33 foot rotor vs 29 foot rotor

[THI]

I have read that some gyro's fly beter with a 29 foot rotor that with a 33 foot rotor. Shouldn't it be the other way around?

[weedy]

I'm no expert, but I think a "rotor disk" loading of about 24-26 edited to 35 pounds per sqr foot is good, being over rotored is worse than being under rotored, Wagtail has all the answers.

[Gyronaut]

I'm no expert either. I also do not wish to get into a debate as to 'this machine is better than that machine' bla bla bla.
For the record, I own a Sycamore and I love Magni's and Gyro's in General. If it flies and its safe I support it. I did most of my own training on Magni's and have over 70 hours of dual (as instructor) on Magni's. I can therefore only compare rotor performance between my Sycamore and the average Magni M16. (Same engines and propellors)

What I do know is this. My machine weighs 80kg's more than the average Magni M16. It has a 33ft rotor and under normal conditions it flies/climbs/behaves the same as the Magni at approximately 200 engine RPM more than the Magni requires. No way would an 80kg pax require only 200rpm more on a an M16. On a cold day, when the density altitude is low it performs even better compared to the lighter/shorter rotor M16's. Dont know if this is relevant but I do know this, if my machine had the same 28ft rotor as the Magni it wouldn't perform nearly as well (if at all). The rotor clearly compensates very well for the extra weight I have to carry.

Len
ZU-RDR

[THI]

Thanks for the info. Please note that I don’t want to compare the different gyro. Not my thing! I’m just curious why some would prefer to fit a 29 foot rotor instead of a 33 foot rotor.

[Low Level]

Although only 14% longer, because on a disc, you have 30% bigger wing. If you need the lift - great, but with it comes extra drag. Therefore if you do not need the extra lift, why bother with extra drag, extra load on the pre-rotator, extra weight, and from what I can understand one has to be extra carefull with the ground handling of the longer rotor.

[Gyronaut]

All very true Low Level. Additional lift at the cost of additional drag of course. In flight though the drag effect may be cancelled due to the reduced angle of attack necessary to maintain straight and level flight. Then again, I dunno, just my opinion and not based on fact.

Ground handling bit me once already when I thought I might get away with coming back on the stick ever so slightly before I had adequate Rotor RPM. Bang, hit the tail. Luckily not a serious accident but you are quite right. When the Sycamore rotor is stationary it can touch the ground. The longer rotor is more difficult to handle on the ground so one has to be extra vigilant. Interesting though, at 110 the longer rotor is coning adequately for flight as opposed to around 140 for the shorter rotor.

I have made a habit of stopping the rotor on the Sycamore totally before doing any ground manouvers that could spoil your day. Gyroscopic precession/effect is stronger with the longer rotor.

[THI]

when I thought I might get away with coming back on the stick ever so slightly before I had adequate Rotor RPM.

While pre-rotating?

Interesting though, at 110 the longer rotor is coning adequately for flight as opposed to around 140 for the shorter rotor.

Do you refer to the rotor "110" rpm?

[Gyronaut]

THI, Yes and Yes.

When pre-rotating:- Stick fully forward; on a Magni - until Rotor RPM is 140; on a Sycamore (with longer rotor) 110 Rotor RPM is adequate and blade flap will not occur.

Capiche?

[THI]

THI, Yes and Yes.

When pre-rotating:- Stick fully forward; on a Magni - until Rotor RPM is 140; on a Sycamore (with longer rotor) 110 Rotor RPM is adequate and blade flap will not occur.

Capiche?

Yebo. I'm still new at this (in training)...I wait until 180-200rpm before pulling back on the stick. What is your take-off rpm on the Sycamore? That should also be lower than the Magni? Magni 28feet;Sycamore 33feet?

[Low Level]

at 110 the longer rotor is coning adequately for flight as opposed to around 140 for the shorter rotor

Interesting - just shows the extra lift.

In flight though the drag effect may be cancelled due to the reduced angle of attack

Also wondered quite a bit about that. Think that is quite a bit of data above our heads, extra lenght, extra lift, extra drag due to lenght, less drag due to smaller angle of attack etc. The maths around that can keep someone a bit busy for some time.

before I had adequate Rotor RPM. Bang, hit the tail

Bought my gyro with that little mark on the tail. Was quite usefull to my instructor in explaining ground handling of a gyro with slow spinning rotor.

What is your take-off rpm on the Sycamore? That should also be lower than the Magni? Magni 28feet;Sycamore 33feet?

I have a 31 foot rotor on my Sycamore. Taking off she wants to fly at around 290 rpm. Straight and level - one up 340 rpm, two up 360 rpm. I understand the MT03 runs easily to 450/470 rpm on their rotors - 27 feet if I'm correct.

I wait until 180-200rpm before pulling back on the stick

I presume you train on an ELA (or MT) with solid shaft pre-rotator. Unfortunately with that you cannot pull back before releasing the pre-rotator. If it has the flex shaft pre-rotator, I was taught to pull back at 130 rpm. Enough lift to clear the tail, and then the thrust from the prop actually helps to spin the rotor up faster towards 180/190 for take-off.

A very ugly pic of the smog over Brakpan, but the rotor rpm - on the far right - while climbing.

[FO Gyro]

Data from my machine:

MT-03 rotor RPM solo: 360-370RPM. MAUW (max all up weight): 400-410RPM.

With the shorter blades, this would make sense.

I also speak under correction, but one advantage of shorter blades is less of a tendency to get that horrible fore and aft violent stick movement one can get when encountering a bad gust. I've never experienced this on my MT-03. It used to scare the daylights out of me. I had it 2-3 times on my VPM M16 (which I understand were prone to it).

Maybe the boffs can help out here. I was a PPL instructor, but rotor aerodynamics is other story. I understand its caused by the centre of pressure moving out of alignment with something (not sure what!), on a slightly unloaded rotor. Loading the rotor and reducing speed seem to stop this immediately.