rjbranchii

I'm hooked on speed now too. I'd like to do fast speed plane. I'd like to get a feeling on some motor recommendations. I'd like to work with about a .90 sized motor. I want the pipe inside the fuse, not outside. I don't mind having a custom header done to get that if needbe or a rear exhaust engine is fine. I've noticed the interesting results with the OS .91 DF on one of the threads, and I have nothing against a motor like that, but I'd like some recommendations for you folks on what "works" to go fast. I'm a competent builder, and have had considerable design experieince and some ideas about what I want to design. But I need to work out the power system from the get go. I've noticed some good motor experience on the group from Bob at Jett Engineering, and I don't mind him or anyone else recommending his stuff if its the way to go. But I'd like some options. So what say ye all?

bob

ChuckL

My Feedback: (2)

Anything with the name Jett will be hard to beat. I have several they are by far the best engines out there as far as I'm concerned, trouble free and they scream. Call Dub he will set you up with an engine that will meet your needs. Besides all that they are built in the USA which counts for a lot in my book, parts are available anytime or service if needed and Dub actually answers the phone.

ChuckL

My Feedback: (2)

If all that isn't enough we have Bob here that will answer your questions with lightening speed, with a wealth of info you couldn't find anywhere else.

f3d

Hallo speedfreaks,

I use the BVM 91 in this class. It is the best speedengine .

Here some pictures with the speed modell for this engine.
The Jett company have nothing in this class.

Best regards Michael

Rudeboy

Unfortunately, as I understand it, the BVM engines are no longer made...?

Perhaps if you shove enough money in Henry Nelsons way he will whip a few up for you...

bob27s

My Feedback: (19)

That is true.... Jett stopped making the Fan 95 engine about 6 years ago when the fan market pretty much vaporized in a cloud of turbine exhaust..... so there is nothing in the line up with that sort of porting and a Rear Rotor design. Same reason Violett/Nelson are not making the engines any longer.

Your aircraft will somewhat dictate the prop sizes that can be used how an engine has to be set up.

The Jett 90LX FIRE is the closest Jett has for your suggested application. Front Intake Rear Exhaust. Combined with a MACs 15cc marine nitro pipe, it can be set up to turn somewhere around 16,500+ on a 10x10 prop. And it will unload well beyond that in the air. That is about the best that can practically be done with a carburated, front intake engine of this size, while still keeping good throttle response and making it fairly user friendly. If it throttle is not really required, it might be capable of a little bit more.

(photo here shows a quiet pipe)

Bob

HighPlains

My Feedback: (1)

I'm not sure that the BVM .91 or any other 15cc engine is really delivering the bang for the buck. If you examine the results for the German "250 MPH" speed event, an airplane with a 6.6 cc engine did 208 mph. The BMV powered model was 14 mph faster or 6.7% greater speed with 2 1/4 times as much displacement. To increase the 6.6cc airplanes speed an additional 6.7% would require a power increase of 22% over the current level.

I realize that the .91 has far greater weight, and the model would also have greater wing area and greater frontal area, but does the .91 fight diminishing returns or is the FAI pylon ship refined to a higher degree?

Razor-RCU

My Feedback: (50)

rjbranchii - What type of speed are you lookin' for? Sport speed plug n play like Jett-50/60/90 or insane, rocket science "I make my own props." speed?

The German speed cup and it's entrants are truly the cream of the speed crop! is that what you are looking for?

James

rjbranchii

Bob27s: Is there a disadvantage to rear intake motors vs front intake? Also, why is it that most props being made seem to max out in pitch in the low teens?

Highplanes: In my experience it is rare that more power does not equate for more speed when things are engineered right. As you pointed out when try to equate percentage power change to percentage speed change, you will not get coresponding % increases. But as much as I enjoyed the MG-B, a Vet with 3 to 4 times the power while it will not go 3 to 4 times the speed, is still a heck of alot faster. I'm interested in the design challenge of going faster. I think the pylon racers are very highly developed. It takes time to get those kinds of results. One advantage of having a throttle and landing under power is that you can fly a heavier wing loading and still land in one piece cause you are not having to glide that much. Higher wing loading enables less drag and higher speeds. Like anything, there are tons of compromises one way vs another. Still its an interesting challenge and the design process is interests me alot. I have alot of experience in computer design in fluid dynamics fields and would like to see how i can apply it to a speed plane.

