I-Love-Jets

Or you could simply copy the blue above posted American Challenge WSR design, which would be perfectly suited for a ducted fan setup. The only additions should be a central rudder and functional elevators plus elongated hull to prevent backward flipping.

I-Love-Jets

An interesting alternative also would be a tandem "wing" hull which would allow functional control surfaces at the front “wing”.

hooty301

The American Challenge WSR design would be very interresting with a ducted fan unit. Do you have any experiance with ducted fans? About how much would I have to spend, the complete unit .91 fan and all?

proptop

My Feedback: (8)

I'm no DF guy, but I've seen "previously owned" stuff selling for around 250.00 to 300.00 and figure all new stuff for about double that...(?)
That's engine/fan unit/pipe
I thought someone else woulda answered you about this by now?

Maybe somebody here will see this and have something to offer that you could use for your project...or check out the RCU Marketplace...you can get some good deals there. (usually )

I-Love-Jets

Well I personally have had gathered experience with OS .91 VR-DF engines and JMP2 pipes in the past (but with Ramtec fan). The OS is a strong and reliable engine as long you keep the mixture rich enough (good visible white smoke trail at full throttle as easy indicator). Recommended glow plug is McCoy MC-9.

Use at least 23% of oil and not more than 10% of nitro. The relatively high amount of oil not only lubes but more importantly cools the engine internals, especially the lower conrod end and the piston. A R/C mixture control is a must in jets but in your 2D application no mixture control and no additional hopper tank is needed.

I-Love-Jets

Good general information on ducted fan propulsion can be found here:

http://www.jethangar.com/Ducted%20Fans/df.html

GENERAL RULES OF DUCTED FANS

Though GDF's and EDF's are intrinsically different in terms of how the fan is driven, the aerodynamics and rules of thumb of the ducted fan remain the same regardless of the power source. We've compiled a list of items and frequently asked questions that should be helpful to you in your DF jet flying. If there is something we missed , please let us know!

How a ducted fan works -
Ducted fans operate as they sound: they are an internally ducted high performance fan which takes in air from the inlet opening and blows air out an exhaust tube to generate forward thrust. The air enters an impeller spinning at high RPM which propels pressurized air out of its exhaust at increased pressure and velocity. A ducted fan thrives on the air fed into it so too little air will starve the fan, thus preventing it from perform at its optimum design point. Conversely, inlets that are grossly oversized can have the same effect: a ducted fan can only process a given amount of air at any particular time, so too much air will hamper performance --not to mention the increased drag from the larger inlets.

Requirements for Ducted Fans -
Ducted fans need inlets sized to approximately 95%-110% of the Fan Swept Area (FSA) with an exhaust outlet sized to approximately 75%-85% FSA. Choking down airflow at the exhaust, increases exhaust velocity. This in turn will increase top end speed of the aircraft. There is, however, a point of diminishing returns: choking down the exhaust too much will back pressure the fan resulting in degraded performance. Additionally, a larger outlet area will increase the static thrust of the system, but lower the top end speed of the aircraft. The areas suggested above represent a good compromise between 7static thrust and exhaust velocity.

Fan Swept Area -
The fan swept area is calculated by subtracting the area of the impeller hub from the area of the inside of the shroud. The equation is shown below:

Duct Area - Impeller Hub Area = Fan Swept Area

External Inlet Shape -
The ideal inlet lip shape for a ducted fan aircraft is a 2x1 ellipse. This represents the most aerodynamic shape while maintaining good airflow into the inlet system.

I-Love-Jets

So unlike turbine inlets, ducted fan units need a cumulated inlet area being approximately equal than the fan swept area. Turbines in contrast can compensate a considerable smaller inlet ducting easily due to their compression wheel.

A properly designed airflow path is a must. So I would not use a separated (twin) inlet but a single “big mouth” right above the cockpit. The inlet design must still allow to use the starter wand for getting the engine started.

