# Rotax 670/gearbox/prop..magic # ?

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##### Well-Known Member
Log Member
I'm hoping someone smarter than myself can help me find the magic # for engine,gearbox and prop combination.
The goal is to acheive maximum static thrust while still being able to maintain a decent cruise speed.
Once my Ragwing Stork is completed it will be my leisurely daily flyer but I would like to be able to repitch the prop and pull it onto the line for STOL contests also....
I've been using a prop calculator but I'm getting some conflicting #'s when plugging in the data.
I understand a small prop gives speed and a large prop gives thrust.....but with a in flight adjustable prop you should be able to find a good compromise between the two....it's not adding up that way though.

Here's the specifics .......
Ragwing Stork : 500 lb empty weight..I weigh 220 lbs....fuel 5 gallons when competing.
Cruise speed should be around 75 mph.

Rotax 670 engine : 100-103hp....Rick claims an honest 100 hp and Mike says another 3 hp with his custom pipe.
6500 rpm max.

E gearbox : ratios 2.68 - 3 - 3.47 - 4...maximum diameter allowed is 72".

The problem that I'm having is I can get 500 - 600 lb static thrust but the cruise speed suffers.
Or I can get a decent cruise speed but static thrust suffers.
I've played with the numbers trying to find a happy median but things start getting weird.....
With an in flight adjustable prop...increasing the pitch does nothing for the cruise but raises the hp required...what's up with those numbers?
I understand you need more hp for pitch but I don't understand why the cruise speed doesn't increase with the pitch change.
Is the calculator not taking into account for the prop unloading with airspeed ?
I'm also finding that the 2.68 gearbox ratio is giving me better numbers than the rest.

Kevin

#### TFF

##### Well-Known Member
Let the static suffer some. Max static pretty much means once the plane starts to move it starts loosing efficiency. You want it to move into efficiency range. I’m just guessing but ground static on something like that should be 80% of max.

##### Well-Known Member
Log Member
There lies the rub.......the static is what I'm after.....but only sometimes.
And you're only talking 10 - 15 feet before breaking ground and pulling throttle back.
I want to be able to pull up to the line at a STOL competition and show them that a $15k plane can compete with their$250k plane.
But I still want a daily flyer with decent cruise.
I've been considering running 2 different props....if I do that....I can get upwards of 700 - 800 lbs static thrust from one prop then change out and fly home with other prop.
It's a thought.....

The #'s on the in flight adjustable prop have me confused though.....increase pitch and cruise stays the same but hp requirement goes up......don't understand that.

Yes,I know I want the best of both worlds.....not impossible.....just need to figure out how to acheive it.

Kevij

#### TFF

##### Well-Known Member
The competition planes with fixed pitch have two props just like people who fly fast planes in races. Regular flying prop and the other one. The fastest Tailwind has a speed prop with 82” of pitch, but his everyday travel prop is a 70”.

That will come down to what you will put up with. Once you’re airframe is outrunning the prop, the prop is a big brake. Like a boat, the airframe wants to plane off at a certain speed. If it never gets there, you are hanging on the prop to stay in the air. The competition flight will be just that. Off the ground and drag around the pattern to the landing. No one cares about how good it flys during that.
Chasing winning is a whole different game than just being in the game.

I never saw it fly but there was a damaged 3/4 Storch at my airport. Rotax 914 and electric prop. There is some video of it on YouTube before it got damaged.

#### poormansairforce

##### Well-Known Member
Just carry an extra blade and go 3 blade low pitch for STOL.

#### n3puppy

##### Well-Known Member
I have a lot of questions about the calculator you (and others) are using- the static thrust numbers are questionable. But I'll start another thread on that

In the mean time - looks like you are using 6500 rpm for you speed calculations
(6500/2.68= 2425 rpm)
But Rotax Rick specs MAX cruise speed as 5500 rpm. And static thrust number 6250-6350

Your cruise speed calcs need to be different than your full power STOL takeoff thrust calcs at 6500 rpm

##### Well-Known Member
Log Member
Just carry an extra blade and go 3 blade low pitch for STOL
The calculations are done with a 3 blade prop......
And it's not as simple as just adjusting pitch........gearbox ratio,engine hp,prop diameter and temp. all come into play.