Razor-RCU: I'd prefer not to have to build my own props, but I have enough laminating experience to be able to do it if its needed. I'd like to pursue off shelf props or custom work by some folks experienced in the field before I would resort to building my own. Still, we do it locally in CL stunt in our area so there is custom carbon fiber prop expertise available if that is the route I found required.

Please someone let me know if the prefered method of answering posts here is a bunch in one, or if mutliple posts are prefered.

bob

bob27s

My Feedback: (19)

The rear rotor design offers performance advantage in exchange for some complexity.

With a front intake, your carb size, and flow passage is limited to how big you can cut the intake port and bore through the crankshaft. Cutting away too much metal results in a strength issue and a broken crankshaft. So you have to balance performance with durability.

The rear rotor disk inlet design is not as restricted. There is direct passage of the fuel/air mix from the carb directly into the the lower crankcase - thus better flow and better resulting massflow. The porting/timing of the disk has fewer structural limitations - it can accomodate some huge intake time again resulting in big massflow. Massflow means more fuel/air. More fuel, means more power (when used properly). Combined with a good tuned pipe, you can make a real nice system. Especially if you are running at a more-or-less consistant rpm (wide open).

Down size to the rear rotor is it is another moving part. Has its own bearing (typically) and a mechanical connection to the crankshaft and crank pin. When producing power like that, its one more thing that can break or go wrong.

rmenke

My Feedback: (13)

You will do well to follow Bob's recommendations with engine choice and direction. First off, you must commit to speed and have the bucks and willing to spend them. The BVM is with out doubt the best 90's size engine out there. They are still available on e-bay, were around $800.00 new, they should be good. The OS 91VRDF is a decent starter engine and will spin a 10 X 10 around 22K, still available from Tower at around $300.00. I have two of them from e-bay in the cabinet waiting for a winter build of something in the crazy area. Mine need to go fast and turn left well as local club pylon is my focus. The ability to Build and design well is a big plus in the game, but do not loose direction overall. You must also fly well. You can always find some old Q-500s around and stuff a 46 in them for practice. The more radical your design approach is, the better you flying skills must be. Remember, you must be able to repeat your run which requires landing in one piece. If you fly off a grass field, you can get away with no landing gear and pickup a free 10-20 mph or more. After reaching 150mph, 10-15 mph starts to come hard. If you fly off a paved field, its hard to keep them in one piece withoug gear. My current "crazy" will have a front retract cowled in with light wire skids at the wing tips and tail. I give it about 50/50 as to working out well. Its hard to retract without a lot of unwanted weight. If you get over 5# and want it to turn well, there are more problems to deal with. The "Jett engilneering" engines are he best bang for your buck, I have them from 35 to 90, all quite good. My lifetime favorite is (believe it or not) the BSE 35. Little sucker is now in its 4th year of raceing and getting stronger all the time. Last club race, took 3 of 3. You are restricted to smaller airframes here and if flying skills are limited, go to the Jett 60LX which is in a 40 frame. It will pull very well and is cost effective. If money is no consideration, go directly to the true jet aircraft. $10,000 will get you a 50# thrust jet, engilne, neat airframe and speeds well past the 200 mph limit imposed by AMA, just for the pushing up on the left side. May be cheaper in the long run. Regardless of your choice, ENJOY. Just my 2cents worth after over 40 years of good times with these things.

rjbranchii

rmenke

Amen to that. If you can't fly it and do it repeadedly you wind up with an interesting excercise but little else. Hmmm, well, you can wind up with something the rest of the world tries to make fly for the next 60 plus years, like the Grandville's famous GB speedster.

bob