I earlier posted that I would begin with a standard Dynamax unit, i.e. without repitched fan blades. However some ducted fan jet pilots have had modified their Dynamax blades meaning they wanted more top speed (higher blade pitch to match the Ramtec exhaust velocity).

But in your case the opposite may apply: If you feel there’s room for improvement after the first speed trials you may need to professionally de-pitch the fan blades slightly to further increase the static thrust. I would contact Tom Cook of JMP for that job. Anyway he claims up to 15 lb of static thrust out of the stock OS91/Dynamax/JMP2 unit. That is already a lot!
http://www.jetmodelproducts.com/dynamax.htm

You won’t get problems with an over-revving OS.91 VR-DF driving a de-pitched rotor (equals less work for the engine to do), because the engine cannot freely unload in your 2-dimensional application.

In order to keep the engine temps low enough I would definitely apply the large engine cooling head for a Dynamax airboat application. Normally this head only is used for pusher type ducted fan (Byron) or pusher propeller application.

Below pictured is the Dynamax fan front:

I-Love-Jets

The JMP2 tuned pipe being muffled produces a nice high performance 2-stroke sound. Compared to other ducted fan pipes it is characterised by a large internal volume of the first tuned pipe chamber which keeps the engine’s combustion temperatures low. Instead of a twin cone pipe design this pipe comes with a deflector plate. Be sure to locate the pressure fitting (nipple) right in front of this deflector plate to guarantee an even and strong tank pressure. Generally never tap within the muffled section of a tuned pipe.

The JMP2 tuned pipe length is fixed and optimised for rpms from 20000 to 23000 in combination with the OS 91. That also is the rpm range the Dynamax works most effectively. So this well-engineered engine/pipe/fan combo has been very successful until the turbine era began that literally displaced all nitro powered ducted fans on the market.

BTW, the total length of the front of the OS .91 engine crankshaft to the end of the JMP 2 pipe is 23.5”.

I-Love-Jets

I would only buy a new never run ducted fan engine to be trouble free right from the beginning. A new OS .91 VR-DF (small or large head) frequently reaches auction prices of maximum 220 US$ at EEE-BAAYY. A fan unit I also would only purchase new to ensure correct balance and no blade nicks or cracks induced by sucked in foreign objects. New Dynamax fans reach prices between 60 and 100 US bucks. Auction prices for a new JMP2 tuned pipe usually range between 50 to 80 bucks. A used pipe 40 bucks at maximum. A used JMP starter wand to be mounted on an regular electric starter does not come too expensive, maybe 20 bucks.

So all in all the entire fan combo will come to about 370 to 400 US dollar (all new never used). One just has to be a little patient to wait for the right auction(s). Fortunately there’s only little demand in ducted fan stuff these days.

I-Love-Jets

I personally would choose the following measurements for the inlet and outlet ducting to hit highest static thrust (see also sketch below):

Inlet ducting area of 110 % of Fan Swept Area = approx 115 cm^2

Outlet ducting of 85 % of Fan Swept Area = approx 89 cm^2

Length of inlet: approx 17.5”

Length of outlet behind fan unit: approx 25”

For the ductings it is most important to keep all transitions and radii smooth to avoid turbulent airflow. The inner surface of the ductings has to be super smooth without any roughness. In your case the positive mold construction method would be convenient. That would be not as time consuming as negative molds and less expensive too.

With a total boat length of about 50” the pitch motions can be reduced effectively at your desired speeds.

The tank position – if possible - should be within the CG to maintain neutral behaviour.

Do you have a large pond at hand for your future speed trials? The actual lake seems a little on the small sideâ€Ĥ

Craig-RCU

That's some great info on ducted fans. I would suggest the shortest ducting possible though, unless you are after a scale representation of the American Challenge WSR design. I have read that long ducting can decrease static thrust by 25%. I have also found this in my own testing http://www.geocities.com/cnhilgers/F86mods.html I would forgo the pain of designing ducting that only adds drag if you are going for all out speed. Check out the Enforcers below. One is powered by an O.S. 91 in a Byro Blaster pusher duced fan and the other is turbine powered. The Enforcers even look like the America Challenge boat if you clip the wings off outside the vertical stabs and add tip-floats.

