Quantcast

Modern day "motorcycle of the air" aircraft class?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Victor Bravo

Well-Known Member
Joined
Jul 30, 2014
Messages
7,699
Location
KWHP, Los Angeles CA, USA
So, what do you think of the spratt wing and fixed rudders?
Were you asking me or Erkki?

Speaking only for me, I don't really like the small tip plates on the end of the rear wing, they will not provide anywhere near as "solid" of yaw stability as larger and higher aspect ratio fins. Taller, narrower fins on the outboard end of the rear wing can work (Ercoupe), but you give away any and all benefit of the prop blast. The prop blast on a large central rudder will be a big deal on a mono-wheel landing gear like what your rendering shows. Even if you have nosewheel steering forward of the main gear, once you raise the nosewheel off the ground during the takeoff roll, you'd be relying on rudder authority from then on. Having the rudder in the prop blast will increase that authority.

If you are referring to the Spratt wing specifically as a free-wing with no direct control input to it, I don't like it at all. The Mignet system of a direct control linkage from the stick to the forward wing for pitch control, I can live with.
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
1,909
Location
Flint, Mi, USA
Were you asking me or Erkki?

Speaking only for me, I don't really like the small tip plates on the end of the rear wing, they will not provide anywhere near as "solid" of yaw stability as larger and higher aspect ratio fins. Taller, narrower fins on the outboard end of the rear wing can work (Ercoupe), but you give away any and all benefit of the prop blast. The prop blast on a large central rudder will be a big deal on a mono-wheel landing gear like what your rendering shows. Even if you have nosewheel steering forward of the main gear, once you raise the nosewheel off the ground during the takeoff roll, you'd be relying on rudder authority from then on. Having the rudder in the prop blast will increase that authority.

If you are referring to the Spratt wing specifically as a free-wing with no direct control input to it, I don't like it at all. The Mignet system of a direct control linkage from the stick to the forward wing for pitch control, I can live with.
I appreciate input from either of you.

As erkki said, a spratt wing is essentially a Mignet type system only with two control linkages so the left and right halves of the wing can be pitched separately when you move the stick side to side. Two-axis with fewer parts than a moving rudder and moving wing would need.
Now, I was under the impression that having the rudder out of the propwash was a good thing since the wash introduces asymmetrical forces on it so you need input to counter it.
 

FritzW

Well-Known Member
Log Member
Joined
Jan 30, 2011
Messages
3,896
Location
Las Cruces, NM
I wanted to mention something about that but I couldn't find a way to say it that didn't sound dirty.
 

pwood66889

Well-Known Member
Joined
Feb 10, 2007
Messages
1,658
Location
Sopchoppy, Florida, USA
Taller, narrower fins on the outboard end of the rear wing can work (Ercoupe), but you give away any and all benefit of the prop blast. The prop blast on a large central rudder will be a big deal on a mono-wheel landing gear like what your rendering shows. Even if you have nosewheel steering forward of the main gear, once you raise the nosewheel off the ground during the takeoff roll, you'd be relying on rudder authority from then on. Having the rudder in the prop blast will increase that authority.
Fred Weick worked a long time at ERCO to get the independent yaw control removed. It involved canting the engine, and moving the "verticle" to the end of the elevators.

" I was under the impression that having the rudder out of the propwash was a good thing since the wash introduces asymmetrical forces on it so you need input to counter it."
Correct, Sock. But wash amount effects control; yet one's throttle may not agree with what one needs from the aircraft.

The `coupe does approach a "motorcycle of the air." "Ask the person who owns/has flown one."

Percy in NW FL, USA
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
1,909
Location
Flint, Mi, USA
To make sure I'm getting the idea across, The broomstick would be two-axis, using the two halves of the main wing for pitch and roll control. The rudders would be fixed, and only there for yaw stability.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,160
Location
Rocky Mountains
Interesting engine about the right size.

Yes it is interesting. Looks kind of like a Honda version of the Suzuki 1L. 50 Hp from the same displacement as the Briggs vertical engines. Being a marine engine it's probably rated for continuous operation?
Nice OEM bosses that could be used for the PSRU mounts.
 

