Prop wash as prerotator?

[BrianInVa]

This video was pointed out to me in a Facebook gyro group. It’s not in English but from what I can gather, it’s an older style gyro and the rotor is hand rotated to get it moving. Then the prop wash is used as a sort of prerotator:

Is this a method that’s commonly known and/ or accepted among owners of older style gyros? Pros or cons?

I think it’s fascinating.

It would eliminate the common concerns about prerotators and their added weight and complexity, at least in the older types where prerotating by hand is still an option.

 

[SpyderMike]

I used to do that with my basic Bensen...it worked fine.

Watch out for rotator cuff injury though. I finally had surgery to fix the torn tendons.

 

[WaspAir]

Hand pat-up of blades used to be common to start Bensen blades moving, but Bensens had nothing to guide propwash to drive further spin-up thereafter.

Cierva had what was called a "scorpion tail" that could be repostioned on the ground to deflect propwash for prerotation about 95 years ago (C.19 Mk I). Does that count as "older style" gyros?

Propwash prerotation works best with a moveable elevator and those are truly rare in these days of rotorhead pitch control.

 

[WaspAir]

P.S. Cierva abandoned that approach quickly in favor of engine powered shaft driven prerotation for later models.

 

[BrianInVa]

In my Brock I can reach the rotor while I’m in the pilots seat. Whether I could actually rotate the prop from that position is another question.

 

[GyroRon]

This is commonly called " hand starting " or " patting up the blades "

The prop wash does help, but only a little. You really need some actual wind going through the blades... IOW's you need there to be at least a slight breeze blowing.

Even with minimal to no wind, if your fast, you can pat up the blades, turn around and sit in the seat and VERY SLOWLY begin to taxi and the blades will come up to speed.

Not all brand rotor blades will do this. Dragon wing rotorblades for example are near impossible to do this. Why? I can't tell you exactly why in a way that makes sense, but in practical terms, even a strong armed man simply can't turn the blades by hand much faster than 60-65 rpm. And some blades, dragons wings especially, need to be close to or over 100 rpm to be able to further accelerate just by the wind being blown through them.

There are other brand blades that will easily spool up from only 50-60 rpm. The big fiberglass " Sky wheels " blades are real good at spooling up from a low hand start rpm. The old metal and likely wooden Bensen blades would as well. I have had experience with the 7 inch chord Sportcopter blades and they will hand start but it takes a lot of skill and trying to do so. There are other brands that will do it as well, I don't have a complete list... But the most common blades these days are the out of production Dragon wings that are near impossible to hand start and the new Razor blades that I have not heard any reports of people trying to hand start those so I can't say they will or won't hand start.

This is one of the gyros I built many years ago and it didn't have a pre rotator and I had sportcopter blades on it and hand started them.

 

[Doug Riley]

You can hand-start Dragon Wings, but it's no fun. I hand-started the 28-footers on my tandem Dominator on a couple occasions. Retreating-blade stall (slang name "flapping") comes on very fast, all at once -- perhaps because DW's are twisted and therefore fly at closer to the same angle of attack outboard and inboard. IOW, the whole blade stalls at once.

I put a minor dent in my rudder performing this trick.

The Bensen gyro was intentionally designed with a mast short enough that most people could reach the blades while seated. This is a lot safer than spinning them while standing (much less standing on the seat!). Several people have mangled their hands in the spinning prop while pushing blades from a standing posture.

My Gyrobee is so tall that I have to stand on the seat to hand-spin the blades, but I keep the engine OFF while doing so, then give the starter a yank after returning to the ground. No way I'm standing on the seat with the engine running.

Propwash can be more or less helpful, depending partly on which way the wash spins, compared to the rotor.

The prerotators on all experimental gyros are really just more powerful versions of your arm -- they spin the rotor only partway up to flight RRPM. The rest of the RRPM buildup is achieved by running forward at a carefully restrained speed (assuming you'd like to use the gyro more than once).

 

[MikeBoyette]

Doug
Main reason they are hard to hand start is the airfoil. It was originally a fixed wing profile I believe originated in the 20’s. It was very slick and low drag therefore efficient. The down fall was its abrupt without warning stall. It was abandoned for fixed wing use because of this danger. Dad thought he had originated this airfoil. Chuck did research on it and found it in an obscure NACA book. In driven and autorotative disks this an acceptable flaw as the blades rarely are in the realm of that stall speed with a good pretotator. When hand starting it shows its abrupt stall like a spiked tail of the Dragon they are named after. It is a finesse thing rather than brute force. I used to watch Craig Wall do it with the disk facing away from the wind then turned in once they achieved RPM’s beyond what arm strength could push them.

 

[wolfy]

Ah thats where the name came from

 

[Jean Claude]

Doug
Main reason they are hard to hand start is the airfoil.

In my opinion, it has more to do with the aerodynamic pitch setting (i.e. from zero lift direction).
A high pitch setting alone explains the greater efficiency of the DW rotor (less profile loss), as well as the difficulty of hand start (low mu of flapping divergence).
Drag discrepancy of airfoils is secundary

 

[Jazzenjohn]

There is a big difference between what is possible, and what is practical. Yes blades can be hand started, and i did it for some time, yes prop blast can help, but from a practical sense, a prerotator is extremely useful and few people don't have them.

