f3d

Hallo speed freaks,

what is the best speed engine if there is no limit in size or price.

The engine is for the German speed cup.
Is it the BVM ( Nelson ) 91 engine or the 6,5 Nelson FI or any Jett type.
Which engine give the best chance to break the 250 mph wall?

Best regards M. Jakob

der_steuermann

My Feedback: (2)

Hi f3d,

good question, hehe... [8D]

Evil Homer

This should bring out some interesting replys - MB

js3

My Feedback: (3)

Yup, MB. But be prepared for the brain damage.

bob27s

My Feedback: (19)

There are not likely any 'production' engines that will achieve this task.

Getting to 250 (consistantly) is going to take a total integration of engine, airframe and prop. It will likely require a larger airframe and engine than what was displayed on the speed event thread.

That FAI/F3D/MB power ship that won this year is about as close as that gets at the moment for that size range.

If they come up with a 'formula' on displacement/airframe size...that may make a different game of it.

I know some of the guys flying the giant scale pylon racers have gotten the aircraft well over 200 mph. Much bigger birds, but with much more capability for HP and huge props.

Variable pitch props may be something that would help get there.......but at 30K rpm, it would be hard to build one that holds together Other than that, building a slippery airframe that will take the largest advantage from an entry dive (and basically ignore the prop/engine) may be the best approach.

Composite structures and very unique designs will certainly play into this speed game. It will be interesting to see what people come up with

Flyboy Dave

My Feedback: (21)

These bad boys....Giant Scale Unlimiteds....go 220+ ....getting one to
go 250 would be a simple matter of throwing a few thousand more $$$
at it, and pumping up the fuel....way beyond the Twilight Zone. [sm=surprised.gif]

FBD.

MOJO65-9or10

???? "Much bigger birds, but with much more capability for HP and huge props" ????

Bob27 - What does the size of the plane have to do with it. I am sure you know. Although I thought that smaller engines usually put out more power by weight than larger engines so I was thinking that a the 40 size planes would be kings of speed. I guess that they might be the kings of speed in terms of "scale speed".

Can you explain why a larger plane is more suited for speed!

Thanks Dave

Flyboy Dave

My Feedback: (21)

....the same reason a full size P-51 racer goes 550 MPH....

....brute horsepower....

Rudeboy

Fluid dyanmics... reynolds number... bigger=faster
Look at full size planes... even the most docile trainer is way faster than your average sport model plane.

As for more horsepower... a full size F4U Corsair has about 2000 hp I think... never seen a quarter scale with 500 hp...

bob27s

My Feedback: (19)

Exactly.

The problem with recipricating engines, is displacement = hp = bigger/heavier.

Plus the aerodyamics as Rudeboy mentioned. We are dealing with small models and relatively 'large' air molecules passing over the wings

Having said that, for the most part, the FAI engine/airframe combination will be hard to beat for speed. Small prop (low frontal area). Keep the overall wetted area lower. An electric version that does not have cooling/drag losses associated with an engine... and is not rpm limited by a mass of moving parts... is probably going to turn out to be the fastest combination.

Power to weight has mainly to do with verticle performance, take-off roll, acceleration, low speed handling, high alpha (stall, G load) and manuverability (wing loading to a large extent). Things associtated with mass. Velocity is not (directly) a function of mass.

Brute power is the key to speed. The lowest possible drag and frontal area on the aircraft is the keyhole. Consider that even the older turbine figters like the F-86 only had about .5 to 1 power. But they were far more powerful and faster than the piston planes (although piston/prop are speed limited by other factors). They also had take off speeds that exceed the top end of what we typically fly at

We commonly build models with 1:1 and even up to 3:1 thrust-weight ratios. Does not always make them fast.

btw... from what I understand, the 1/4 scale engines don't produce 500 hp, but I think some of them are sneaking up in the 35 hp range.

I have no doubt someone could build a big bird as shown in the photos above that would hit 250 in a straight line. As noted, would take the $$$ for airframe, prop and engine development.

MOJO65-9or10

I figured you all would have some good answers and as usual I wasn't let down.

Thanks Dave

der_steuermann

My Feedback: (2)

OK, brute power is needed [sm=punching.gif]

But let's come back to the original question still waiting to be answered.

Mr. f3d needs your judgment!






So, does one of the actual ultra mega hot .40 pylon FAI engines break the 250 barrier easier than the hottest .90 sized ducted fan engine?

Needing the smallest possible frontal area we assume that both engine sizes will be used in combination with a 'zero drag' pure speed airframe design known from the German speed scene (accordingly no F3D pylon airframe).




Maybe we should go deeper into some engine and prop data to be able to objectively compare both engine setups.

RPM: Which maximum rpm does each engine reliably run unloaded?

PROP EFFICIENCY: Keeping that particular max rpm in mind, which is the maximum prop diameter allowing still subsonic prop tip speeds for each engine size?

TARGET SPEED = 250 mph: Which 'realistic' prop pitch should be picked to reach (or even exceed ???) the 250 mph goal for each engine size?

