Propeller built like a tailrotor. 3

[RoarkS]

Okay so on top of being an engineer and an A&P I also have my Helicopter and Airplane private pilot certs.

So here I am sitting next to the tailrotor assembly of an MD-500. I go for a bit of a walk and I can check out the tail rotor on a Huey. They are basically designed the same so lets go with that.

Now lets go have a look at a constant speed airplane prop... complete mess. Hand me a manual and yes, I can take it apart and put it back together successfully. Stick me in a plane with one... and that "prop" lever had me baffled. I get it manifold pressure, throttle... ehhhh.... what's going on. No idea.

Back to the helicopter... Okay, engine RPM in the green, rotor RPM in the green, Manifold pressure/%torque, TIT... Push more pedal (more pitch) or pull collective... everything goes up. Makes sense. I like it.

So what's stopping me from building a propeller on an airplane, basically identical to a tail rotor assembly, that has a manual pitch control? Is there something obvious I'm missing? Seems a whole lot simpler.

Second to that... Helicopters in their infinite wisdom like to be able to flap or Teter. Way I figure is it takes stresses out of the structure that a propeller would typically fight... and essentially get rid of P-factor?

I see this as a win-win. What am I missing?

I tried to google, but my google-fu failed me.

 

[verymadmac]

I recall reading that early Supermarine spitfires where fitted with manual control 2 position propellers, quite a number were lost due to pilots attempting to take off / go around with the propeller in course pitch.
Tail rotors don't lend themselves to simple automation like a propeller being subject to quite horrible aerodynamic conditions and force demands. Do Tail rotors actually cone, or is it only main rotors. A coning propeller would be subject to wing upwash field which I image could make it somewhat unstable or atleast generate some interesting vibrations. There is also the cost issue,the ability to lag/lead or coning makes for a complicated structure.


Of course you can actually get manual control propellers if its an electric one like this one here.
Link

 

[IRstuff]

The simple answer is that tail rotors are not designed to propel an airplane; if they were, they'd look like an airplane propeller. Form follows function.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[VEBill]

Just a related tidbit:

The very interesting airplane called 'Draco' has used Reverse Pitch during flight to slow his forward speed (to 80kts?) during steep descent (38000fpm?) to match flight profile so that a pair of wingsuit jumpers can hang onto his wingtips.

Ref:

https://www.youtube.com/watch?v=Jdccr7qm5NA

(Apologies in advance that you'll spend the rest of the day watching other 'Draco' videos. As mentioned, a very interesting aircraft.)

 

[RoarkS]

Thanks for the replies!

@verymadmac

I will have to look into that! Once upon a time in a propeller class I remember hearing a quip about 2 position props... I personally thought that was a great idea and is what led me down the rabbit hole.

I didn't know they were on spitfires... which oddly enough I was really looking hard a spitfire propeller blade blueprints not too long ago.

I had not heard about taking off in "course pitch" accidents. Makes sense... maybe rig in a squat switch that forces takeoff pitch?
----------------
I don't know much about electric props other than folks that had them on old beechcraft STC them for CS props. If being a 2 speed prop is all they are doing why not make a manual one? Should be simpler/cheaper?
----------------
Honestly someone a lot smarter than me probably figured this out a long time ago, but I still want to go through the thought experiment. I've often thought It would be neat to have a propeller "collective" control... twist throttle and pitch control on the pull. I imagine it would feel as instinctive as a helicopter. It could be a horrible idea, I don't know. I just really have a hard time wrapping my brain around the constant speed concept... I've understood it for a few minutes after someones explained it but then it escapes me as not being intuitive.
---------------
Do tailrotors cone... Not enough to count as far as I know. Coning happens because of blade flex on teter and rigid main rotors... I've never seen a fully articulated tail rotor that I can think of, which would also be the only reason for lead/lag to occur. I'm talking a simple teter hinge blade arrangement. As far as aerodynamic forces... I was once told that the Huey Tail sucks up nearly 200 horsepower... That's basically a Cessna.
---------------
Agreed, tail rotors are not designed to drive an airplane. But I'm not giving up on such a high level assumption. I have the gut instinct that tail rotors are a "more advanced" technology and possibly nobody ever tried it on a plane that I know of cause what they had was pretty good and things were certified as such (ie fixed timing mechanical dual magnetos)

Rotor blades flap to fight Dissymmetry of Lift to balance the system out, because the first helicopters didn't have strong enough... proper materials and hydraulic flight controls to allow for ridgid rotor systems... thus ingenious and elegant designs resulted. I think propellers may be brute force overbuilt systems just because they can't flex just a little bit.
--------------
You bet I would have a detented position for reverse pitch! I flew a glider with airbrakes/spoilers a while back... WOW! I wish more GA planes had airbakes, lined up for final doing something stupid like 120 came over the end of the runway... I was thinking what's he doing... going around in a glider... BOOM airbakes... we were on the ground. Amazing. If it was manually controllable I could ease in just a little bit of reverse pitch. how cool would that be!? I imagine if you gave the system to the right nutcase in a Pitts he could probably figure out how to fly backwards coming out of a tailslide.