 

[ultracruiser41]

There is a big difference between what is possible, and what is practical. Yes blades can be hand started, and i did it for some time, yes prop blast can help, but from a practical sense, a prerotator is extremely useful and few people don't have them.

We’re all a little older and wiser
….and our shoulders hurt

 

[Jazzenjohn]

In certainly older, and my shoulders do hurt, wiser... well, maybe, maybe not. I often can't remember why I went into the kitchen, or the garage...

 

[Doug Riley]

If you want to fly an honest Part 103 gyro, you are likely stuck with hand-prerotation. A prerotator will put you over the weight limit -- at least with conventional Erector-set drill 'n bolt construction.

 

[Sv.grainne]

In certainly older, and my shoulders do hurt, wiser... well, maybe, maybe not. I often can't remember why I went into the kitchen, or the garage...

But at least you remember that you went!

Bobby

 

[BrianInVa]

If you want to fly an honest Part 103 gyro, you are likely stuck with hand-prerotation. A prerotator will put you over the weight limit -- at least with conventional Erector-set drill 'n bolt construction.

I recently saw a photo of a chrome moly gyro with a Rotax 503. It was around 210 pounds, well under part 103 range. It would still be under with a 582.


I wish I could find more about the diameter and wall thickness of the chrome moly tubing required.

 

[Aerofoam]

Maybe this needs it's own thread, but I have had this idea for a long time and will build it, but will throw it out there for anyone who wants to build it themselves.

The prerotater gear is replaced with a pulley sheave with about a 3" to 4" inner diameter. Behind the pulley sheave is a mounted pulley to change the direction down the mast. There is a sleeve along the mast and another pulley at the keel with a sleeve to the tail.
You thread a 1/8" Amsteel line through the sleeves and pulleys, then pat the rotor backwards to wind the line on the sheave.
Anchor the tail to a bush, or stake and push the gyro forward to take out the slack. A firm acceleration will unwind the line spinning up the rotor
with more power than any available pre rotater and the line wil exit out the back. 50 to 70 ft. of this line is only a couple dollars when you buy it by the spool and you could carry a couple extra lines with you. You could even carry an extra stake.
The weight of this gadget should be under 5lbs. and you leave the line and stake behind....
Yes it's a little effort to wind it up, but not more then a few minutes and the pre rotation will occur within about 30 to 40 ft with a better result than a prerotator and probably, since you are already moving forward during the spin up, no dead band reversing the air flow from thrusting to auto rotation...
This idea originated almost 30 years ago from when I used a bobbin on one of my RC gyros in the same manner, but would just stand on the line and then accelerate.
It was a tractor, so it didn't need the pulleys and sleeves to keep the line out of the prop....

 

[XXavier]

It might work...

My very rough calculation, assuming zero rotor blade drag, for an ELA with a 100 HP engine and a rotor inertia of 190 kg.m^2, results in a run of approx. ten meters for a pre-rotation of 150 RRPM...

[I assume a constant prop push force of 2500 N. As the rotational energy of the rotor would be 24000 J @ 150 RRPM, the prop push should be sustained for ~10 meters in order to transfer those 24000 J to the rotor...]

Human pull worked for early Cierva autogiros...

 

[wolfy]

Maybe this needs it's own thread, but I have had this idea for a long time and will build it, but will throw it out there for anyone who wants to build it themselves.

The prerotater gear is replaced with a pulley sheave with about a 3" to 4" inner diameter. Behind the pulley sheave is a mounted pulley to change the direction down the mast. There is a sleeve along the mast and another pulley at the keel with a sleeve to the tail.
You thread a 1/8" Amsteel line through the sleeves and pulleys, then pat the rotor backwards to wind the line on the sheave.
Anchor the tail to a bush, or stake and push the gyro forward to take out the slack. A firm acceleration will unwind the line spinning up the rotor
with more power than any available pre rotater and the line wil exit out the back. 50 to 70 ft. of this line is only a couple dollars when you buy it by the spool and you could carry a couple extra lines with you. You could even carry an extra stake.
The weight of this gadget should be under 5lbs. and you leave the line and stake behind....
Yes it's a little effort to wind it up, but not more then a few minutes and the pre rotation will occur within about 30 to 40 ft with a better result than a prerotator and probably, since you are already moving forward during the spin up, no dead band reversing the air flow from thrusting to auto rotation...
This idea originated almost 30 years ago from when I used a bobbin on one of my RC gyros in the same manner, but would just stand on the line and then accelerate.
It was a tractor, so it didn't need the pulleys and sleeves to keep the line out of the prop....

View attachment 1162033

Why?
Besides the weight advantage, simple pre rotators that can do the same rrpm as this can be really simple, fairly light and you can take it anywhere.

 

[Aerofoam]

Why?
Besides the weight advantage, simple pre rotators that can do the same rrpm as this can be really simple, fairly light and you can take it anywhere.

40 lbs of pre rotator verses about 3 lbs. is the difference between Part 103, or not on many light gyros....

It also has the possibility of shortening the take off run because of no flow reversal....