Gentleman, please start your (engines) pocket calculators

HighPlains

My Feedback: (1)

You should shoot for an easier target than 250 mph, say 400 kph [8D]






Yeah, I know.

Bob.R

My Feedback: (2)

der_steuermann - we are waiting for you to start volume production of an 8.75x12 carbon fiber prop so we can slap one on a BVM ducted fan motor.

Cyclic Hardover

My Feedback: (3)

Dave I have a picture of that plane from a a couple yeasr ago. Can you get me a name and number for that thing. CH

Cyclic Hardover

My Feedback: (3)

You know, I think I could take a perfectly good 60-90 sixe Extra and bash it into something very close to this. Put a Midget type cowl up front. What do you think.

Rudeboy

IMHO a properly designed, developped, tested, refined... whatever, speed plane on a nice hot DF engine will be faster than something with an MB Profi or similar. It has to be! But it is going to take time and perseverance.

I think Bobber is on the right track with his 8.75x12... maybe one could bring the diameter down even more if the plane was good enough. The extremely small diameter will also aid in the "terminal power dive" department.

250 mph is within reach on something like a BVM 91/96 engine or even a cheapo VRDF 91... again, it has to be!
Somebody should contact Nelson to ask what these engines can take. I take it they are running in the 22-23K rpm range on a Ramtec, and a little faster on the Viofan... so unloaded rpm must be in the 25-26K rpm range, right? So how much more can they take on a properly set up pipe system (short) and a nice little prop... diving in from 500 feet up?

Steuermann: if you have a 8.75x12 prop mould at hand, i'll take some of those props if you ever decide on a new batch... I could make my own, but buying something close and sanding is faster and cheaper...

Just my 0.02

der_steuermann

My Feedback: (2)

Hi Rude,

it would be a pleasure to support your new properly designed, developped, tested, refined speed project with a matching CF speed prop.

However, it is fundamental to know the specs of your speed plane as well as the desired engine/pipe combo in order to determine the correct prop size. Precise adjustment...

Living in Belgium you maybe intend to be present at the next Speed-Cup in Germany? [sm=tongue.gif]

BTW, already tested your Stinger .40 plane?

Rudeboy

MY new speed project? There is no project at this time. Just a lot of ideas in my head.
But I will make something like this one day. And I intend to do it the hard way: trial and error. I think I will learn more that way, and it will be a good test of my insights into this matter.

I looked up Achmer... that's only about 350 kilometers from where I live... so attending (and participating in) the next "Home of Speed" is definitely an option.

The Stinger 40 isn't finished yet. Can't wait to try it though... it really thik it's going to be fun. And with the BSE Jett it will really look the part too!

Razor-RCU

My Feedback: (50)

Finally someone mentions it- using a BVM-91/96 with a prop. I pulled my OS-77VRDF out of my A-4 and keep thinking I should bash it onto my Cermark F-20 or a 40 sized mustang or something.... The difference is, I have NO idea what I am doing-

Hmmmm...

Gold

You know why Giant scale extreme speed planes have potentially higher top speed than QM-size one?
Because frontal area (and hence drag) is proportional to the square of the dimension of the plane and volume (hence approximately mass) is proportional to the qube of the dimension. Simply put, smaller planes have relatively more drag than larger planes at high speeds. Add to that the typical case that larger planes have less material than smaller ones (less density) - they are relatively lighter fro their size.
Regarding thrust/weight of 1/2 of some fast jets: When a jet has this static ratio, its high speed ratio will be better because compressibility becomes dominant beyond 0.3M. Any prop plane has narrow range of prop efficiency, ultimately limited by tip sonic speed. Variable pitch (or very high pitch) pospones the effect and allows high efficiency at high speeds, but not jet plane speeds. So no 3D plane with 3D prop will have a good T/W at 100MPH. Prop efficiency and thrust drops very much at high speeds.

bretthan

What about building a gear drive for your motor. couldn't you get the rpm/torque that you need from that.

P.S. I have no experience in speed planes, so please don't bash me all to hell if this is the stupidest idea you've ever heard

bob27s

My Feedback: (19)

Gear drive........its possible. Full scale aircraft of any substantial HP are geared engines. Utlized the HP created at reasonable engine RPM, while keeping the prop and tip speed in an efficient range.

Cyclic Hardover

My Feedback: (3)

Well some helicopters have a gear reduction. Take a Huey and the direction of the power. You got the engine, a drive shaft goes "forward into the transmission which also houses the rotor system. On the bottom of the transmission is another drive shaft going aft to the tailrotor, passing through a few gear boxes along the way.

You have rotor rpm which i forgot is in the 300's somewhere and engine rpm which is up around 6800rpm

I am only guessing if the rc helicopters have similar system

r-c-guy

My Feedback: (37)

Hi,
I have been doing the same thing. i fly a highly modified Dago Red .40 with a stiletto appearance - no airscoop, retracts, etc and a streamline cowl. I have used the .65 vrdf, 77 vrdf and 91 vrdf (all os). The 77 gives the best all around performance and i can break 200 mph on radar. I have used 11x8 or 9 apc props and trimmed them.

randy