# The Prop windmilling on RX1/Apex engines

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#### TFF

##### Well-Known Member
In an STOL plane doing bush flying, you are in the dirt. You accept that. Best glide is for someone 5000 ft AGL. Under 500 ft You dump the nose over and in 20 seconds your on the ground. Hopefully you were looking for places to land the whole time you were low. What you need to know is your airplane and how it acts; then you can make the right choices or know the cost of that 150ft beach you want to land on.

If you have altitude, a stopped prop is great. A friend glided a 172 5 miles to the airport. A Pitts glides down 1000 ft a minute. A STOL plane flying on the prop decent, just dorks in; no time to get the speed back even with a stopped prop. You got to know your airplane. Most of the last 15 years I have been in helicopters. Any glide is a luxury in one of those.

#### BJC

##### Well-Known Member
HBA Supporter
A free windmilling prop, although not all ideal, can be the lesser drag option than a fixed pitch prop at a full stop.
“Can be” infers that not all situations are the same. Can you provide more details of a free windmilling prop that has less drag than the same prop stopped?

Thanks,

BJC

#### cohocarl

##### Member
there is a sprag clutch....between the engine and propeller .... so the prop spins freely when not powered....
I haven't had my 2nd cup of coffee yet this a.m... In an engine out situation comparing drag from a sprag equipped-completely free to spin prop and a conventional direct drive turning over a non running engine...would one produce more drag than the other? I would assume the sprag equipped would be less.

#### Pilot-34

##### Well-Known Member
Windmilling turning the engine over is the aeronautical equivalent of a truck coming down the mountain with the Jake break on
With a free wheeling Propeller it’s the air motive Equivalent of kicking the truck into neutral and holding a couple of baseball bats out the windows

#### GTX_Engines

##### Active Member
The only issue ... I’ve come up with so far with the Yamaha conversion is that the prop windmills if you have an engine shutdown

I didn't write this as a critique ..everyone involved in the Yamaha conversion is doing out of love of aviation...volunteering time and expertise...

I'm not an expert on windmilling props...so I post this here for others to chime in and educate me on this issue....I'm not that familiar with rotax engines either... do they windmill on some of their gearboxes also?

To this point it’s not been an issue as nobody yet has had an inflight engine outage...

But as of right now there is no easy means to stop the prop ... other then installing an electric constant speed prop so you can feather the propeller...

Everyone involved has answered all my questions and the Facebook site even compiles all known failures of parts or issues...it’s a good community

Any question I had were answered on the Facebook Yamaha conversion site in minutes...I texted Teal Jenkins with some questions and he responded within an hour...
There may be some misconception about Yamaha conversion engines here.

ALL YG2 and YG3 engines must have a clutch to start the motor, since it won't rev through to a smooth idle when hard coupled to a prop. It's like trying to start a bike while in gear.

Next, there is no need to use a clutch with a YG4, neither RX1 nor Apex. Those engines start and spin up just fine with the prop hard coupled to the engine.

Some guys like the ultra smooth start and use a clutch anyway, however. The last couple of YG4 builds I did for customers were all sans clutch YG4 installations. I like them better since it eliminates one more single point of failure.

I have experienced and routinely practiced engine out landings with gyrocopters using Yamaha as well as other types of engines coupled through sprag clutches that allow windmilling. It is no big deal, really. I guess if you are critical for reaching the LZ it may be a problem, like flying over water trying to reach the beach!

#### rv7charlie

##### Well-Known Member
The gyro is pretty much coming down with little forward motion anyway if the motor stops, right? Not enough forward speed of the a/c to generate any drag. Completely different situation with faster homebuilts like RVs, Midget Mustangs, etc; even the cleaner light sport qualified a/c. The spinning prop blocks air. Faster homebuilts have even had cooling issues when the prop's root end doesn't have enough pitch for the higher speeds; the spinning prop actually blocks air inflow to the cooling inlets.

The faster the prop spins (when being driven by the air inflow), the closer to a solid disc it will appear to the air. On the other hand, the slower and higher drag the airframe itself is, the less effect there will be from the freewheeling prop. The gyro & trike guys would probably never notice the extra drag, but as the engine migrates to faster a/c, it could be a more significant issue.