I-Love-Jets

Hooty,

I forgot to post a high performance pusher prop setup of a buddy of mine. He used this combo for a fast delta aircraft. His components are:

OS .91 VR-DF
Byron ducted fan tuned pipe
Zinger pusher prop 11x10 (shortened diameter to 10”)

But in your application a drawback of such a setup could be a tailheavy airboat. However the wide bladed (# P505) 11-7W Zinger pusher prop, to be cut down to 10“ dia, could work fine in an air-speedboat at about 21000 rpm. The entire tuned pipe plus engine (except for the large cooling head) could easily be fully cowled, while adding NACA inlets and outlets for cooling.

hooty301

Yes there are some real lakes near by, the only reason I use this pond is theres access to a boat for recovering reasons. This spring Ill have to get a raft and some paddles and hit the big lakes.

Ive been watching Ebay for a OS.91 fan motor no luck yet, Ill probably just save a bit and buy new. I did see a .80 Rossi fan motor, what do you think about these. If im right rossi doesnt make airplane engines any longer, when I go to there site they only shows car engines. Also found a Rossi tuned muffler on Ebay for a .60 the flange looked like it would bolt up to my OS 61 but dont know for sure, Ill have to look into this.

Thanks for all the info.

hooty301

Went to the pond yesterday and tested some smaller dia. props (11/5apc and 11/6apc) with no gain in speed, lost about 8mph. The 12/4apc seems to be the best prop for this boat however did get about the same speed with a 12/5 apc if I could get a perfect run from one end of the pond to the other. Also tried a 12/6 master airscrew scmiter series and got very close in speed to the 12/4apc. The 61 wings the master airscrew at 14,250rpms which is about the same rpm as the 12/5apc and kind of shows how much these(master airsrew) props flex.

I-Love-Jets

Well, the Rossi .80 DF is out of production since about 10 or 15 years. For that I would not buy such an engine which has been discontinued long time ago. Same applies to the 13 cc ducted fan engines made by Picco, OPS, CMB, Webra, K&B...

BTW, the Rossi .80 DF features a quite bad balance. Rossi simply took the .61 ducted fan crankshaft without adding enough counterweight to compensate a „heavy“ .80 sized piston. Accordingly this engine is characterised by very “rough” running attitudes. The Rossi carburettors for DF application generally are less user friendly than the OS 9B carb of the OS .91 VR-DF being on of the best carbs on the market.

So the #1 choice for you still is the OS .91 VR-DF Large Head which also is considerable stronger than all (discontinued) .80 ducted fan engines. The Nelson / BVM .91 or .96 would also be a good alternative, but these are twice as expensive as the OS and spare parts are rare.

As for your OS .61 side exhaust sport engine, I personally would not spend more money in your actual running airboat which seems to have reached its dynamic limits. A completely new boat design (stable runs without backflipping) + power unit (about 12 to 15 lb of static thrust produced using a comparable small prop/fan diameter) is the only key to reach your desired speeds.

MasterAirscrew props – as you already found out – are much too flexy for any kind of serious speed application.

hooty301

The only reason Im looking for more performance out of my .61 is for the snow record which is set at 44mph .I have been designing a boat similar to the canyard hydro teather boats just for snow.

For the American Challenger I would really like to keep it under 40 inchs and ten pounds. Would this be to small for the .91 fan unit to be efficient and what kind of speeds do you think could be had? Also what speeds with this setup in a actual plane?

I-Love-Jets

OK, although it doesn’t really make sense to post ducted fan powered speeds here (it's like comparing apples with oranges ):

The Ramtec fan combined with a high performance airframe such like the Trim Spectre reaches about 200 to 210 mph on a good day. The stock Dynamax fan, due to its lighter pitched blades, achieves “only” 180 to 200 with the same airframe but makes more static thrust. Both OS .91 / JMP2 driven.