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,909
Location
capital district NY
2 Blade Propeller dia. = 1.83 times the cube root of the bhp.
3 Blade Propeller dia.= 1.5 times the cube root of the bhp.
Does anyone know for a 4 Blade Propeller dia the multiplying factor of the cube root of the bhp?
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,010
Location
US
2 Blade Propeller dia. = 1.83 times the cube root of the bhp.
3 Blade Propeller dia.= 1.5 times the cube root of the bhp.
Does anyone know for a 4 Blade Propeller dia the multiplying factor of the cube root of the bhp?
Maybe a hint or clue about where this comes from? Is this some sort of rule-of-thumb for picking a prop diameter? I would think airspeed would need to be included somehow for this sort of thing to be useful.
 

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,909
Location
capital district NY
Maybe a hint or clue about where this comes from? Is this some sort of rule-of-thumb for picking a prop diameter? I would think airspeed would need to be included somehow for this sort of thing to be useful.
Dan Raymer's SIMPLIFIED DESIGN FOR HOMEBUILDERS page 23. And yes there is another formula with airspeed to check tip speed on the same page.

I believe what he his saying is that these are the practical diameters the hp is going to be able to use. Then check to see if it works with the airspeed.

I don't think airspeed is all that much of an issue in practical terms for what I am playing around with, it is mostly if I can reduce propeller dia. and also it may just be easier to build a four blade prop than a three blade prop. A four blade prop can be as easy as laminating two, two blade props together. Then it becomes whether they are more efficient parallel like the Lazair's or placed in some kind of X configuration or just plane ol' square to one another.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,010
Location
US
Dan Raymer's SIMPLIFIED DESIGN FOR HOMEBUILDERS page 23. And yes there is another formula with airspeed to check tip speed on the same page.

I believe what he his saying is that these are the practical diameters the hp is going to be able to use. Then check to see if it works with the airspeed.
Thanks for the reference. It is probably a useful starting point, but maybe not equally applicable across all HPs. For example, for a 75HP engine and a 2 blade prop, it gives a result of 7.7. If that is "feet" then it would be a 92" diameter prop, and there's no 75HP engine I know of that turns a prop that long.

With regard to helical tip speed, we do care about it, but doesn't change a >whole< lot with airspeed. The circular speed of the prop continues to be much more important in this regard than the speed of the airplane for most GA applications.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,502
Location
97FL, Florida, USA
Dan Raymer's SIMPLIFIED DESIGN FOR HOMEBUILDERS page 23. And yes there is another formula with airspeed to check tip speed on the same page.
My copy (this is the first time I’ve referred to it - need to scan it soon) says the fourth root rather that the third. That is better. The 10 foot diameter for my airplane would necessitate an entirely new landing gear.


BJC
 
Last edited:

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,909
Location
capital district NY
Thanks for the reference. It is probably a useful starting point, but maybe not equally applicable across all HPs. For example, for a 75HP engine and a 2 blade prop, it gives a result of 7.7. If that is "feet" then it would be a 92" diameter prop, and there's no 75HP engine I know of that turns a prop that long.

With regard to helical tip speed, we do care about it, but doesn't change a >whole< lot with airspeed. The circular speed of the prop continues to be much more important in this regard than the speed of the airplane for most GA applications.
Sure but he is saying 7.7 feet is the biggest dia. that in practicality a 75hp engine is going to turn, then check tip speed and reduce dia.

V (tip) = The square root of (V squared + {[pie x n x D]squared}) ; D in feet, V in feet per second, n=RPM/60.
My copy (this is the first time I’ve referred to it - need to scan it soon) says the fourth root rather that the third. That is better. The 10 foot diameter for my airplane would necessaries an entirely new landing gear.


BJC
Crap is says to the fourth, I was looking at the fourth, guess I was interpreting square (four sides) to cube or something equally silly. It would be easier if I could type the proper formula instead of having to word it.
 

Dart

Active Member
Joined
Apr 9, 2015
Messages
38
Location
Near Nelson B.C.
I'll start by pointing out that I'm ignorant of the facts around prop design, but have some impressions that are not aligned with what I'm reading here. I'd thought the general assumption was that a larger prop, aerodynamically is always better, and for practical reasons they are sized down? Moving lots of air more slowly and all that?
 
Top