Dominator 2 place center of mass

[jm-urbani]

Hi All,

where is situated the 2 places dominator center of mass when only one person is flying on it ?

regards

jean michel

[MrGrey]

The simple answer is below the rotor head somewhere in the back seat. When you add a passenger it does not affect nose down or up as the passenger is nicely placed within the CG envelope. I had a two place Dom and w/ passenger it hardly made any difference in the hang test.

[groundhog]

I believe the answer is do the hang test to be sure.

[jm-urbani]

High Guys,
thanks for replies, however
I should have mentioned the vertical position of the center of mass sorry
does it remains above the thrust line ?
regards
jean mich

[MrGrey]

High Guys,
thanks for replies, however
I should have mentioned the vertical position of the center of mass sorry
does it remains above the thrust line ?
regards
jean mich

That can only really be determined by a double hang test with a useful load. All of it depends on many factors... engine type size, pilot size, passenger, single/double seat tanks, pod/no pod, rotor head placement, and the list goes on. I gave the simple answer of CG...as a passenger sits nearly directly under the rotor head and does not create a significant nose up/down tendency....best to let aeronautical guys to take it from here... I just know how to fly them. :)

[Doug Riley]

I flew my Tandem Dom with both a light 2-stroke (Rotax 618) and a light (but heavier than the 2-stroke) 4-stroke (Rotax 912S). I used a 70" prop on both, so that the thrustline location was in the same place.

With both engines, the aircraft had a very noticeable nose-up trim change when power was increased -- enough so that it slowed down if forward stick pressure or in-flight trim were not applied. It had the opposite reaction when power was reduced below cruise: back stick or back trim were needed to avoid speeding up.

Since the H-stab had zero incidence, CG above thrustline was almost certainly the cause of this behavior.

[StanFoster]

Doug- I had the same experience with my SparrowHawk. It was obviously LTL, and in my opinion too low. Had I still had it I may have tried to get it more CLT. Any throttle chop and you were headed down and it took a lot of backstick pressure turning.


Stan

[Doug Riley]

Stan: The Dom required backstick in turning,too. It should. All of these reactions we're mentioning are indications of pitch stability with respect to power setting and rotor disk AOA -- a good thing.

Of course, a good thing can be overdone. I agree with you that overdoing it is not desirable. In a power-off descent, though, my Dom would stabilize at about 10 mph over (the previous) trimmed airspeed. It wouldn't simply speed up and up. Rotorcraft in general should NOT speed up endlessly in any circumstances, because rotor blowback increases, casuing disk AOA to increase and snub off the process.

If a rotorcraft DOES demonstrate airspeed runaway, it has a design problem. Something is overwhelming the natural, healthy tendency of a rotor to return to a lower airspeed. Such behavior can be related to a pod that pulls the nose down or some weird flow hitting the H-stab from below.

In the case of the Dom, I believe the airspeed increase power-off was caused by the RTV locating itself significantly aft of CG, in order to balance out the low thrustline. Once power is cut, there's nothing to hold the RTV back there, but it stays there just long enough to pull the tail up. This, in turn, decreases the rotor disk's AOA and speeds up the aircraft. Things reach a new equilibrium at a higher airspeed when additional flapping and blowback occur and when the RTV has moved forward, probably right to the CG.

[StanFoster]

Doug- Back stick is necessary of course in a turn...but I am talking a LOT of backstick was required. Also...you cut the throttle...the nose would drop like it should...but I am saying drop a LOT. If I would do nothing to the backstick, it would keep picking up speed until VNE was reached...but I would cry uncle before that and give it backstick.

Stan

[Doug Riley]

Yeah, Stan -- hence perhaps the infamous B-Days crash that we're both thinking of. There is something very unhealthy going on in a rotorcraft that overcomes its own rotor blowback and behaves the way you describe.

Since the thread is about the tandem Dom, however, I want to repeat that, at any rate, no "airspeed runaway" occurred in mine. The speed-up was merely annoying, requiring a pull on the trim switch when powering down, if one wanted to hold airspeed. If you didn't pull the switch or hold back pressure, your landing approach would be at 75 instead of 65.

[StanFoster]

Doug- I regret never double hang testing my SparrowHawk. It was after I sold it that I started finding the CG by double hanging. It must have been several inches LTL. Stan

[Chuck Roberg]

Doug- Back stick is necessary of course in a turn...but I am talking a LOT of backstick was required. Also...you cut the throttle...the nose would drop like it should...but I am saying drop a LOT. If I would do nothing to the backstick, it would keep picking up speed until VNE was reached...but I would cry uncle before that and give it backstick.

Stan

Stan, In all the different Sparrow Hawks I have flown. I noticed the same thing. I just got used to it and automatically added back stick on throttle reduction.

[Doug Riley]

I think that's what others have reported, Stan. It's true of the Dominator as well. I never double-hung mine, either, because I knew what I would find: several inches of LTL.

[jm-urbani]

I flew my Tandem Dom with both a light 2-stroke (Rotax 618) and a light (but heavier than the 2-stroke) 4-stroke (Rotax 912S). I used a 70" prop on both, so that the thrustline location was in the same place.

With both engines, the aircraft had a very noticeable nose-up trim change when power was increased -- enough so that it slowed down if forward stick pressure or in-flight trim were not applied. It had the opposite reaction when power was reduced below cruise: back stick or back trim were needed to avoid speeding up.

Since the H-stab had zero incidence, CG above thrustline was almost certainly the cause of this behavior.

off topic sorry, Ernie emailed me he did not used 912uls on his 2 places doms ... but only 914's, maybe he understood 912 (80hp) when I asked about the 912uls (100HP)

what was your 618 power ?

was it enough ?

and what about yours 912 uls is it ok for your Dom ?

on his site Ernie states he mounts hirth's (they are 100 hp it think not more?)

regards

jm

[Doug Riley]

JM: My Dom was the heavier model -- 3" deep extra-long front keel, pod, hydraulic prerotator, large radiator, dual strobes, dual airspeed and engine tach, intercom, two seat tanks with covers and 28-foot blades. The 618 (73 hp)was more than enough power solo. I could only carry light passengers on a hot day, however.

The 912S (100 hp) was quite adequate. I flew passengers up to 250 lb., although I did not fill the fuel tanks on those flights. I weigh about 180 lb.

The Rotax 912S was 100% reliable. I would be very reluctant to fly passengers, much less students, with a Hirth. They have a terrible reputation.

[Early Bird Dave]

Mine has a Sub 4 and I do get a nose drop but from what I recall I need to add a slight nose down with engine out to increase speed when in trim flight.
I do not get Doug's" speed up" unless I nose over.
Mine is the heavy loaded model at approx. 600 lbs. empty. 27 foot blades.
I need very little trim solo and I weigh 220. I add on trim with passenger.
Very stable.

[Doug Riley]

Dave's Dom. is much more heavily loaded than mine. Mine weighed about 495 empty with the 912S, I weigh about 175 and I had a 28-foot rotor. It felt like an ultralight when I flew it solo -- especially back when I had the even lighter 618.

[Murray Barker]

Heres one with a Rotax 912.

[Penguin]

@Murray - that's a brilliant photo!

[Murray Barker]

Photo taken by Sam LaBruna during the Australian fleet survey.
This was a mandatory Thrustline /COG survey for gyro's registered in OZ.

Some owners stuck their head in the sand but overall we got good results.