[Jettison]

I Have the Hotel Whiskey Aviation ER tanks installed in my RV-7. They say on their website that spins are not recommended due to a lack of flight testing by them. Does anyone have experience with this? And what would be the concern? Clearly not aerodynamic, but maybe additional weight out near the wingtips? pretty tough to do aerobatics without at least doing spins in initial training. Thoughts?

 

[Mel]

I Have the Hotel Whiskey Aviation ER tanks installed in my RV-7. They say on their website that spins are not recommended due to a lack of flight testing by them. Does anyone have experience with this? And what would be the concern? Clearly not aerodynamic, but maybe additional weight out near the wingtips? pretty tough to do aerobatics without at least doing spins in initial training. Thoughts?

There is a person who, inadvertently, performed acro with full ER tanks on his RV-8 and noted no adverse issues. I will not divulge his name, but he is a regular here.

 

[Freemasm]

You'll have an increase in angular momentum from the added weight/distribution. I can't imagine not having enough aero surface to arrest the spin as there's no related changes there. That said, expect more time for such an arrest to execute; thus, more altitude lost.

 

[Jettison]

There is a person who, inadvertently, performed acro with full ER tanks on his RV-8 and noted no adverse issues. I will not divulge his name, but he is a regular here.

Did that include spins?

 

[rocketman1988]

You'll have an increase in angular momentum from the added weight/distribution. I can't imagine not having enough aero surface to arrest the spin as there's no related changes there. That said, expect more time for such an arrest to execute; thus, more altitude lost.

The change in moment of inertia can make a huge, and largely unseen, difference. In fact, there is the potential to make even a simple spin unrecoverable. Without running numbers or practical data, it is impossible to know for sure.

Be careful out there...

 

[Mel]

Did that include spins?

If is my understanding is that it did. But I won't speak for the man.

 

[Ed_Wischmeyer]

It's been way too long since I studied all that in school, but there's a part of me that wonders about spins with partial tanks, not just full tanks. Angular momentum vs total mass...

 

[pa38112]

My belief is that empty ER tanks would not change the aircraft enough to cause a problem. Full ER tanks could.

 

[Jettison]

My belief is that empty ER tanks would not change the aircraft enough to cause a problem. Full ER tanks could.

Yeah, the website says the whole system only weighs 15 pounds……

 

[Mel]

If is my understanding is that it did. But I won't speak for the man.

My belief is that empty ER tanks would not change the aircraft enough to cause a problem. Full ER tanks could.

This is the accepted opinion.

 

[Freemasm]

The change in moment of inertia can make a huge, and largely unseen, difference. In fact, there is the potential to make even a simple spin unrecoverable. Without running numbers or practical data, it is impossible to know for sure.

Be careful out there...

Trust me I'm not downplaying the possible effects. It would be reasonable to assume the added weight and associated distribution would flatten a spin to some degree. That said, there's no foreseen reason the rudder can't effect the same counting force and that the vehicles CG is still within limits. The eyeball/common sense/shade tree mechanic effect was already mentioned, or:

Impulse (change in momentum delta (F x v)) = F x delta T

F is constant so delta T will increase proportionally with the associated Impulse.

Higher momentum -> longer time -> leave yourself more recovery altitude as a bare minimum.

If my thought process is wrong, there are plenty of technically talented people here that will @#$%! slap me, deservedly so.

All that said, Any expansion of the demonstrated operational envelope puts the pilot (back) into the test pilot category.

 

[rocketman1988]

Trust me I'm not downplaying the possible effects. It would be reasonable to assume the added weight and associated distribution would flatten a spin to some degree. That said, there's no foreseen reason the rudder can't effect the same counting force and that the vehicles CG is still within limits. The eyeball/common sense/shade tree mechanic effect was already mentioned, or:

Impulse (change in momentum delta (F x v)) = F x delta T

F is constant so delta T will increase proportionally with the associated Impulse.

Higher momentum -> longer time -> leave yourself more recovery altitude as a bare minimum.

If my thought process is wrong, there are plenty of technically talented people here that will @#$%! slap me, deservedly so.

All that said, Any expansion of the demonstrated operational envelope puts the pilot (back) into the test pilot category.

I think you would find it a bit more complicated than that, which is why I said there can often be unseen factors; in general, changing just one parameter in design can affect many other parameters some of which may not be intuitively obvious.

I definitely agree with your statement about becoming a test pilot, especially when the changed parameters may affect the controllability of the aircraft.

 

[rvbuilder2002]

Trust me I'm not downplaying the possible effects. It would be reasonable to assume the added weight and associated distribution would flatten a spin to some degree. That said, there's no foreseen reason the rudder can't effect the same counting force and that the vehicles CG is still within limits. The eyeball/common sense/shade tree mechanic effect was already mentioned, or:

Impulse (change in momentum delta (F x v)) = F x delta T

F is constant so delta T will increase proportionally with the associated Impulse.

Higher momentum -> longer time -> leave yourself more recovery altitude as a bare minimum.

If my thought process is wrong, there are plenty of technically talented people here that will @#$%! slap me, deservedly so.

All that said, Any expansion of the demonstrated operational envelope puts the pilot (back) into the test pilot category.

I don’t think math can always tell the entire story, which is why the math is done first, and then ( careful and incremental) flight testing to prove the math, is done second.

 

[Freemasm]

I don’t think math can always tell the entire story, which is why the math is done first, and then ( careful and incremental) flight testing to prove the math, is done second.

When did I say the OP wouldn’t have to validate any theory? I stated some expected effects from the new weight and its related distribution. Always better start somewhere other than a blind position. I offered some expected, theoretic effects where no one else has. I’ll stick with my approach and expected results and heed the vague “expect the unexpected”. If someone offers some other expected, quantified effects, I’ll gladly listen and hopefully learn.

 

[Daida]

For my better understanding, how are the ER tanks connected to the fuel system?

Talking about momentum (rotation) and counteracting (opposite rudder) while spinning.
If the tanks (ER and standard) are just connected to each other, they (ER) won’t be/stay empty during spinning!

The centrifugal force from spinning rotation would make the fuel move into the ER tanks, thus changing the position of the rotating mass more to the outside. Whilst i would expect the rotation speed to be maintained (aerodynamically caused) and the mass being further away from the axis, the total energy of the system (momentum) is increased.
As your possibilities to counteract this momentum, opposite rudder (rudder size), does not change, -as stated above- this puts you back in the test pilot position!

A single pound more on the wing tip will be probably not noticeable, 100 pounds most certainly will. But as you never know what the “critical mass“ is and when it is out there (depending on the inner tanks connection and flow rates to the ER’s), i would leave this to people who are professionals in the flight testing business.
There are tons of other factors to be considered as well and also tons of calculations which ought to be done.

When the ER tank manufacturer does not recommend due to lack of flight testing, i wouldn’t do it.

edited: just checked on their website, there is no direct (open) connection between the ER tanks and the standard. There’s supposed to be a pump which acts as shutoff valve when switched off. So when installed as instructed, there will be no fuel flow into the ER tanks due rotation.
My assumptions above would still be accountable for people who do the complete “wet wing” approach.
For the ER tanks from the OP, you still add weight on the outer-most position to the wing (15 lbs total).
I still wouldn’t do it.