Kevin

##### Well-Known Member
Log Member
In the mean time - looks like you are using 6500 rpm for you speed calculations
(6500/2.68= 2425 rpm)
But Rotax Rick specs MAX cruise speed as 5500 rpm. And static thrust number 6250-6350
You are correct......
But here's why I'm using 6,500 as my base rpm.
I talked with Rick about the engine he is going to build for me and I was completely honest with him.
I explained to him what I would be doing with the engine and that I wanted an honest 100 hp from it.
He said he would advance the timing a bit,do some head work and a pipe from Mike Hair....honest 100 hp,no problem.
I talked with Mike about his pipe and he claims 103 hp...but I digress.
I also explained to Rick that I would be tearing the guts out of the engine for 20 - 30 seconds at a time if I lined it up to compete.....Rick says it will handle that with no problem.

The 670 is actually capable of a lot more rpm and hp than we use in aircraft.
In the snowmobiles I think they run upwards of 9,000 rpm and around 115 - 120 hp.
They are de-rated in aircraft use for reliability.
So........I don't think using 6,500 rpm as my baseline is too over the top.

There are some guys actually building the 670 into a 720 or 740 using Polaris pistons and they are seeing 130 -140 hp out of the engine.

Kevin

##### Well-Known Member
Log Member
Your cruise speed calcs need to be different than your full power STOL takeoff thrust calcs at 6500 rpm
Yes,I realise this......
That's why I'm trying to find a happy median with the whole combination of engine,gearbox and prop.....and that's also why I'm going with an adjustable pitch prop.
The problem is that I'm probably going to wind up with 2 different diameter props to acheive good STOL & decent cruise figures.

Kevin

##### Well-Known Member
Log Member
I have a lot of questions about the calculator you (and others) are using- the static thrust numbers are questionable. But I'll start another thread on that
I've never messed around with a prop calculator before so I don't know that I don't know.......
I need to pick a gearbox ratio and prop fairly soon so I've been running numbers like crazy trying to find a combination that suits me.

What do you see wrong with the #'s ?

Kevin

#### TFF

##### Well-Known Member
If you are going to complete, you want the engine load max at rotation or load just below max. Once leveled out, it’s going to overspeed. That is what climb props do, yours will just be extreme. If the engine overspeeds in climb, you will not be using the 100 hp; the extra horsepower is overpowering the prop instead of making more thrust. Prop calculators are really top speed. They are not set for transient speeds.

I can see something as extreme as top speed set to 15 miles an hour above stall to be about where climb pitch would end up.

#### n3puppy

##### Well-Known Member
What do you see wrong with the #'s ?
Pick a prop - Diam, Blade#, RPM. CF, temp
Keep those 5 numbers the same
Then try 4 or 5 differnt pitch numbers for that same prop
Static thrust remains the same -1 inch or 33 makes no difference to static thrust
Doesn't seem right

Started a new thread with more details

#### poormansairforce

##### Well-Known Member
The calculations are done with a 3 blade prop......
And it's not as simple as just adjusting pitch........gearbox ratio,engine hp,prop diameter and temp. all come into play.

Kevin
I understand that. Someone suggested an extra prop but you can't carry a 3 blade very well. Fly 2 blades for cruise and carry an extra 3 blade hub and blade for STOL. Or just carry 3 longer blades to swap in for STOL. BTW, take that prop calculator with a grain of salt. Hope you figure it out.

##### Well-Known Member
Log Member
Someone suggested an extra prop but you can't carry a 3 blade very well.
Actually,I can.......very easily.
Google Ivoprop and see the individual blades that bolt to the hub.

BTW, take that prop calculator with a grain of salt.
Yes sir......I'm doing exactly that.
There's a few #'s that have come up that make no sense to me.....hoping someone can help clarify things.

