6G's at what speed?

[BillL]

I hear the Hartzell blended airfoil is good for 6 g's. OK -Fine, well what is the composite good for with a 2.5" spacer? For gyroscopic bending moment in a loop or pull out, the equation is moment=inertia*angular velocity (prop)*angular velocity(of planes path). OK, yes we need the off set from the base of the prop flange, and the equation modification for a two blade prop. But, I calculated the rad/sec for differing G loading upon entry of a loop (for instance). If the calcs are correct, then it seems the plane speed is a big factor for 6 G capability. The same bending moment on the crank is at 4 G's, 140 mph, or at 6 G's at 210 MPH. So the big question - at what speed is this calculated? 6G's at maneuvering speed?

I will measure the Moment of inertia via the bifilar pendulum method, but that just gets me in deeper in the unknown. Can anyone help with the conditions under which the official G limit (for the prop and engine flange) is determined? and who does it, Lycoming, or Hartzell, or the airframe designer?

Enlightenment would be greatly appreciated. Thanks!

 

[sandifer]

I guess I'm unclear on the purpose of the exercise, and the supposed 6G limit on the prop. Are you thinking only in terms of gyroscopic load on the prop hub and crank/flange due to the rotation rate of the propellor plane? If so, then a loop (regardless of airspeed and G load) would not produce the largest gyroscopic load. A simple hammerhead will induce more gyroscopic load on the crank than you could ever achieve from a looping maneuver, simply due due to the difference in rotation rate. Are you simply interested in the mathematical exercise, or do you actually have concerns about gyroscopic loads on your crank when doing aerobatics in an RV? Unless you're doing snaps, tumbles, or full-power flat spins, I would not worry about the gyro loads, even with a metal prop and a light crank flange.

 

[DanH]

Pretty sure you want pitch or yaw rate in radians per second rather than angular velocity of the flight path.

Bifillar suspension for MMI is good, but an email to Les at Hartzell would be faster.

In the context of an RV I guess a flat spin would be worst case. Unless you can find somebody with a measured rate I'd suggest 2700 RPM and 1 rotation per second, or MMI * 283 * 6.28

 

[BillL]

. . Are you simply interested in the mathematical exercise, or do you actually have concerns about gyroscopic loads on your crank when doing aerobatics in an RV?

Yes, Eric, I do have concerns about basic aerobatics. I was told that there was 4G "limit" using the 2.5" extension, regardless of the aerobatic maneuver. I have a new M1B and it has a .440" thick flange. I am simply trying to get some numbers to quantify the comparison between the BA and composite. It does not have to be accurate, or guaranteed, but would sure make be feel better than "you are probably OK". I have never done aerobatics, and don't plan on learning the extreme maneuvers, but wish to understand what limits exist for my configuration. Primarily, just the effect of the prop extension, as everything else is pretty much standard.

This is one of those things that don't get documented openly, feel free to contact me in other ways.

 

[sandifer]

I understand the issue with extra load on the crank the further the prop gets spaced from the hub, but there are so many variables, I don't understand how this can be boiled down to a simple G-load limit issue. As you're already aware, the gyro load depends on the pitch rate, which depends on airspeed for a given G-load. And prop RPM is another variable that significantly affects gyro load on the crank. Metal vs. composite prop makes a huge difference. And again, you could be as careful with the G as you want on the pull into a hammerhead, but you'd then load your crank more during the pivot than you could during any sort of pull, up to 6G even. Don't all aerobatic model RVs use a 2+" prop spacer? Never heard of anyone with an RV cracking their crank flange because of aerobatics.

In the context of an RV I guess a flat spin would be worst case. Unless you can find somebody with a measured rate I'd suggest 2700 RPM and 1 rotation per second, or MMI * 283 * 6.28

That would be one helluva flat spin. Flat spin rotation in acro planes is typically quite a bit slower than normal power-off spin rotation. The Pitts flat spins with power around 150 deg/sec.

On that note, anyone actually induced an honest flat spin with power in an RV? Just curious. Never thought to try it when I had my RV-3. Never heard of anyone doing this in an RV. For acro-type planes with no spin quirks, flat spins are no big deal.

 

[BillL]

The 6 G title was to get some attention to the subject. It is much more involved than that, but your discussion has helped define how the problem can be approached. Pitch and yaw rates are certainly the way. Oh - Dan, the plane angular velocity is in rad/sec. You probably remember our discussion about this at last years Osh/VAF beer tent.

1 rev/sec is 60 rpm, quite a spin!

My take away is to think about what aero maneuvers are in scope and what pitch and yaw rates apply along with engine rpm.

I had talked to Les when I was making my prop selection and will renew that conversation, maybe I can just get the moment ratio of the two props since the stresses will be proportional to that anyway. That way he does not have to give proprietary information away.

Thanks for the discussion.

 

[F1Boss]

GEEZ!

Hey Fellas:

I had the same conversation with Les re:F1 Rocket and the longer M hubs used on those ships. It came down to: the ship won't produce enough rotation in any plane to cause a problem, even with all the levers forward. This is in reference to an 80" 2 blade 8475 prop (BIG ol' blades).

Since you are using the shorter C hub with shorter, lighter blades, my bet is your margin is fine, no matter what you do.

Agreed: the hammerhead is the closest you would get to the rotational limits.

BillL: my guess is you are an engineer. Quit worrying and finish your project.

Carry on!
Mark

 

[BillL]

Hey Fellas:


BillL: my guess is you are an engineer. Quit worrying and finish your project.

Carry on!
Mark

Guilty as charged!

That is the kind of answer I can live with, and just enough understanding to work. I shall build on!

Thanks to all.