props for high altitude

[dalbert02]

Hypothetically, if one were to attempt an altitude record in the appropriate air space, with FAA notified, with NOTAM issued, etc, etc, what would be the most ideal prop? Assuming a very light wing loaded F5J type glider with a 3.2m wingspan (120"), what would be the recommended prop for altitudes above 50,000 feet? Ideally, I would assume a variable pitch prop would be recommended, but to reduce complexity a fixed pitch prop would be used for a first attempt. Motor selection, I would assume, would need to be a very low kv with a gear reduction to spin a very large prop, but that is just my assumption. Thank you for any ideas, hints, or suggestions.
-dave

[Brandano]

I'd try to gain altitude on a smaller prop while carrying up a much larger one feathered, and then either start the larger prop on its own motor or switch over the motor to the larger one using a clutch mechanism. Incidentally, I think the altitude record for prop driven plane is still held by a pre-WW2 Caproni machine that used an oxigen supply to the piston engine's carburetors. Maybe an electric plane could claim it back now?

[vintage1]

50,0000 feet? blimey. Is there enough air to keep a plane up anyway?

I think you wouldn't need variable pitch.

Motocalc allows you to enter different altitudes: Id start there. I suspect you want a prop that overloads the model down low, so you simply run part throttle till its up there.

But what a challenge. Its unlikely you can do more than 1000 fpm climb..thats 50 minutes!!! So you need to carry a huge pack as well.

Sounds like a very very strong and light glider is what you want: Huge pack and enormous prop too.

Thee'rs jet streams up there as well..might get broken up in CAT..

[markdrela]

Hypothetically, if one were to attempt an altitude record in the appropriate air space, with FAA notified, with NOTAM issued, etc, etc, what would be the most ideal prop? Assuming that the tips don't go supersonic at altitude, the aerodynamically-ideal prop is nearly the same for any altitude. The blade airfoils do want to be tweaked so they work OK at low Reynolds numbers at altitude, but that's a minor difference. The RPM and the power, however, want to increase as 1/sqrt(air_density).
As you know, this aero-optimum solution is not compatible with the motor-optimum solution, which is a fixed RPM and power. The only way to match them up without compromising either the prop or the motor efficiency is with continuously-variable gearing.

Variable pitch is an alternative, but it will cost some efficiency and can only go so far. I did a variable-pitch prop design (no gearbox) for a 65 Kft UAV some time ago. At sea level takeoff in the lowest pitch, the prop was so far off design that the outer 20% of the blades were actually generating negative thrust. Even a 2-speed gearbox would be a huge improvement.

[ghoti]

A small battery pack could be used if a balloon lofted it to altitude . Bill

[JetPlaneFlyer]

Hypothetically, if one were to attempt an altitude record

You would really have to go some... NASA took their Helios electric/solar power R/C aircraft to nearly 97,000ft altitude in 2001: http://www.nasa.gov/centers/dryden/history/pastprojects/Erast/helios.html

Which by the way is the absolute record for a prop driven aircraft :) ... Twice the altitude of the Prop/piston engine Caproni ;)

Steve

[jfrickmann]

Another way to change the gearing would be to change the pack tension (voltage). Either with electric relay switches, reconfiguring your pack from, say, 1S2P to 2S1P at some point. Or, to vary continuously, an efficient switching regulator on the pack.

[JetPlaneFlyer]

Another way to change the gearing would be to change the pack tension (voltage). Either with electric relay switches, reconfiguring your pack from, say, 1S2P to 2S1P at some point. Or, to vary continuously, an efficient switching regulator on the pack.

Or a much easier way to control voltage... just use a standard ESC ;)

[ghoti]

It was my belief that standard brushless ESC circuitry changed frequency, not voltage. Is this correct? Bill

[Brandano]

I believe any ESC nowadays will just change the frequency. Brushless controllers will change the frequency of th single phases, while brushed controllers will alter the PWM signal to the motor. Most speed controllers for brushed motors control the speed via a PWM signal because it has the advantage of giving much the same torque at different speeds. Maybe not essential for planes, but it's the only way to slow-start a wheeled vehicle.

