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[Uriah]

After a little research, it seems the basic cause of ground resonance is a dynamic interaction between the rotor forces and the ground contacts, namely asymmetric lift and flapping moment of the rotor disk, causing a feedback loop that increases the magnitudes of the forces. Flapping moment is property I can set in the rotor, so I will have to experiment with changing the value.

This video provides a good illustration of the  dynamics involved in mast bumping and flapping moments.



I was surprised by how much force ground resonance can exert and resulting catastrophic failure of the airframe, such as in the case with this CH-47.



I also plan for the rotor blades to have a damage model, resulting in physical damage and affecting the aerodynamics forces of the flight model if they collide with the ground.

Another thing I need to be able to model is turbine compressor stall. I was recently able to get an example of a flight model which uses a turbine engine, however I'm still having issues with this example in terms of the a system called "rpm-governor", which causes power loss when collective is increased.

Regards,
Uriah

[krcm1981]

Good thing about the 64 and 60 because they are rigid rotor systems you won't get mast bumping, but flying the TH-67, OH-58A/C and UH-1 it was a serious concern and had to listen for a high pitch ping/ring. Never got into a mast bumping situation so I don't know exactly what it sounded like.

[Uriah]

Hey there,

Good to see you around!

It is great there are so many pilots frequenting the OT forums! You flew the UH-60A primarily if I remember correctly? Don't worry, I haven't forgotten about the Black Hawk! In fact, much of the work I've done on the Apache flight model has been applied to the UH-60 FDM as well. I'll try to get a new versions up of the flight models for both the AH-64 and the UH-60 sometime this coming week. It would be great to get your feedback.

I certainly need to do more research about the aerodynamic differences between each type of rotor system; fully articulated, semi-rigid and rigid.

From my understanding, in a rigid rotor system the flapping moment occurs by means of blade flex instead of articulated hinges. Specifically, with a rigid rotor system, does the rotor disk's collective lift vector change angle relative to the airframe in the direction of cyclic, or is it only the pitch/roll angle of the aircraft that changes?

With a semi-ridged or articulated system, the rotor disk tilts in the direction of cyclic, so does differential blade flex due to cyclic pitch have the same effect with a ridged system? It seems logical that the load, and therefore the flex, on each blade changes with cyclic pitch, and if forward cyclic is applied there will be an average differential between the fore and aft blades.

If anyone knows of any sources for aerodynamic data or a mathematical model of the Apache, it would be extremely helpful. I have found some, such as the basic specifications for the main rotor and tail rotor, but am lacking sufficient quantitative data. I have enough data on the turbine engines and APU, but have almost nothing about the gear ratios for the transmission/gear box. I'm also having a hard time determining the center of gravity location and the moments of inertia (empty weight) for the AH-64D Longbow and/or AH-64E Guardian.

Best regards,
Uriah

[MrWohoo]

Oh god i love cheesy US Army training videos

In all seriousness, that video teahced me more than i already knew, so thats good for a change.

The rotoblade damage model would be, quite frankly, quite fun! having good fun with that damagemodel in DCS flying the helicopters. So i reckon it'll do good for all

Now when you say turbine compressor stall, you mean total engine faliure, following autorotation ?
The RPM-Governor is especially handly when flying with a slingload or heavy gross weight, so i do think that is an requirement, especially in combat enviroment (not quite sure, but its logical (Too lazy to look up the facts atm ))

Anyways, we had a good chat about this, still amazed theres someone out there who actually knows what hes talking about

Cheers Uriah and KW71 !

[Uriah]

Turbine compressor stall occurs when the power demanded to maintain the required rotor speed is greater than the engine can supply, which can be a function of change in collective over time, among many other factors such as altitude or gross weight. In the event of engine compressor stall you would decrease collective, to take some of the load off the turbine, so the speed of the compressor and the rotor can pick back up, then collective would be increased again to gain lift, or else you would get into a situation where autorotation would be necessary.

Anyways, we had a good chat about this, still amazed theres someone out there who actually knows what hes talking about

Actually, I learned much of this from krcm1981 here, and my limited understanding of physics and aerodynamics!  Getting feedback from pilots is very critical, maybe more so than having fancy data, because a lot of the technical information available is difficult to understand without practical experience.

Best regards,
Uriah

[krcm1981]

This guy breaks down helicopter mechanics pretty well, give these vids a view
https://m.youtube.com/playlist?list=PLZSdN_I4WlGvdepmdlhZpJTTF_meQb4kL

[Uriah]

Nice thank you! Great resource.

I'm going to have to take a lot of notes. 

Regards,
Uriah

[KW71]

Hi, krcm1981!!

"U.S. Army CW2 Ret.
153DD UH-60A/L Aviator MedEvac" 

Being in the U.S. Army and entering in this forum, is like being a doctor and entering in  an old people's home!!   

So, doctor krcm1981, do you think you could helpme with some photos, so I can model things accurately? I have tons of pics, but there are always those hidden corners or unseen angles that prevent me to finish things the way I should.

[krcm1981]

Hi, krcm1981!!

"U.S. Army CW2 Ret.
153DD UH-60A/L Aviator MedEvac" 

Being in the U.S. Army and entering in this forum, is like being a doctor and entering in  an old people's home!!   

So, doctor krcm1981, do you think you could helpme with some photos, so I can model things accurately? I have tons of pics, but there are always those hidden corners or unseen angles that prevent me to finish things the way I should.

Hey KW71,
Unfortunately the few pics I have left after a computer crash are not that great, I never thought to take pics of all kinds of things while I was actually flying the 60(it was just a job), I regret that every day now because I realize how special it was and the memories that were once everywhere in my head are fading from memory faster and faster as the years go by. Also to clear up info for everyone, I was not a Dr, After flight school I took the MedEvac Pilot/operations course at Fort Sam Houston because I was going to a MedEvac unit. My specific MedEvac unit didn't have any Q models just A models, a lot of the material gave specifics on the Q(Designated UH-60 for MedEvac mission). We still had the carousel with four litter shelfs and most if not all medical equipment for the medic to use but didn't have most of the customizations of the Q airframe/interior. For any potential future Army Aviators specifically High School Student getting ready to graduate, you can go through the High School to Flight School program the Army has, that's the route I took. If anyone is interested I can start a off topic discussion, let me know. I'll also post 60 pics in the 60 thread here.

[KW71]

Ha ha! Yep, I guess is a common misconception... In this case I thought "People in the US Army should be walking between all kind of war vehicles every day"... Not at all, of course... I used to work for a telecommunication company (organizing the radio bases setups), and my friends use to ask me for good cheaps phones... Of course I had no idea...

Thanks anyway krcm1981, and welcome aboard!!!

[KW71]

A glance to the new tail rotor.

[bongodriver]

is the lamination detail at the rotor roots actual polygon detail or bump mapping? it looks superb.

[KW71]

Hi, bongodriver!   Is bump mapping.

[Revolver]

hehe ... yet a "detail freak" ... wish every success Amigo.
The court, we are on the road to explore soon with a properly equipped Apache to OT-Earth. 

Thank you for your effort.

[KW71]

Ha ha!!! Thanks, AH-DG!!!