You should be able to find controlled studies measuring the difference in drag between a stopped and spinning prop.
Charlie

##### Well-Known Member
“Can be” infers that not all situations are the same. Can you provide more details of a free windmilling prop that has less drag than the same prop stopped?

Thanks,

BJC
I was inferring that you have drag either way; perhaps I made a misleading statement. Different propellers with different airfoil shapes, flat plate drag area and such. I never said that you have less drag in a windmilling state as opposed to static. A faster rotating prop is like a larger flat disc to the wind as opposed to a slower/nonrotating. More flat plate area = more drag, right? Different props have different drag profiles. That was what I was trying to word out, so I apologize...and will eat my crow with salt and pepper!

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##### Well-Known Member
You should be able to find controlled studies measuring the difference in drag between a stopped and spinning prop.
There are many. I never truly saw the theory in motion until I started flying helicopters and performing autorotations. When you pitch up at the end and build more revs, you slow down significantly.

So I do agree with you that the higher the RPM, the higher the drag. I'll have to pull out my Hoerner book and get his take!

#### BJC

##### Well-Known Member
HBA Supporter
Thanks, I was just looking for clarification; no apology needed.

My engine failure response in the Sportsman includes pulling the propeller RPM control all the way out (low RPM). It really does make a difference. If I were high, I would slow below best glide speed to stop it.

BJC

##### Well-Known Member
Thanks, I was just looking for clarification; no apology needed.

My engine failure response in the Sportsman includes pulling the propeller RPM control all the way out (low RPM). It really does make a difference. If I were high, I would slow below best glide speed to stop it.

BJC
All good. That was the technique I was taught in a Piper Arrow for the commercial check ride way back when!

I plan to use a Sensinich 3-blade on my bird...but a windmilling prop, especially at a higher rpm, will indeed be an airbrake of sorts!

Do you know of anyone who makes a folding prop that can handle 100-150hp? I would love to evaluate one!

#### way_up_noth

##### Well-Known Member
I think my best bet is to use another gear box on the Yamaha engine....one that gives me a stopped prop in an engine shutdown situation.

Im new to the whole Rotax world...so anyone who wants to chime in with recommendations or links....is very welcome

does rotax sell just gear boxes?

Im looking for something that can handle a lightweight ground adjustable prop with edge protection...nothing too heavy...

#### TFF

##### Well-Known Member
The gearbox doesn’t have the clutch in this application, it’s the adapter.

#### way_up_noth

##### Well-Known Member
The gearbox doesn’t have the clutch in this application, it’s the adapter.
sorry lots of questions....

ok...I dont know much about the adapters....I assuming the earlier ones had no clutch and Teal is making the new ones with the sprag clutch ?

is it Skytrax and Mohawk the only companies making adapters at this point.....what are your thoughts on the Mohawk gear box...I was taking a look at the Mohawk site and it looks like they have a spag clutch also in thier adaptor

how does the engine itself play into this...as I understand it has some internal dampening...how does that work?

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#### dino

##### Well-Known Member
HBA Supporter
A dead engine with a sprag clutch will windmill at much higher speed than a direct coupled engine which is slowed by engine breaking. The increase in drag is significant and decreases your chances of reaching a suitable field for landing.

#### daveklingler

##### Well-Known Member
A faster rotating prop is like a larger flat disc to the wind as opposed to a slower/nonrotating. More flat plate area = more drag, right?
Is this true? I would think that the flat plate area would be the same with either a non-rotating prop or a fast-rotating prop.

The increase in drag would come from the work being done to turn the rotating prop, work being force x distance, in addition to the increased propeller tip drag from vortex generation. Or so I think. Please correct me if I'm wrong.

#### rv7charlie

##### Well-Known Member
What keeps a gyro up? Or a helicopter in autorotation?

You can test for yourself; rig a small fan with the blades locked to prevent rotation with a tension scale as a tension element, and check the tension with it out the window while driving your car. Then unlock the blades so they freewheel, and run the same test at the same speed.

#### way_up_noth

##### Well-Known Member
What keeps a gyro up? Or a helicopter in autorotation?

You can test for yourself; rig a small fan with the blades locked to prevent rotation with a tension scale as a tension element, and check the tension with it out the window while driving your car. Then unlock the blades so they freewheel, and run the same test at the same speed.
from what ive read a freewheeling prop has the same properties as a parachute of the similar size....

