Gotchas with the Carmichael racer

I'm just playing around at the moment... using #s from roncz spreadsheets and w/b of me and a rotax 582. NLF414 14% thickness. pod was roughed out to fit me in a supline (think asw glider) position with 4" for a helmet, then crudely filleted @ 286mm or so. 6" boom just because redneck engineering likes the number.

The janowski j5 style of engine mount might work, but the open space behind was meant for fuel too, but I could probably fit a lot in the wing - but that also increases build complexity.

I still want to do the fold-a-plane / makerplane style cnc cut foam & vacuum bagged prepreg construction. Done so many dtf RC planes it just warms the cockles of my heart to think of doing it full-scale.

But roncz puts the cruise at 114, not exactly what I was shooting for...


 

In real life, these little pod-and-boom pushers look to me like they should go fast, but I'm usually disappointed by the true numbers. I suspect the rear of the pod where there's a lot of stuff happening is the challenge.

 

Yeah that's pretty much why that space is open right now. That's why the 582 is pushed back so far versus trying to integrate into the pod. haven't even started to ponder wing root fillets / interference drag. Pretty sure I have a couple papers here in my stack about fuselage stations and wing pressure gradient and making them work together.

My thought was to eyeball something between laminar magic and the J5 getting the airflow off the fuselage cleanly but pushing more of the prop out into clean air. To avoid the issues like shown in the decalage angle thread, which seems to be where the high wing suffers most, but the boorabee actually does better than most

 

Perhaps get the prop up as high as practical to get as much of the propeller disk above the pod or pod+wing and into the clean air above as possible? FWIW, the Cessna folks did some head scratching when the C-337 performed better on the rear engine than the front engine. There were theories that the rear prop was more efficient and produced more thrust or that the "scrubbing" drag from the front prop's slipstream on the fuselage was the issue. , Tuft testing and other assessments indicated the issue was the smoothness of flow at the back side of the pod. When the rear engine is running, the reduced pressure in front of the rear prop disk helps smooth the flow over the rear fairing. When the engine is stopped, that flow becomes more turbulent and draggy.

 

Why not convert your 582 Rotax into a 670 Rotax ?
582 dimensions with 30 more hp.
Couldn't do anything but help the cruise number that you're disappointed in.
In other words.......cheat a little bit and stack the odds in your favor from the start.

Kevin

 

Or are they slower than expected because they have small engines? A rotax 582 is 48kW (64 hp) and maybe of the “large” engines on these planes. An O200 like on Cassut is around 70kW (90hp). Designing around a 100kW engine might make a faster small pod and boom pusher airplane.

 

Well like the laminar magic it did 127 on 30hp, if strojnicks curve was right - it should do 200 on 50.

I picked a 582 because I needed a baseline, and switching to the vw isn't out of the question. But I'd rathwr hit the numbers with aero choices and design than raw hp

 

If you look at the LSA/AULA etc designs out there, even when you go up to rotax 912s - you're not getting much more than 100-120mph cruise out of them. Now most of the designs predate lsa rules with the 120kt cap so ... (?) And they arent faster outside the USA where the rules don't put a Vne cap...

So... Is it designed limitation? An overall shape that's creating the issue (Carmichael said 330mph for his designed theory)

Then again I'm realizing roncz is probably not a correct guesstimate as it's likely predicting turbulent flow not laminar flow (and yes I'd have to seriously work on it to get there)

 

All the yellow in the picture is assumed laminar. That is where the speed comes from which means accurate and stiff, smooth airfoils and no ripples in the forward section of the pod. Look up the VMAX Probe to see what could have been.

 

I agree that a small pod and boom plane looks like it should be highly efficient--the (possibility for) laminar flow over the pod, very small wetted area going back to the tailfeathers, etc. And yet the planes that are setting speed records with small engines are generally tractor-engined conventional layout planes with the fuselage sitting in the turbulent flow of the prop--and doing 210+ MPH on 65 HP. They just have a lot of attention to detail in their design and construction.
I'm just wondering where the pod-and-boom counterparts to the AR-5 are. Maybe folks just aren't building many pod-and-boom designs, but in the quest for records people generally do their research and do what it takes. There seems to always be some reason the pod and boom design just got unlucky ("Oops, the airfoil caused a crash." "Well, if my plane had more HP it could set a new record." etc).

 

A pusher engine/prop is sometimes in slowed or even reversed air (see Eugene's tuft photos)
So that's a concern for enough cooling. And those big scoops are not low drag. Might want a positive pressure cooling fan.

 

Most of the higher end gliders are good examples lol.

I'm leaning towards it's that behind the pod area where the engine usually is that's the issue. Pretty much why it's fun to play with it for the moment -

But then again most of the time it's like the cgs hawk and Challenger crowd who fly these where speed isn't much of a factor. Other than the powered gliders it's Hershey bar wings with round tube struts - shape chosen more for ease of/cost of construction more than anything. So I wonder if it's ever been even the focus?

A standard tractor you've got a very paint by numbers approach to cleaning things up - but this has in some ways it seems a bigger challenge to overcome.

 

if you have the budget see if Dave Lednicer would work up a CFD model. Given his previous published work he might something he would share with you.

 

I think some combination of Mike Arnold’s videos can give some topics to think on. Specifically, the one where he gets into some of his interference drag reduction theories will show how the rear pod fairing of a low or mid-wing pusher would probably get pretty nightmarish pretty fast if an ultra-slippery airframe was the goal.

 

I'll have to go hunting for his videos.

Yup that area is a nightmare, box is a roughed out 582. I can mount like the J5 but fairing it won't be easy.

 

Look for The Arnold Company on YouTube. When Arnold died he put his videos there.

 

So yeah the whole rear of the pod is a biggish issue. Mr Arnold really does explain it nicely.

Ridiculous question time - if I started a naca duct roughly inline with the leading edge, continuing through to the thickest part of the wing root - then let it exit behind the pod? Probably possible to do the same on top of the pod - naca duct starting just before the heighest point of the canopy - run them back into the pods wake area... There would be duct losses but - scavenge at high pressure release into low pressure zone...

 

I suspect simpler approaches will offer more payback.
1) You can accomplish some of this with the cooling air you need to bring in anyway, if it enters and leaves in the right
way/spots.
2) If there's room for the exit duct that you want to use, then presumably there would be room to simply reduce the size of the rear fairing and reduce its departure angle from the airstream at the edges of the pod (i.e. make the taper more gentle). That, together with some vortex generators to help keep the flow attached as it moves aft, would probably be just as effective, lighter, and easier.

 

The widest part of the pod should be at the wing trailing edge. Or not close to the wing at all.
Air can't stay attached around the backside of a pod and wing combined.

 

Haha, I'm starting to think Bruce had the layout right to start with