littlepiston

My Feedback: (12)

i dont think the toki engine will spin the kess fan that fast you would be luck to get 13.000 rpms out iof it heheheh'


you guys use formulas i use try it and see hwta happens
if all the munber and charts were correct there would not ba any reason to have all these engine manufactors

watt_the!

ok.. here goes.


if you want to know if your engine will work for a fan, do this...


start with your engine's claimed hp at revs.

look to see where it sits on the power (load) chart for the fan you want to use. if it is below, it still may be ok, but it'll only do the revs that the line crosses at.

if it is above, then you need to make sure that it is above with some decent margin, else it may actually be above at the end point, but somewhere before that, it may actually be below the curve.

it is difficult to guess engine power curves, but you can use 2 stroke theory somewhat. also, the more peaky the engine power, the steeper the engine's power curve.

i'd say that the picco is line ball. i'd be testing that baby before going to all the trouble of installing it.

watt_the!

piston..

we tested it on the bench two days ago,

you go ahead with your project mate... i'm here trying to help.

sigh.

we derive our formula from tests.. and btw, the theory is just about spot on. so if it looks ok from theory, we dont test. if it looks lineball, we test.

if it doesnt look ok from theory, we dont even try it.

ok?

Dave03B

What what will i be looking for when i test it? The thrust levels?

watt_the!

purely revs.

i'd say you'll get around 22k revs.

use the same slope from the chart as the os, then draw a line from your picco's 26k and 2.6 hp, and see where it intersects the power curve.

littlepiston

My Feedback: (12)

ok what the hell what you are saying is the a tine .18 size engine will out turn and spin the blades off the rk720 if so YYYYY do the 46 mod to the a7 corsair just order a rk720 fan unit ??
and instal the tine engine on the power hungry fan unit hehheheh i would like to see that

Dave03B

Hang on, i forgot. My 740 has already been run with a picco 45. By previous owner who built the fan unit. I ain't going to worry with charts and stuff. I'll let you know how it goes, once i've got the spares from Kress. J Kress, can you email me an estimate of how long the parts will be?

Cheers
Dave

watt_the!

i give up.

you all know that things are designed before they are built dont you?

the building you go into are designed from material properties before they are constructed?... they dont build a building , then load it with ppl to see if it falls down...

you guys go ahead.

do what you have to.

all i can say is that i've been a practicins gengineer for 15 years in defence, aero and construction in design and other fields, and this aint BS. i was thinking that i could help out fellow modellers in terms of giving some time and money saving advice built on test and principle.

from what i can see is that ppl arent convinced for some reason, do i need to post the derivations from first principles?... i.e from kinetic theiry, starting at newton's laws?... you do agree with newton don't you'all?

or do you want to drop an apple from a tree and see how long it takes to hit the ground to test his universal laws?...

we wouldnt even be able to design the aircraft you are building without designing them. you dont just put a wing on a stick then test it to see if it works... you calc CoG, select airfoil based on properties, etc... then build, then test to verify.

i'm out of this topic.

yours in frsutration,

tim.[]

watt_the!

im saying that the toki 0.18 with 30% nitro and properly tuned pipe stays above the rk720 load up to 29k.

so, yes.



BTW John, the fan coefficients dont coincide with this graph.. the graph shows extrapolated thrust at 29k as over 5.5 lb. the coefficient shows 4.5 lb.

Dave03B

All i know is my Rk-740 powered by a picco 45 Df engine will fly a ducted fan airframe. It's already been done by the bloke who built the fan unit. Thats why i bought a picco 45, because this particular fan unit had been built around the picco engine.
I'll be honest, I'm not really all up on these power curves info etc..., and after knowing that my fan will work with my engine i'm happy.

Thanks to all for your advice.

littlepiston

My Feedback: (12)

um we have a feiw ppl like you at work them guy are prety smart BUT NO COMMON SENCE AT ALL HEHEHEH NEWTON WHO IT HE I STILL THINK HE IS WRONG OR SHE? I TRYED TO TEACH ONE OF THEM HOW TO RIDE MY F WHEELER AND HE BROKE THE GEAR SHIFTER TRYING TO STATED HEHEHEHEH THE KICH STARTED IS ON THE RIGHT SIDE HEHEH

watt_the!

now you're just being insulting.

no common sense?

please show me where this is evident?

this stereotype of a nerdy engineering geek drives me nuts. not all are geeks, in fact most modellers are geeks- engineers or not. What strikes me is that modellers are quite arrogant and although given some sound advice, they'd rather find out the hard way, than accept someone's input.