[vintage1]

It was my belief that standard brushless ESC circuitry changed frequency, not voltage. Is this correct? Bill


Umm. Its really very complicated. It does pulse width modulation to sort of control the voltage..thats the throttling bit, and then the frequency of commutation is advanced until the motor runs up against its KV limits for that particular reduced 'voltage'.

If you like its like having a brushed controller, with the output fed sequentially to the poles to do the commutation. Its no different in principle to a brushed motor - just the commutation is done electroncally instead of with brushes.


In this application I'd use something like a Hall effect current sensor and a uProcessor to limit throttle at low altitudes.

[dalbert02]

You would really have to go some... NASA took their Helios electric/solar power R/C aircraft to nearly 97,000ft altitude in 2001: http://www.nasa.gov/centers/dryden/history/pastprojects/Erast/helios.html

Which by the way is the absolute record for a prop driven aircraft :) ... Twice the altitude of the Prop/piston engine Caproni ;)

Steve

True... But maybe an amateur record could be broken.
-dave

[jfrickmann]

Or a much easier way to control voltage... just use a standard ESC ;)
I think that this is less efficient, as they "pulse" the current at a high rate. Maybe one of these (http://www.rcgroups.com/forums/showthread.php?t=788065) ESCs will be better.

[JetPlaneFlyer]

PWM is effectively voltage control... or at least it controls the 'average' voltage. Capacitors are used to damp down the 'ripple'.
I believe the PWM is a more efficient means of voltage control as it generates much less waste heat than old fashioned methods. I cant see any reason at all to put a secondary means of voltage control in the circuit, what would you gain?... It would just add weight which is the last thing you would want for an altitude record.

I'm not sure if standard battery driven electric would be the way to go to attempt an altitude record. Carrying the weight of the required MASSIVE battery would increase the wing loading which would limit the altitude attainable. Maybe if you used multiple small batteries and had a mechanism to drop the exhausted packs?

So what is the 'amateur' record?

Steve

[dalbert02]

So what is the 'amateur' record?

Steve


According to the FAI website:


Category F5: Radio Controlled Flight
Sub-class F5 Open (Aeroplane, Electric motor S (rechargeable sources of current))
N°173: Gain in altitude : 3 418 m (11,207 ft)
Date of flight: 02/10/2004
Record holders: Giorgio AZZALIN (USA), Simone AZZALIN (USA)
Course/place: Ponte Vedra Beach, FL (USA)
Database ID 9857

-dave

[macboffin]

The Boeing "Condor" with three stage supercharged piston engines, span of over 200 feet, got to 98,000 ft about twenty years ago with BIG three bladed props.

[dalbert02]

Even a 2-speed gearbox would be a huge improvement.

Any pictures or diagrams you care to share on gearboxes you or stuents may have used? I have access to CNC tooling, but not sure how to make something so small, light and practical.
Thank you kindly,
-dave

[JetPlaneFlyer]

The Boeing "Condor" with three stage supercharged piston engines, span of over 200 feet, got to 98,000 ft about twenty years ago with BIG three bladed props.

The info I've found say 67,000 ft ceiling :confused: e.g. : http://aviationtrivia.info/Boeing-Condor.php

[Brandano]

Looks like the highest flying single engine aircraft altitude record still stands for the Caproni Ca.161 at 56,032 ft in 1936 ( http://en.wikipedia.org/wiki/Caproni_Ca.161 ). the Rans RV4 is probably the highest flying mass produced aircraft, but only reached 46,919 ft according to http://aviationtrivia.homestead.com/rv4.html . Other single engine drones have flown higher, but I suppose that not having to carry with them the life support apparatus for the pilot makes things a bit easier.

[macboffin]

The info I've found say 67,000 ft ceiling :confused: e.g. : http://aviationtrivia.info/Boeing-Condor.php My info from Boeing publication

[Brandano]

Ah, an unbiased source....
Did they register it with FAI?
Btw, this is the list of records for landplanes, piston engined:
http://records.fai.org/general_aviation/current.asp?id1=21&id2=1