Gyros on "real" heli's

Gyros on "real" heli's (Full Version)

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Rawlie -> Gyros on "real" heli's (5/21/2003 8:18 PM)
Does anyone know whether "real" helicopters (as opposed to R/C helicopters) also use gyros to hold the tail straight? Or is this merely a R/C heli item? Presumably, if so, then in a real heli its up to the pilot to keep the nose straight with his rudder controls? Just a thought :)

GRANT ED -> Gyros on "real" heli's (5/21/2003 8:52 PM)
Some 100% scale helicopters do indeed have this type of feature. Quite a few larger helicopters B412, S76, come to mind have a thing called Stability augmentation. This with out going into to much detail is part of the autopilot/control system although it is on all the time. It also stabilizes pitch and roll as well as yaw. Most smaller (read single engine) helicopter's don't have this feature. Bearing in mind these advanced helicopters can have an autohover function on there autopilots anyway.

Rawlie -> Gyros on "real" heli's (5/21/2003 11:51 PM)
Thanks for the info GRANT ED. I guess there's always more to it than meets the eye .....

Rawlie -> Gyros on "real" heli's (5/22/2003 12:16 AM)
GRANT ED Seeing as you seem to know quite a bit about this stuff, maybe you can answer another question that's been tickling me: In a model heli, do the Physics "scale down" proportionally from a normal helicopter. For e.g. if my model heli is a 1/16 version of a real heli, will the model heli fly approx 1/16th as fast and are the forces in proportion? As far as I can tell, the only major difference would be that R/C helis are relatively overpowered and light compared to normal heli's, due to 2 stroke engines and reduced fuselage etc. But if you compare rotor RPM, both model and R/C helis operate in the 1500-1700 range as far as I know. Any thoughts Grant/anyone? :rainfro:

TCA-Eng -> Gyros on "real" heli's (5/22/2003 1:25 AM)
Rawlie I do not know anything about the real heli, but I can tell you as an engineer, nothing "scales down" in a direct proportional manner when it comes to size. When you double a size, most times the reaction forces go up by a factor of 4 (it is a squared relationship). This is why the larger model Heli's are a little more stable that smaller ones....

extremeflying -> Gyros on "real" heli's (5/22/2003 2:43 AM)
i think they use Gy601 with 9252 combo ;)

Dyehard -> Gyros on "real" heli's (5/22/2003 7:06 AM)
Rawlie, full size helis only turn the rotor head in the 300 to 400 RPM range. Very few things scale down from full size aircraft to models, whether heli or airplane.

venture32os -> m/a tach (5/22/2003 9:53 AM)
I have used m/a tach on r/c and full sized helis. He was right about rotor speed, they both fly at about 1500 to 1700. I dont think a heli would lift off at 400 to 500 unless it had about 40 deg. pitch.

Dyehard -> Gyros on "real" heli's (5/23/2003 9:00 AM)
Venture, you better have your tach checked. I flew in a lot of UH-1s in Vietnam, they turned the head around 330 RPM. I have also flown in several Bell Jet Rangers. They have a rotor speed of around 400 RPM. Full size helicopters don't need to turn the rotor as fast as we turn our model to develop the lift they need for basically the same reason that my full size Champ doesn't need to turn it's prop at 12000 t0 15000 RPM like a model would. As lifting surfaces get larger, they develop morer lift at much lower speeds. How air effects surfaces as it flows over them is expressed as Reynolds numbers, those numbers are vastly different for models and full size aircraft.

rscamp -> Gyros on "real" heli's (5/23/2003 9:04 AM)
One thing's for sure about a model heli. Is is WAY heavier for its size than the full scale. This also contributes to the "faster than scale" speed. Rob

GRANT ED -> Gyros on "real" heli's (5/23/2003 3:42 PM)
Rawlie. To answer your question. No the physics don't scale down. TCA-Eng is right when he says the reaction forces go up by 4 when you double the size of something. Also when you half the size of something its weight reduces by a factor of 8. This is the reason why a 1/10 scale heli weigh's less than 1/10 that of the full size version. Add to that the area of a control surface reduces by a factor of 4 if you reduce the scale by 2, shows that nothing is equal. The same basic pricipals apply to both full size and model heli's but there are many variable to consider when thinking of the forces involved. Iv'e worked on Bell 206's and from memory their rotor RPM was about 400ish. Regarding the speed of a model compared to the real thing. The deciding factor has to do with the main blades. As a heli comes close to it's VNE the advancing blade will be near it's critical mach number while the retreating blade will be close to its stall angle. It is because of this that real heli's all don't get much over 150 knots. I'm assuming its the same limitation for the models.