I for one, go with kress's charts, as i accept that Bob knows his stuff and i can bank on the info supplied. This is why i dont have to go through the same analysis and test regime he has obviously done, plus his 50 years engineering experience. I humbly accept that this guy is an expert, and I will use his talent to my advantage.

it's as simple as that. And regarding the common snese comment, it is amazing that it is actually you, yourself, who is not showing common sense. you go build that ladder you've been planning- with your test regime of climbing to the top to see if it holds your weight. lol.

thanks for the insults, i'm really out now....

littlepiston

My Feedback: (12)

[quote]ORIGINAL: what_the?!

ILL TAKE THAT you not that bad after all

littlepiston

My Feedback: (12)

i have commone cence i would not do the latter trick but one that other hand thats why well have you guys to build and try these one a computer cad drawings and so forth

littlepiston

My Feedback: (12)

were we talking about thew rk709 or the rk720??? i was talking aboutthe kr720??

JKress

My Feedback: (6)

The calculators are “reliably conservative”.
There a loss factor of %25 in the calculator.
Multiply 5.5 * .75 = 4.1(lb)
Very close to the calculator’s result of 4.5(lb)

watt_the!

JKress

My Feedback: (6)

http://kress-jets.com/Calc-720G.htm

Here you can enter RPM or BHP

JKress

My Feedback: (6)

Don't know if my first try to post this worked, so I'm trying it again...

Bob Kress writes:

The recently published RK-720G BHP and thrust information was for the fan part of the solution. That is, it defined the BHP required to spin the rotor at a given RPM and the installed thrust to be expected from a model having a long, fighter-type duct system with typical inlet losses.

The fan thrust presented in the graph included with the RK-720 instructions reflects an optimum fan where the ducting is short and the inlet an efficient bellmouth. The BHP numbers are for the K&B 3.5cc engine circa 1980. The BHP is engine power output, not fan power required as mentioned earlier.

PigHeaded

My Feedback: (8)

Hello,

What I would like is for someone to explain these charts in detail. I would like to know why 2.2 bhp at 22k RPM is a magical number for the RK-740? I would like to understand these charts better than I do now. I think that if more people understood them there would be fewer people asking "can this engine work". If they can plot the curve they will know. I assume the problem with the buggy engines is that they have a steep power curve and are behind the fan power curve and therefore can not get up to speed due to lack of torque at low RPMs. I would like to know where you can find performance charts for different engines. The 46VX for one. It states 2.5bhp at 23K, but is rated to 28k. What is the bhp at 25k? or 15k? The Picco is another engine I would like to know more about. It says 2.3 at 26.5k. Based on the Kress chart is would need @3 bhp at 26k to stay on the power curve. But it wont do that, so at what point does it cross the fan power curve? 20k? 21K? From what I understand the Picco will never reach 26.5k, but what will it reach? How can you determine this with out physically testing the engine on the fan?

Sorry lots of questions, but have a vested interest in this information.

Thank you,

Jerry

watt_the!

Hi Jerry, I dont mind answering this, as long as ppl dont see it as a flaming opportunity and a chance to nit pick. It can be very complicated, but it can also be kept relatively simple.
The thing is, that engine manufacturers dont publish their power curves. it gives fodder for competition. However, if you can get them dyno'd, then all is there to see.
Here is a dyno image from a test done on a sirio 15 engine.


Now, the Sirio is advertised as
Practical rpm range: up to 41,000
Max. power output: 1.57 bhp

From this marketting info, you can see how misleading it can be. Where does it do 1.57hp?, at 41k?... 20k?... who knows... they dont supply the dyno chart.

So how do you select an engine to match a fan then?..

Well, you need to know both the engine's power at revs, and the fan's load at revs. Thanks to Kress, his fans are known. WemoTec also used to have similar charts at their site. Ramtec etc dont have them.

Once you know the fan, you need to make sure that your engine NEVER has less power than the fan load, throughout the range- or at least up to your intended operational point. This is because, where your engine power crosses the fan load curve, is where your revs stop.

For instance, if i bought a fan called SUPERFAN 101, and it's load in hp at 20k was 1 hp, at 24k was 1.5 hp, at 30k was 2.3 hp and at 40k was 4 hp, then any engine i select for this fan needs to give me that power at those rev points..at least.

