Wing and prop gaps

elippse · #1 08-20-2010, 03:20 PM

I just got off the phone with a friend that flies an RV-6. We were discussing the sealing of the gap between the propeller blade and the spinner, and I was commenting on how that seal is as important as the seal between the wing and fuselage of a plane. He related to me how he and another were flying along one day and this other fellow told him he was suddenly getting a lot of noise and vibration. My friend took a look and saw that about 18" of the rubber sealing strip had come loose on the back of the left wing root ahead of the flap. He then related how another who flies an RV told him that the rubber sealing strip on one wing came off in flight and how the plane became hard to control.
On both a propeller and a wing it is important that the pressure differential between the top and bottom surfaces be maintained from tip to tip over the fuselage or spinner. If the rubber sealing strip that seals the wing to the fuselage comes off, that wing panel will have two tips; one at the tip and one at the root. The mass that the wing intercepts in flight will no longer be a tube that is the diameter of the wingspan and whose length is the forward speed, but instead has a mass of about twice the diameter of one wing panel and the forward speed, and so you are left with less than half of the mass flow and you will have to operate the plane at over twice the AOA in order to remain aloft.
He found that on sheet 46 of his RV-6 plans, rev. 92, it gave no dimension for how large the gap was to be. On his plane, it was 5/16" and the sealing strip went up 3/8" on the fuselage. Now consider this; in the cowling you have the flex strips curved onto the inner surface of the cowling in such a way that the pressure differential plasters the seal against the cowl. Now look at the wing. The inside of the wing is at a higher pressure than is the top surface, so wouldn't it make more sense to have the rubber sealing strip on the top of the wing bent down against the fuselage rather than up so that the pressure would hold it tightly? And when you pull more Gs, the pressure rise will go up proportionately with the G load. Maybe some of you had seal leakage in flight and didn't know it when doing aerobatics with resulting unusual flight characteristics!
Here's one fellow telling about two others that have had the strip come off in flight with troubling results; what are the odds of that? And since those plans give no idea of how much gap the rubber strip should seal, maybe some who trimmed it wider might have problems in the future, especially when the seal hardens with time and weather exposure. But to get back to where I started, it is just as important to seal your prop to your spinner so that you get the best efficiency from it. I tell customers to keep the prop-spinner gap no larger than 1/8" and to put adhesive-backed 1/2" foam around the blade just inside the spinner so that centrifugal force will hold the foam against the spinner and form a tight seal. In that way your prop will continue the pressure differential up and over the spinner from one blade tip to the other. And please don't argue about what the pressure is inside the wing or spinner; those strips wouldn't have been extruded without a force pushing them out. And no, suction is not a force!

pierre smith · #2 08-21-2010, 05:22 AM

....would the foam allow blade movement on my -10 with its two-bladed blended airfoil and still maintain a seal? How many degrees does a blade travel from flat to full pitch on the IO-540?

Thanks,

elippse · #3 08-21-2010, 10:48 AM

Quote:

Originally Posted by pierre smith

....would the foam allow blade movement on my -10 with its two-bladed blended airfoil and still maintain a seal? How many degrees does a blade travel from flat to full pitch on the IO-540?

Thanks,

Hi, Pierre! The use of the foam is for FP props. I have a friend, who on my recommendation, made a rotary seal from the blade to the spinner on his race plane; he picked up 5-6 kt, about 2% more speed. This is like getting 6% more power for free! Rocket Bob was going to send me one of the blades from his CS prop and I was going to either make a mold or give him the dimensions for one to form a streamlined cuff. Once you have a cuff with a filled-in flat bottom, it is not much trouble to build up the spinner below the blade's angular travel, and with felt on the bottom flat surface of the blade rubbing against the spinner, form a rotary seal; we've made one!

mcencula · #4 08-21-2010, 08:19 PM

Quote:

Originally Posted by elippse

<snip>
On both a propeller and a wing it is important that the pressure differential between the top and bottom surfaces be maintained from tip to tip over the fuselage or spinner.
<snip>

Paul,

Somequestions about this statement:
Regardless of what you do with sealing the prop to the spinner, isn't a prop going to behave as two independent "wings" rather than one which spans the spinner?
Are you suggesting that sealing the prop to the spinner eliminates the hub vortices?
Are you trying to carry the pseudo-elliptical loading from one tip across to the other, or do you expect pseudo-elliptical loading to be applied to each blade independently resulting in a no-load section at the hub?