I-Love-Jets

So let’s compare the below shown .90 sized purpose built speed plane (50” of fuse length) with a purpose built .90 sized speed-airboat in terms of total length requirements:

In both cases the elongated design helps much to reduce the pitch moments. Such designed speed planes do respond “cool” on elevator even at speeds exceeding 200 mph. Maybe you should have a call to Andy Brown of CMD or to the tether speed boaters who will confirm you that at a certain ground speed on the pond a certain minimum hull length is essential. Also you can compensate an unfavourable CG position - which in your case will most likely be higher than in the record holding Eagle SGX for example - by simply elongating the hull to your needs to get the moment arms right again.



For that it will be essential to keep the hull as long as possible.

The longer – the better!

I believe that you won’t have problems staying under 10 lb of weight at 50” of length. An elongated American Challenger front part won’t weigh that much.

Have you tried the WOF construction method for instance or other (glassed) sandwich designs in the past? That really cuts down weight without hurting stabilityâ€Ĥ

hooty301

I understand what your saying now about the length issue and Wow over 200mph from the .91 fan set up, thats incredable. With this set up in a boat maybe half that could be had, what do you think?

Ive really been studing the American Challenge design and wonder if this canyard would be the most efficient. Ive been looking at some other designs like the one that holds the record for fastest boat in the world the Spirit of Australia. The American Challenge looks like it would stay on the water and not have a problem with flippng due to the wing surface in the rear of the boat. I would like to talk with the people who designed this boat and get some deminsions and find out about where the balance point should be,that is if it was ever even built. It was supposed to get the record in 2003. Has there been anything else about this boat?

Is the pic of the jet one of your planes I would like to see more, thats a nice one.

hooty301

Ive been looking on the web for some layouts of some bottoms of fast canyard hydros and regular hydros. You guys have been finding some good looking designs and posting them. I was wondering if anybody has seen any bottom veiws If so I would be very intrested in seeing them. Also any other boat designs that I could get ideas from would be helpful too.

I-Love-Jets

This is not my Trim Spectre. So here’s one more, this time a nice “tigermeet”. I personally do prefer scale jets.

I-Love-Jets

Hooty, because you most likely are one of the the first individuums using this propulsion for a speed airboat application nobody knows the final speed potential. But from the Dynamax static thrust numbers terminal speed should be very satisfying (de-pitching of the blades remains as option).

What you need is a really large pond.

I-Love-Jets

The bottom layout of the latest Ken Warby design seems to be even more promising, meaning more stable roll axis:

http://www.kenwarby.com/Aussie_Spirit.htm

IMHO this one below, the “older” Quicksilver design of Nigel Macknight, would be easier to build for you (no monocoque-like hull) than the American Challenge or the Aussie Spirit:

http://news.bbc.co.uk/2/hi/uk_news/1568630.stm
http://www.solarnavigator.net/quicks...eed_record.htm

hooty301

I would like to build with the full ducting, seems it would be more arodynamic and help with speed, definatly looks better. In the artical it does say something about trading static thrust for dynamic thrust. It seems if the duct was the right dia. and consistant it would be efficient wouldent it? Maybe not quit as long though.

Craig-RCU

My ducting was built in three sections to make it possible to install in the fuse of the F-86. When I tested the static thrust on a test stand, I found the more ducting footage assembled the less the static thrust. My ducting was designed with 100% fan's swept area from inlet to outlet to maximise static thrust and I still tested significant losses just from air friction against the interior of the ducting. If you design ducting that necks down to 75-80% fsa you will increase exhaust velocity at the expense of static thrust. There was no similiar trade-off for me since my ducting was 100% fsa from nose to tail.

I would think that with a boat, you would want to start with maximum static thrust (ie. no ducting, just a nice inlet lip and maybe an engine fairing like the BVM Viofan picture below). Then, try adding ducting later to see if it increases speed performance.