I may go out and buy two engines. THE "RIPSNORTER" AND "BAD****" 0.21 ENGINES. Ripsnorter states 1.8 hp at 28k in it's marketting gaff. BAD**** states 1000 hp at 40k. That **** is one hell of a beast right!?...should be able to rev the SUPERFAN to a million revs right?....

well it turns out that at 20 k the **** gives 1.2 hp, so no probs so far, it'll spin the fan to 20k, and at 24k it gives 1.6 hp, still no probs, then at 30k it gives 2.2 hp. Well guess what?... it cant spin the fan any faster than at 30k revs..even less actually. this is because it doesnt have the power at 30k to push the fan any faster. So the extra 998hp is useless, even though at 40k it more than enough to spin the fan.

On the other hand, the ripsnorter, with its modest 1.8 hp perfectly matches the fan, and can get to it's 28k no problem.
In this case, the **** still turns the fan to 30k or so, resulting on more thrust, but doesnt come close to what it can do. The ripsnorter is right on the design. The determining factor is that the **** costs $5000 and the ripsnorter $50.

And so on.

Now, as a general rule, when unsure, you need to assess the likelihood of your engine being able to produce enough power at all the rev range for the fan you've chosen. That is why it is a good idea to give yourself some room for a safety margin.
Regarding the kress fan, you need to make sure that you have more than

you need to have an idea that your engine can meet all the fan load throughout the range. That means at 18k, you need 0.4 hp, at 20k, 0.6 hp, and 24k 1hp or so.

So if you have an engine that states 1.6 hp at 30k, then chances are that it wont have 1 hp at 24k. For the higher revving engines, you can assume that if the power is at 38000 or so, then at 20k it is going to be pretty low. We recently testing a hyper 21 8 port turbo which states 2.5 hp at 35k revs. Plenty for the rk720 right?

Wrong.

It gives 0.93 hp at 25k. which means it wont go any faster than 25k or so with the rk720. We tached this and repeated it for verification.

What is interesting, is that the hyper 21 is around $150US. Another engine we have states 1.9 hp at 30k. And is priced at under $100US. Guess what?..perfect match. runs to 30k no problems. So in this case the cheaper solution is best.

Another thing, the novarossi is 44000 and 3 hp... wow!!!. $400US...wow!.. does it match the rk720.. we doubt it. It is very likely that this engine is also very peaky and does not deliver the goods at 25k revs. We wont even try it, as it is likely that it will not perform.

Another great example of this has been around the DF community for years and is a bit of folk lore or urban myth. the NovaRossi 105 powerhouse. Claims 7 hp at 28000 rpm.
So this baby will blow the os 91 out of the water right?... well yes, and no.

If you strap the Rossi to the Ramtec, or any other fan designed for the OS 91 power curve, you will find that the Rossi doesnt deliver the 4.8 hp at 24k that the OS 91 does. The fan needs this power at 24k in order to spin. In fact, the Rossi doesnt deliver the 4 hp needed at 22k.

So we end up with the rossi being stuck at under 22k, and all experts claiming that it's 7 hp and 28k is BS. Hardly conclusive to come to this summary. what is happening is that same as what has happened with the hyper 21.
Should you change the fan design so that lower power is absorbed at these revs, then you'll find the rossi spinning to 28k and the os stalling at perhaps around 25k (due to the reduced load).

This is a very interesting engineering problem.

If you're unsure about things, there is a rule to apply.

An increase in revs (i.e. from 22k to 23k ), which is a factor of 23/22 = 1.045, gives a thrust increase of that number squared... ie 1.045 squared = 1.0929 ( so if you had 10lb thrust at 22 k, then at 23 k you'll have 10 x 1.09 = 10.9 lb. Similarly, the power needed to get there is cubed!.. i.e. 1.045 cubed = 1.14 times what was used at 22k. So if you needed 1 hp at 22k, then to get to 23k you need 1.14 x 1 hp = 1.14hp.

This is the thrust/momentum theory. It is for static conditions. It can be used for dynamic conditions, but that's another story.

This stuff can be viewed at our website at www.skunkworx.com.au

Please post further questions if required. I'm only trying to help fellow modellers..not be a know all.

Jackjet

My Feedback: (1)

I for one believe what Mr.Kress says about his fan(s) - after all - he did MUCH of the design work on the full size F-14 Tomcat - please correct me if I am in error........

Jackjet

watt_the!

you are correct.

please remember the grumman 688 vtol, the lunar lander and was vice president of grumman advanced aero projects.

all this theory was taken from works Bob did..to the letter.

he has some very good references and software also.

we have studied this work and it cant be flawed from first principles... it is sound engineering, design from theory, then backed by verification. apparently the thrust values were obtained using fish scales/spring balance.

here's a pic of our setup.