P.S. Sorry for the thread drift...I'm just starting to learn about this stuff.

Thanks,

elippse · #5 08-21-2010, 09:52 PM

Quote:

Originally Posted by mcencula

Paul,

Somequestions about this statement:
Regardless of what you do with sealing the prop to the spinner, isn't a prop going to behave as two independent "wings" rather than one which spans the spinner?
Are you suggesting that sealing the prop to the spinner eliminates the hub vortices?
Are you trying to carry the pseudo-elliptical loading from one tip across to the other, or do you expect pseudo-elliptical loading to be applied to each blade independently resulting in a no-load section at the hub?

P.S. Sorry for the thread drift...I'm just starting to learn about this stuff.

Thanks,

Hi, Mike! I'm note sure what you mean by "pseudo". Do you use that word as a descriptor to imply that the lift may not be precisely elliptical, with some percentage of departure from a true ellipse? In the program that designs the propeller, and the program that goes through the same equations with a simple model of the plane, the assumption is that the pressure carries across the spinner from tip to tip. That means that the mass-flow must be based on this total area otherwise the performance of the blade would suffer, and it would not meet its estimates, which it does. Have I any way to prove this is taking place? No, other than the fact that my prop designs perform exactly as predicted! I don't think this would occur if the lift went to zero at the hub/spinner. We are all just learning!

Greg Arehart · #6 08-21-2010, 10:22 PM

Quote:

Originally Posted by elippse

He then related how another who flies an RV told him that the rubber sealing strip on one wing came off in flight and how the plane became hard to control.

This is curious to me as earlier this year I took off from Ely and had a nasty banging noise. Upon returning to the airport, it was obvious that it was the wing root sealing strip. Since it was late in the day and I didn't want to take the time to pull the gear fairing and wing root fairing to reinstall the sealing strip, I just pulled the sealing strip off and flew home. I noticed no difference in performance or handling, other than some extra wind noise.

I won't argue that sealing is not a good idea (how about that for a confusing double negative?), but it really surprises me how such a small gap could cause any significant handling difficulties. My disclaimer is that I'm flying a 9 whereas this person was a 6 but....

cheers,
greg

pierre smith · #7 08-22-2010, 05:19 AM

....on my -10, nor do any of them...I think, instead, the fairing almost touches the fuselage with no appreciable gap. I don't see why the 2 seaters can't also do this.

Best,

Ron Lee · #8 08-22-2010, 06:53 AM

Pierre, my question is what is the most aerodynamic (least drag) way to finish the junction between the wing and fuselage? Should it be a curved surface of TBD radius?

aerhed · #9 08-22-2010, 08:05 AM

I think it's been shown that it takes a very correctly increasing radius at the root to beat a sharp corner for drag. Does anyone out there have a root fairing that beats the stock joint?

hevansrv7a · #10 08-22-2010, 08:25 AM

Quote:

Originally Posted by elippse

....
I have a friend, who on my recommendation, made a rotary seal from the blade to the spinner on his race plane; he picked up 5-6 kt, about 2% more speed. This is like getting 6% more power for free!...
... In the program that designs the propeller, and the program that goes through the same equations with a simple model of the plane, the assumption is that the pressure carries across the spinner from tip to tip. That means that the mass-flow must be based on this total area otherwise the performance of the blade would suffer, and it would not meet its estimates, which it does. Have I any way to prove this is taking place? No, other than the fact that my prop designs perform exactly as predicted! I don't think this would occur if the lift went to zero at the hub/spinner. We are all just learning!

Predictions and spanwise theory notwithstanding, the q at the hub is a great deal less as a result of the much lower air velocity vector (which is the right triangle net of the rotational speed - less - and the forward speed - the same). With that as a "given", what could possibly explain a 6% gain in THP from that region alone? Is there that much area? Is the pitch affected that much? Inquiring minds want to know.

For instance, with a 72" prop at 2700 RPM at 200 MPH TAS the sum of the 2 net airspeeds-squared for the first 27% of the blade is only 5.9% of the total for the blade, using 8% intervals from 19% to 99%. This ignores the values inbetween and it ignores chord although on some of Paul's designs, chord is lower at the root than at mid-blade and a little outboard of there. This is rather crude, but it puts the question in perspective. - how to explain a 6% gain in THP?