that should work as long as you keep the impedances (of all the "slices") identical.

A problem I can foresee is the heating of the internal (uncooled) webs. I'd be interested in seeing how your experimental model holds up after some long hard running.

Nice work, as always.

L Cottrill

[PyroJoe]

Hi Larry,
Good point, but the slices are only for visual aid, so each body can be visualized.

The point is that there are no webs or internal dividers. The bodies have been blended into one duct!

Somewhat like taking a look at valved engines through valveless glasses so to speak.

James D. last version of Lady Anne has two intakes, IMO Lady Anne makes much more sense as two blended bodies.
The thrust levels somewhat give it away also, going from near 2lbs. to over 4lbs.

I built two different FWE before this last caliber type engine, but at the smaller scales I tend to not do so well with them. If I would have faired better would be reporting on a liquid fueled Anne by now. When I find time I may give it a go with a tailpipe of 1.5" minimum diameter, propane is so fussy at 1.25" . At the moment, the back log of work is to an extreme.

[larry cottrill]

Gee whiz, Batman! I totally misunderstood what you were driving at. Now I get what you mean by "blended". I think I was pretty tired when I saw this and reacted in my customary knee-jerk manner.

In my opinion, no matter how much you split up the intake area, you will not have real FWE action in the chamber if the cone gets too "stubby". I think you'll start getting increased harmonic generation, and there will also be some separation of the backflow (from the tailpipe) along the chamber walls (which may, of course, not mean anything ;-). But, I have to admit, it's worth trying. After all, one of the main things I was after in the first place was "compactness".

Also, I no longer like the idea of "linear" FWEs with flat front plates -- I think reasonably long front cones are vastly better.

L Cottrill

[PyroJoe]

As fate would have it, that is nearly what I have in process at the moment. I had to shorten the CC on a caliber type engine, and cut most of the cylinder section away, so now I'm looking at nearly a FWE chamber with multiple inlets in the headplate.

Its not as fun to fabricate multiple intakes, but it appears to be one of the few methods that work when trying to create a more compact engine. Where the intake port opens onto the CC I create a sharp and perfect 90 degree angle. It would probably work even better if I passed the intake pipe some distance into the CC, but my chambers are already on the short side.

[Mike Everman]

Love to see someone else experimenting with this! Mine experiments were pretty conclusive, documented I think in my build log.

Long and short of it was a triple that yielded not 3x thrust of it's single parent, but 3x divided by sqrt3. (pocketjet 4.5 lb, hoping for 10.5 lb on the triple, getting just under 6 lb). The follow-on experiment was to be running 3 of the separate motors in close proximity, and what the interference hit is. I'm going to predict that it's the same loss because of this interference, even if you get them synched in three-phase. Anyone that has tried synching just two separate motors will know that it's not easy, and you're not sure if you have even done it!

One of the things I was last playing with on the subject was a "wye" fueling stinger, that I'd hoped would help push the fuel around in a three-phase way to help with synching. Still have not done that.

My experiments were inspired by Bill Hinote's conjecture on this type of scaling for achieving shorter motors. I think he even made a motor that was 1/4 of a Kentfield four-intake.

[metiz]

Love to see that engine build, great design. BUT won't those cc's overheat where they touch? best to leave a small gap in between

[PyroJoe]

Mike,
What was the proposed reason for the lower yield in the triple in comparison to the single(x3)?

[Mike Everman]

Yeah, a little space in between is essential, I agree.

As to the why of the low thrust, I have no idea, other than the fates wouldn't make it so easy on us, would it? M. confirmed the result after the fact with his method. Everything was the same, though, frequency, starting, turn-down ratio, etc. I tried to preserve the average taper angle in the rear cone, and average radial-inward distance at the cc, hence the elliptical cc. Perhaps a different result if it were cylindrical, but I couldn't think of a way to maintain the cone half angle.

The next test actually was to make a 2x, and see if the resulting thrust is 2x/sqrt2, or, oddly, about 6 lb again. Wouldn't that be something? Can't be right...

[PyroJoe]

Yeah something sounds a little off there with the 2x/sqrt2.
Maintaining the original cone angle is a bit of unknown, my intuition is to increase the angle but maintain the original cone length, trying not to exceed 20 degrees overall. Several of the valved engines that exceed 20 degrees tend to have a radius from the cone to tail.

[PyroJoe]

One of the few valved engines that approximates a single body, throws its single valve and continues to run valveless.

Nanosoft's Craft Jet
viewtopic.php?f=2&t=995

albeit this was on acetylene.

[Mike Everman]

One thing I did with valvers was to analyse them the same way I do a valveless. If the proper radius inlet is used, the acoustic stations on the length(say, Bailey or Red-head), the throat, length and acoustically corrected length of the intake, it looks exactly like a valveless, with an inlet area that is slightly off. Any good valver can take a proper valveless intake of the same acoustic length and work.
The big radius of the typical valver has an acoustic length that is far forward of the physical inlet plane, like at about 10% of the total length, which is one possible good intake length.

I've been going over my data for my pocket jet scaling builds and finding interesting things. I'm plotting some things up for discussion. I think the aspect ratio is going to measure heavily into the derating equation when blending multiples of known good engines. I do wonder if the SFC is improved one way or the other. I can't guess which way, though.