 

[Compositepro]

Helicopter blades (main and tail rotors) fly into the wind on one side of the disc and with the wind on the other, requiring cyclic pitch changes and flexibility. Airplane propellers do not. Flexible blades are complicated and have limited service life. I'm sure there are many more subtleties involved.

 

[rb1957]

before they had "constant speed" propellers (that use beta control enable the prop to turn at a constant speed to match thrust required with engine power)
they had constant pitch propellers (as they sound, with a fixed pitch/beta and so a fixed thrust curve).
in the interim they had "two speed" propellers that had two pitch/beta positions (and, yes, accidents as pilots get used to the new technology … just as they had (and have) with that new fangled retractable landing gear).

so you could have manual pitch control but for 99/99% of the time you ask yourself "why not automatic ?" This could have to do with being a helicopter pilot (as opposed to a fixed wing pilot) … I think you helo guys think about forward thrust differently to fixed wing guys … I think you need to extract a component of main rotor lift to be your thrust vector, whereas a fixed wing pilot doesn't … the fixed wing engine is doing one job, much simpler than the helo.

for your second point, I think you're asking to mount the propeller on a gimbal so it's always pointing into the airflow. Maybe a good idea, but I think the mechanical losses and the complex mechanism would outweigh any benefits.

another day in paradise, or is paradise one day closer ?

 

[RoarkS]

Thank you for pointing that out. Both of you are correct... helicopter main rotor and tail rotor when flying (vs hover) are perpendicular to that of a propeller... and that is the reason for flapping features. When hovering... they are at their highest power output acting as thrust devices, which is what I akin my thought experiment to. I have seen the flapping teter mechanism react to wind gusts on tail rotors in hover, so I'm not convinced it's worthless... but it's certainly not it's main job. Maybe that's justification enough to leave the flapping alone. I wish there was an easy way to see what it would do to p factor though... I cant get that out of my head.

@Compositepro...
Oh I'm totally against flexible blades. Main rotors blade flex (Coning) is unavoidable, as their rigidity is defined by them spinning... way more complicated especially in rigid rotor heads I don't want to touch that with a 10ft pole. I don't think it's an intended design consideration in most tail rotors... I think those are as rigid as possible much like a prop.


@rb1957... not to get off topic... but you're using Beta in a manner in which you sound like you know what it means... I would like to hear your definition. All I was ever told is: Beta is the ground operation pitch control mode allowing for reverse thrust on a PT6. I never got to the bottom of why it's called "Beta".

Back on topic... pesky landing gear.

"Why not automatic"... because automatic is expensive, complex, more maintenance and did I mention expensive.
I'm talking a cheap GA Vans RV-8. A good constant speed prop is nearly $15k!? I spend that much on the entire airframe. I would love a FADEC controlled engine with Autothrottle coupled autopilot... but I don't have that kind of money.

I fly around all day with a "climb" prop. Would be nice to get out and put a "cruise" prop on, or make a 2 position... or why do that when an infinitely variable "tail rotor" system is sitting right next to me saying here this design works (but leave out the flapping). A few hours at my friends CNC shop and I bet it would be good to go.

 

[IRstuff]

Because a tail rotor is going to suck at giving you any thrust for a fixed wing aircraft.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[RoarkS]

IRstuff...

Obviously blindly transplanting a tail rotor out of an MD500 onto the front of a GA airplane would be ill advised.

-The mechanical pitch change linkage design is interesting. I've not seen a mechanical pitch change propeller.

-Musing the applicability of the teter mechanism is interesting, especially with p factor. So far the consensus is that's not it's job.

-further speculation that the teter mechanism may relieve traditional propeller stresses allowing for significantly lighter construction. Google a picture of a UH-1H Huey Tail rotor...It's original design was 386 foot-pounds (ft-lb) of torque (122 shp at 1,654 rpm). 170 horsepower at full deflection (I just found that in a military report) Horsepower is horsepower. Thrust is Thrust. That's the power of a small airplane. Shorten those blades up, with a proper airfoil, crank up the RPM... I don't understand the difference. Is it simply Tail rotor blades are life limited to 2500 hours VS a GA prop that's based on inspection, and blending limits... so their safety factor is lower? I don't understand where the beef of a prop went.