Kevin

##### Well-Known Member
Log Member
Pick a prop - Diam, Blade#, RPM. CF, temp
Keep those 5 numbers the same
Then try 4 or 5 differnt pitch numbers for that same prop
Static thrust remains the same -1 inch or 33 makes no difference to static thrust
Doesn't seem right
That's part of what I'm talking about...
Different pitch numbers and the static thrust & cruise stays the same with the hp required going up or down.
Doesn't make sense to me.....

Kevin

#### n3puppy

##### Well-Known Member
That's part of what I'm talking about...
Different pitch numbers and the static thrust & cruise stays the same with the hp required going up or down.
Doesn't make sense to me.....

Kevin
Makes no sense to me either. - That why I question if any of its numbers- hp, speed etc are any good.

The chart below is from the Ultra Prop website.
Kinda confusing because it shows 2 -3-4 blade props all lumped together.
BUT you can clearly see when you change pitch - the thrust changes too
Its another data point showing the calculator numbers are suspect

If it were me - I'd give up running in circles trying to make a decision based on that calculator. The numbers can't be trusted.

Ultra prop now has 66inch props that can handle over 100hp.
Would be interesting to call them and see what they would recommend based on your parameters 100 hp, 2400 rpm, max static thrust with acceptable cruise of 75mph
If you are lucky -they have already created a chart for the new props too

Last edited:

##### Well-Known Member
Log Member
Would be interesting to call them and see what they would recommend based on your parameters 100 hp, 2400 rpm, max static thrust with acceptable cruise of 75mph
I had the same idea a few years ago when I was shopping for a prop to put on a friends plane.....called a couple of prop manufactures to see what kind of numbers that they were seeing with their props and asked what they recommended for his aircraft and particular set up.
The short answer is that the reply I got from 3 different manufacturers was "f***ed if I know" .
But they wanted to take his money and were more than helpful listing half a dozen ways that he could get the money to them.
That really didn't instill any confidence in either of us pertaining to their companies......not knowing what their own product is capable of doing.
He ended up buying a used prop from a guy that had a very similar set up and he has been happy with it for the most part.
However,I've always wondered if there was better performance to be had .....if only the companies had known the product that they were producing and selling.

Kevin

#### poormansairforce

##### Well-Known Member
Actually,I can.......very easily.
Google Ivoprop and see the individual blades that bolt to the hub.
Ok, after re reading that I had to laugh at myself. Especially after suggesting the same thing right after that. Like I said in the other thread the calculator seems to work inside a certain window.

#### n3puppy

##### Well-Known Member
I had the same idea a few years ago when I was shopping for a prop to put on a friends plane.....called a couple of prop manufactures to see what kind of numbers that they were seeing with their props and asked what they recommended for his aircraft and particular set up.
The short answer is that the reply I got from 3 different manufacturers was "f***ed if I know" .
But they wanted to take his money and were more than helpful listing half a dozen ways that he could get the money to them.
That really didn't instill any confidence in either of us pertaining to their companies......not knowing what their own product is capable of doing.
He ended up buying a used prop from a guy that had a very similar set up and he has been happy with it for the most part.
However,I've always wondered if there was better performance to be had .....if only the companies had known the product that they were producing and selling.

Kevin
Contacted Competition Aircraft today.
www.competitionaircraft.com
Their UltraProp II will handle 100HP

Asked them for prop charts like I showed before and a prop recommendation based on your parameters from above
Rotax 670
100hp @6500 rpm
2.68 gearbox
2425 prop rpm
Desired prop characteristics
Max static thrust for STOL competition
75mph cruise to get to competition site

Here is their answer - (BTW they use pitch degrees, not inches)

"I do not have charts you can use for the U/P II, but I have lots of dyno data. For 100 HP at 2450 Prop RPM, our 4-blade at 66 inches and with 12 degrees looks good. Static thrust is over 450 lb. That is the limit of our dyno measurement equipment. You can fine tune the prop with pitch block changes.

Best regards,
Bob Davis"