Im not knocking what the yamaha communtee is trying to do...but since there has not been an engine stoppage yet...I guess nobody is intrested in this until you have to glide a plane in but the prop now working against you...

#### TFF

##### Well-Known Member
When you power the engine back past drive, it’s not windmilling any different than if it was off. It’s normal operation.

#### GTX_Engines

##### Active Member
sorry lots of questions....

ok...I dont know much about the adapters....I assuming the earlier ones had no clutch and Teal is making the new ones with the sprag clutch ?

is it Skytrax and Mohawk the only companies making adapters at this point.....what are your thoughts on the Mohawk gear box...I was taking a look at the Mohawk site and it looks like they have a spag clutch also in thier adaptor

how does the engine itself play into this...as I understand it has some internal dampening...how does that work?
Mohawk Aero has a variety of gearbox options, as well as clutch and no clutch. They always have, and they always will, since 2012. These are now available to work with RX1, Apex, Phazer, Nytro, Attak, Vector, Rage, Mountain...in other words ALL Yamaha Genesis (YG) engines.

In some cases, such as the Arrow PSRU, the clutch is inside the gearbox, and is a sprag like that used in automatic transmissions for autos. In most other cases, a clutch is an option, and becomes the shaft coupling between the engine and the gearbox. If a clutch is not used, then a rubber flex shaft coupling is used as the coupling. This allows imperfections in axial alignment up to easily .030", as well as radial alignments of incredible angles. In fact, the Rotax rubber donut is the coupling for the rear half shaft on early 1960's Lotus and other small, ~100HP range Euro sports cars from the 1960's. Porsche only ever used CV joints.

The much beefier AK7 (an Airgear Type "B" PSRU) gearbox Mohawk Aero uses on nearly all of the YG3 and YG4 installations sold has a BMW drive shaft harmonic damper for the flex shaft coupling between the PTO and gearbox, and is sturdy enough that spinning up the prop from start and then flying will not tear it up like the smaller Rotax donut. Rotax and AK7 rubber donuts are a completely different size and are not interchangeable. Mohawk does offer a special adapter that will allow the use of the larger BMW coupling on the smaller Rotax C.

With a Skytrax (which can be confusing, since when you do a google search you come up with a company based in London that rates international airline companies, while the guy who builds the Yamaha stuff using this name has no website) RX1 PSRU adapter you are limited to the Rotax C gearbox only - which was designed for use with their 582 65 HP 2-cycle motor. You MUST use the RK400 centrifugal clutch with this set up because if you try to use the Rotax rubber donut (what is technically called a flex shaft coupling) supplied and intended by Rotax, the shear power of the 140 HP RX1 Yamaha will shred the donut in less than 10 hours.

There are other problems associated with this equipment, some of which were only discovered this year. One of those is that it is impossible to align the clutch drum with the hub due to the imperfections of the Rotax PSRU drive flanges which are usually at least +/- .015" out of tolerance, whereas is must be +/-.0005 for proper clutch wear. That pinion flange must be re-machined to the finer spec.

Now where was I....

With the old Arrow gearbox the pinion gear bolts directly onto the YG4 taper shaft PTO. I make the adapter such that the gearbox then bolts onto the engine crankcase. The sprag clutch is inside, slap on a prop and away ya go.

Teal took this design and added a third gear to the mix, combined it with the crankcase cover and that is the gearbox he now sells.

Mohawk Aero (MAC) also uses a number of different clutches which are NOT in the gearbox like this, but replace the shaft coupling. THese include the popular RK400 from Air-Tech, Inc. the MAC GT3, which has a sprag element inside, the MAC GT4 which has a roller ramp element inside, the Air Trikes "BMW" centrifugal clutch, and the Tango Gyro centrifugal clutch. MAC partners with Air-tech, Inc., Air Trikes, and Tango Gyro in each case, and enjoys a cozy relationship with each.

That should answer the first question.

I've already answered the next question. But to say it again, YES:

MOHAWK AERO and Teal Jenkins are the only vendors for YG4 conversions.

MOHAWK AERO and Tango Gyro are the only vendors for YG3 conversions.

MOHAWK AERO is the only vendor for YG2 conversions.

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