Dyehard -> Gyros on "real" heli's (5/24/2003 6:20 AM)
I know designers would love to raise the rotor speed of full size helis, as that would be the easiest way to raise top speed without worrying about retreating rotor stall, but can you imagine the forces on a full size rotor head at 1700 RPM. I don't know if there are any materials that could stand that kind of pull and still be light enough to allow it to be used on something that will become airborne.

venture32os -> optical tach (5/24/2003 8:44 AM)
Well tell me how a optical tach works on a rc heli but not a full size heli. It works on all engines with blades that run in the 1000 to 1900 range.

GRANT ED -> Gyros on "real" heli's (5/24/2003 9:48 AM)
At 1700 RPM rotor speed for a full size heli the advancing blade would be well and truly supersonic. Not a good thing.

Dyehard -> Gyros on "real" heli's (5/24/2003 10:29 AM)
Venture, the MA optical has a upper limit of 1990 RPM and probably a low limit also, I've never checked mine to see how low it will go. I dought it was made to check the speeds that full size helis turn their blades. I did a quick check on some sites, the fastest head speed I found for a full size heli was the Rotoway Exec, a small kit built two seat heli. It had a 100% blade speed of 520 RPM. As helis get bigger, which means almost every other full size compared to the Exec, the blade speed gets slower. If you have any dought about our figures, stop by your local airport, almost everyone of them has a heli or two sitting around. Most will be happy to show off their machines and you can ask the pilot of those helis what max head speed is.

LEDZx -> Gyros on "real" heli's (5/24/2003 10:33 PM)
Apart from Grant Ed's statement on the main blades going supersonic, you must keep in mind there is a tailrotor spining behind you at nearly 3 times the speed. Now in a full size heli, the tip speed on the tail rotor blades are below mach 1, but not by very much, in some helis, a rpm increase of 10-15%(maybe a bit higher?) can send the tail blades supersonic, which as we all know releases a shock wave which could , and has happend once before as far as I know, damage the main rotroblades( because they are so close ). Strange that Dyehard should mension it, but we have an Rotorway Exec 90, soon to be converted to a Jet Exec (turbine powered). Cheers LEDZ

Raptor^Norway -> Gyros on "real" heli's (5/25/2003 12:50 AM)
Full size helis rpm is between 300 and 400!! Why is that so hard to understand? Have you guys ever been overflown by a full size? Does it really sounds like it is secs away from tearing itself apart? Screeeming rotorblades at 1700?? No! You hear a low freq. flap flap flap (wich comes from the main rotor turning at 300-400 RPM). I am a helicopter airframe mechanic, and I KNOW what I'm talking about! Listen to Dyehard wich is a full size PILOT! He definately should know what he's talking about, and he does.... GRANT ED is also worth listening to! The blades on a model needs to, and can due to weight and span, go at 1700! On a single engine private airplane e.g. piper PA-28 (archer) the propeller RPM is 2600 MAX! On a model airplane the propeller spins at 10-15K RPM! Helicopter aerodynamics is EXTREAMLY complex, if you look at the whole picture! And if you give yourself some time to read some books about the subject you would be amazed by the fact that these machines actually manages to fly at all. Does not intent to step on somebody here, but it is more to full size vs. models than most average flyers probably don't not know. I don't know why your tach reads incorrect, but it sure is incorrect if it reads 1700 on a full size!

Galifrey -> Gyros on "real" heli's (5/25/2003 2:45 AM)
extreme, does a 601 work with a 9252? thought you had to use a 9251.... ;)

Homebrewer -> Gyros on "real" heli's (5/25/2003 5:23 AM)
UH-1/H Normal Rotor RPM 334 rpm, no "gyro" to hold the tail straight. Coordinated movement with collective pitch and anti-torque pedals accomplished this. It was natural to push a little left pedal in automatically when you pulled pitch. CH-47D RRPM at 100% = 225 rpm. Tandem rotors practically canceled torques out but design of this puppy makes it very unstable (not impossible) to fly. AFCS tames this beast which likes to fly sideways or swap sides in forward flight if you fly AFCS off and aren't up to snuff and on your toes.

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