PigHeaded

My Feedback: (8)

Hello what_the?! ,

Thank you for the valuable information on reading these fan charts. But I am still left with several questions.

1. Since there is no way to get engine performance charts, how can we determine correct engine/fan combinations without physical testing?

2. The Picco 45df will turn the RK-740, but at what RPM? It will not get to 26.5k, but what will it get to? How can we predict this?

3. The Kress charts and specs show that the OS46vrdf will turn the fan at 23k. But to do this the engine would need to produce around 2.7bhp to stay on the fan power curve. I do not believe this engine can do that. How is it possible to turn at 23k and be behind the fan power curve?

4. How can we predict what the OS46vxdf will do using the RK-740?


Thank you,

Jerry

watt_the!

1. Since there is no way to get engine performance charts, how can we determine correct engine/fan combinations without physical testing? There are three ways.. all of which arent ideal.
The first is to dyno test your engine.
The second is to select an engine, note the claimed hp at revs, then use an existing dyno chart from a similar engine and draw the same curve (slope) to fit your engine...
The third is to take your engine's max power and revs, and compare that to the fan's power required at those revs. If the fan needs more power, then your engine probably wont work. This works on the principle that the fan's load curve will always be steeper than the engine's power curve.

For instance, the hyper 21 claimed 2.5 hp at 35k. Now, we know that the rk720 needs around 2.8 hp for 35k. So somewhere on the curve it's either gunna be real close, or going to stall.


2. The Picco 45df will turn the RK-740, but at what RPM? It will not get to 26.5k, but what will it get to? How can we predict this?

This is basically the same method as with the rk740.
We use a pretty coarse method where we take a fairly typical power curve and simply apply percentages to it. It has proven to work very well.
So we start with the claimed max power at those revs., we then calculate the revs backwards from that at these points.. 85% of those revs, 75%, 50%, 25%,12.5% finishing at zero.
We then use the following %'s and apply them to the claimed hp, starting from the max power and corresponding to these points, they are 75%,55%,25%,22.5%,12.5% and zero. So you now have your x and y coordinates to draw on top of the fan chart.
something like what you see attached.


The 46VX for one. It states 2.5bhp at 23K, but is rated to 28k. What is the bhp at 25k? or 15k?

We can do this with the 46VX. We know that at 100% it is 2.5bhp at 23k. So we plot those points and the answers for the x axis (revs) are 23 x 0.85= 19.55k, and so on.. (17.25k, 11.5k....) We then do this for the bhp and get 1.875 @ 19.55k, [email protected]. Now, as we know that the max revs for the OS is greater than the fan load at 23k, it is very likely that it will get to 23k. After that point, there will be a drop off of power, so it is going to intersect with the load curve somewhere after 23k.

The Picco is another engine I would like to know more about. It says 2.3 at 26.5k. Based on the Kress chart is would need @3 bhp at 26k to stay on the power curve. But it wont do that, so at what point does it cross the fan power curve? 20k? 21K? From what I understand the Picco will never reach 26.5k, but what will it reach? How can you determine this with out physically testing the engine on the fan?
You'd use the same method. At 85% of revs (i.e. 0.85 x 26.5k =22.5k), you'd likely get 75% of max power, (or 0.75 x 2.3hp = 1.72hp). So it looks like the picco wont get to 22.5k with the rk740 either. So the next point is 50% revs, or 0.5 x 26.5 = 13.25k revs, where it would likely generate 55% power, or 0.55 x 2.3 hp = 1.265 hp. Now the rk740 shows that the power required at 13.25k to turn the fan is well below 1 hp. So you will turn the fan faster than 13.25, but less than 22.5k. You could simply draw a line between these two points and see where it intersects the fan curve. Somewhere around the 20k mark.

3. The Kress charts and specs show that the OS46vrdf will turn the fan at 23k. But to do this the engine would need to produce around 2.7bhp to stay on the fan power curve. I do not believe this engine can do that. How is it possible to turn at 23k and be behind the fan power curve? The OS 46 is only behind the curve up to 21k. The very steep line is the fan load, and the curve drawn that eventually decays after 22k or so revs, is the os 46's power. So the OS and the chart show that it will do only 21k with the rk740. I'm sorry, I was using the kress OS data, if your new OS can do 2.5hp at 23k, then it may be able to turn the rk740 to 22k revs.

4. How can we predict what the OS46vxdf will do using the RK-740?..done.