FYI, tail rotors are typically much simpler than main rotor heads...except the teter hinge type. I'm not talking swashplate or cyclic control, lead lag dampers, individual blade flapping. At this point just simple collective pitch control.

 

[rb1957]

I've used "beta" (possibly incorrectly) to mean propeller pitch angle control, google "propeller pitch control".

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

Hi Roarks,
I tried to read the replies you've received so far, and either I have read your question wrong, or everyone else has.

Your tail-rotor mech is a good example, IMO, but for some reason it may have lead others astray. IIRC many heli tail-rotors have a teetering hub to reduce forces from precession but this is not a feature to be concerned about in the aircraft-propellor combination. Doesn't really need to be considered since fully-rigid hubs are the norm on all propellor-driven aircraft and you aren't asking questions that challenge that anyway.

So it's not TLDR, it's TLRSTRIS: Too Long, Read Some, the Rest is Sidetracked

I take your question as (paraphrasing) "why use a complex and heavy mechanism to assure constant propellor speed if a pilot could be given a control that would allow direct selection of the propellor pitch, using a simpler and lighter mechanism?"

OK good question. The pilot wants to "set it and forget it" so that's the main thing. Excessive propellor speed is dangerous, and normal prop operating speed is about 90% of their maximum operating speed. The margin is not that great, but the range of speeds of an aircraft is very wide: 50 knots to 200 knots is a 1:4 ratio. You may have experience flying with a fixed-pitch prop, and may have had a moment or two when the flight instructor pointed out that while diving you have to retard the throttle of the prop will overspeed. That's because a prop speed trimmed for a slow aircraft speed will increase if the aircraft increases, and the margin to allow the prop speed to increase is very small compared to the aircraft's ability to speed up.

So if you gave yourself a lever to manually control the pitch, you'd still have a new lever in the cockpit, but it's not the "RPM" lever, now it's the "pitch" lever. You would use it to set RPM, but as you sped up or slowed down, you'd have to adjust it again and again. So you're stuck with the worst combination: more cockpit work, an extra lever, and more chances to do something stupid.

Second thing to consider is relative inertia, or angular momentum, of the various systems.
[ul]
[li]Rotorcraft: Main rotor inertia >> engine inertia >> tail rotor inertia[/li]
[li]Aircraft: Propellor inertia ~~ engine inertia[/li]
[/ul]
I don't need any more math to know that I should be careful to compare their structure or control mechanisms fairly.

Third thing: question on my part - Doesn't the MD500 have an Allison turboshaft engine? That might have a speed governor on it (correct me if I'm wrong).

Fourth: Huey T53 engines are rated between 1200 and 1800 HP, so yeah, the tail-rotors take about 200 HP for themselves, easy, in a hover.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[VEBill]

Another tidbit:

An Airboat prop on a bush plane, for max acceleration.

Mike Patey:

https://www.youtube.com/watch?v=stkvTQ1lCSw

 

[rb1957]

I can't imagine pilots would want to manually control propeller pitch and engine rpm (separately). They had trouble enough in remembering to select the correct pitch for landing.



another day in paradise, or is paradise one day closer ?

 

[berkshire]

There are some propeller's made by Hoffman in Germany that have mechanical pitch control , they have fine ,coarse, and feather settings, operated by a hand lever. They are used on light aircraft and Motorgliders as an alternative to expensive controllable propellers.
B.E.

You are judged not by what you know, but by what you can do.

 

[IRstuff]

Deleted, yes, it was in response to a silly spammer.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[RoarkS]

Here I am over here hoping I was getting more input

 

[VEBill]

The Draco airplane (mentioned previously) crashed. Extreme weather. Nobody hurt.

https://www.youtube.com/watch?v=NfctWAoDXvs

 

[beaklio]

I'm just interested in aircraft engineering topics to learn more info and to be impressed with some projects. This thread made me curious with your skills and knowledge. I look forward hearing about your next updates. good luck to this project.

 

[Compositepro]

Beaklio, welcome to Eng-Tips. I hope to see you become a valuable contributor to the forums here, but you have made a number of posts like the one here which basically say, " Hi, I'm here and listening, too". This does not contribute to the discussion. Bear in mind that there are thousands of other engineers that follow these forums.